Re: FETCH FIRST clause PERCENT option

[Jaime Casanova]

On Tue, Jan 26, 2021 at 07:29:11PM +0300, Surafel Temesgen wrote:
> On Mon, Jan 25, 2021 at 2:39 PM Kyotaro Horiguchi 
> wrote:
> 
> > Sorry for the dealy. I started to look this.
> >
> >
> Hi kyotaro,
> Thanks for looking into this but did we agree to proceed
> on this approach? I fear that it will be the west of effort if
> Andrew comes up with the patch for his approach.
> Andrew Gierth: Are you working on your approach?
> 

Hi Surafel,

I'm marking this one as "returned with feedback". Then you or Andrew can
submit a new version of the patch for the next CF.

-- 
Jaime Casanova
Director de Servicios Profesionales
SystemGuards - Consultores de PostgreSQL

Re: FETCH FIRST clause PERCENT option

[Jaime Casanova]

On Thu, Mar 25, 2021 at 11:15:10AM -0400, David Steele wrote:
> On 1/26/21 11:29 AM, Surafel Temesgen wrote:
> > 
> > On Mon, Jan 25, 2021 at 2:39 PM Kyotaro Horiguchi
> > mailto:horikyota@gmail.com>> wrote:
> > 
> > Sorry for the dealy. I started to look this.
> > 
> > Hi kyotaro,
> > Thanks for looking into this but did we agree to proceed
> > on this approach? I fear that it will be the west of effort if
> > Andrew comes up with the patch for his approach.
> > Andrew Gierth: Are you working on your approach?
> 
> [Added Andrew to recipients]
> 
> Andrew, would you care to comment?
> 

Hi everyone,

This is still marked as needs review, but I think it should be marked
as "Returned with feedback" or "waiting on author".

suggestions?

-- 
Jaime Casanova
Director de Servicios Profesionales
SystemGuards - Consultores de PostgreSQL

Re: FETCH FIRST clause PERCENT option

[David Steele]

On 1/26/21 11:29 AM, Surafel Temesgen wrote:


On Mon, Jan 25, 2021 at 2:39 PM Kyotaro Horiguchi 
mailto:horikyota@gmail.com>> wrote:


Sorry for the dealy. I started to look this.

Hi kyotaro,
Thanks for looking into this but did we agree to proceed
on this approach? I fear that it will be the west of effort if
Andrew comes up with the patch for his approach.
Andrew Gierth: Are you working on your approach?


[Added Andrew to recipients]

Andrew, would you care to comment?

Regards,
--
-David
da...@pgmasters.net

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Mon, Jan 25, 2021 at 2:39 PM Kyotaro Horiguchi 
wrote:

> Sorry for the dealy. I started to look this.
>
>
Hi kyotaro,
Thanks for looking into this but did we agree to proceed
on this approach? I fear that it will be the west of effort if
Andrew comes up with the patch for his approach.
Andrew Gierth: Are you working on your approach?

regards
Surafel

Re: FETCH FIRST clause PERCENT option

[Jaime Casanova]

On Mon, Jan 25, 2021 at 6:39 AM Kyotaro Horiguchi
 wrote:
>
> Sorry for the dealy. I started to look this.
>
> At Fri, 25 Sep 2020 12:25:24 +0300, Surafel Temesgen  
> wrote in
> > Hi Michael
> > On Thu, Sep 24, 2020 at 6:58 AM Michael Paquier  wrote:
> >
> > > On Mon, Aug 10, 2020 at 01:23:44PM +0300, Surafel Temesgen wrote:
> > > > I also Implement PERCENT WITH TIES option. patch is attached
> > > > i don't start a new tread because the patches share common code
> > >
> > > This fails to apply per the CF bot.  Could you send a rebase?
>
> This still applies on the master HEAD.
>
> percent-incremental-v11.patch
>
> The existing nodeLimit passes the slot of the subnode to the
> caller. but this patch changes that behavior. You added a new function
> to tuplestore.c not to store a minimal tuple into the slot that passed
> from subnode, but we should refrain from scribbling on the slot passed
> from the subnode. Instead, the PERCENT path of the limit node should
> use its own ResultTupleSlot for the purpose. See nodeSort for a
> concrete example.
>
>
> +   LIMIT_OPTION_PER_WITH_TIES, /* FETCH FIRST... PERCENT WITH TIES */
>
> That name is a bit hard to read. We should spell it with complete
> words.
>
> case LIMIT_INWINDOW:
> ...
> +   if (IsPercentOption(node->limitOption) && 
> node->backwardPosition
> ...
> +   if (IsPercentOption(node->limitOption) && 
> node->reachEnd)
> ...
> +   if (IsPercentOption(node->limitOption))
>
> I think we can use separate lstate state for each condition above
> since IsPercentOption() gives a constant result through the execution
> time. For example, LIMIT_PERCENT_TUPLESLOT_NOT_FILLED and
> LIMIT_PERCENT_TUPLESLOT_FILLED and some derived states similar to the
> non-percent path.  I *feel* that makes code simpler.
>
> What do you think about this?
>
> regards.
>
> --
> Kyotaro Horiguchi
> NTT Open Source Software Center
>
>

Hi,

I was testing this and found that it doesn't work well with
subselects, this query make the server crash:

"""
select * from pg_type where exists (select 1 from pg_authid fetch
first 10 percent rows only);
"""

postgres was compiled with these options:

"""
CFLAGS="-ggdb -O0 -g3 -fno-omit-frame-pointer" ./configure
--prefix=/opt/var/pgdg/14dev/percent --enable-debug --enable-cassert
--with-pgport=54314 --enable-depend
"""

attached  is the stack trace

-- 
Jaime Casanova
Director de Servicios Profesionales
SystemGuards - Consultores de PostgreSQL
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
set = {__val = {0, 140724437079104, 2, 6, 6458975, 94647092334544, 
4611686018427388799, 139701007526566, 0, 281470681751456, 0, 0, 0, 0, 0, 0}}
pid = 
tid = 
ret = 
#1  0x7f0eacb55535 in __GI_abort () at abort.c:79
save_stage = 1
act = {__sigaction_handler = {sa_handler = 0x0, sa_sigaction = 0x0}, 
sa_mask = {__val = {0, 0, 0, 0, 0, 139701005283317, 2, 3760846768312216920, 
7018409443653412194, 94647092334544, 
  7003717752975484000, 0, 3976935554753230848, 140724437079344, 0, 
140724437080208}}, sa_flags = -1101967408, sa_restorer = 0x0}
sigs = {__val = {32, 0 }}
#2  0x5614beb8d44e in ExceptionalCondition (conditionName=0x5614bed22e7b 
"limit->consttype == INT8OID", errorType=0x5614bed22b76 "FailedAssertion", 
fileName=0x5614bed22c44 "subselect.c", lineNumber=1594)
at assert.c:69
No locals.
#3  0x5614be8df87d in simplify_EXISTS_query (root=0x5614bfecf4b8, 
query=0x5614bfe038e0) at subselect.c:1594
node = 0x5614bfed0190
limit = 0x5614bfed0190
#4  0x5614be8df49b in convert_EXISTS_sublink_to_join (root=0x5614bfecf4b8, 
sublink=0x5614bfe036e0, under_not=false, available_rels=0x5614bfecf1d8) at 
subselect.c:1420
result = 0x5614bfecf940
parse = 0x5614bfe039f8
subselect = 0x5614bfe038e0
whereClause = 0x5614bfecf8b8
rtoffset = 22036
varno = 22036
clause_varnos = 0x18
upper_varnos = 0x20
#5  0x5614be8e3d8d in pull_up_sublinks_qual_recurse (root=0x5614bfecf4b8, 
node=0x5614bfe036e0, jtlink1=0x7ffcf6155a48, available_rels1=0x5614bfecf1d8, 
jtlink2=0x0, available_rels2=0x0) at prepjointree.c:458
sublink = 0x5614bfe036e0
j = 0x0
child_rels = 0x5614bfecf940
#6  0x5614be8e38cc in pull_up_sublinks_jointree_recurse 
(root=0x5614bfecf4b8, jtnode=0x5614bfecf3d0, relids=0x7ffcf6155ab0) at 
prepjointree.c:275
frelids = 0x5614bfecf1d8
newf = 0x5614bfecf928
jtlink = 0x5614bfecf928
f = 0x5614bfecf3d0
newfromlist = 0x5614bfecf8d0
l = 0x0
__func__ = "pull_up_sublinks_jointree_recurse"
#7  0x5614be8e36ce in pull_up_sublinks (root=0x5614bfecf4b8) at 
prepjointree.c:214
jtnode = 0x5614bfecf6e8
relids = 0x0
#8  0x5614be8c9e6a in subquery_planner (glob=0x5614bfe2acc0, 
parse=0x5614bfe03

Re: FETCH FIRST clause PERCENT option

[Kyotaro Horiguchi]

Sorry for the dealy. I started to look this.

At Fri, 25 Sep 2020 12:25:24 +0300, Surafel Temesgen  
wrote in 
> Hi Michael
> On Thu, Sep 24, 2020 at 6:58 AM Michael Paquier  wrote:
> 
> > On Mon, Aug 10, 2020 at 01:23:44PM +0300, Surafel Temesgen wrote:
> > > I also Implement PERCENT WITH TIES option. patch is attached
> > > i don't start a new tread because the patches share common code
> >
> > This fails to apply per the CF bot.  Could you send a rebase?

This still applies on the master HEAD.

percent-incremental-v11.patch

The existing nodeLimit passes the slot of the subnode to the
caller. but this patch changes that behavior. You added a new function
to tuplestore.c not to store a minimal tuple into the slot that passed
from subnode, but we should refrain from scribbling on the slot passed
from the subnode. Instead, the PERCENT path of the limit node should
use its own ResultTupleSlot for the purpose. See nodeSort for a
concrete example.


+   LIMIT_OPTION_PER_WITH_TIES, /* FETCH FIRST... PERCENT WITH TIES */

That name is a bit hard to read. We should spell it with complete
words.

case LIMIT_INWINDOW:
...
+   if (IsPercentOption(node->limitOption) && 
node->backwardPosition
...
+   if (IsPercentOption(node->limitOption) && 
node->reachEnd)
...
+   if (IsPercentOption(node->limitOption))

I think we can use separate lstate state for each condition above
since IsPercentOption() gives a constant result through the execution
time. For example, LIMIT_PERCENT_TUPLESLOT_NOT_FILLED and
LIMIT_PERCENT_TUPLESLOT_FILLED and some derived states similar to the
non-percent path.  I *feel* that makes code simpler.

What do you think about this?

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

Hi Michael
On Thu, Sep 24, 2020 at 6:58 AM Michael Paquier  wrote:

> On Mon, Aug 10, 2020 at 01:23:44PM +0300, Surafel Temesgen wrote:
> > I also Implement PERCENT WITH TIES option. patch is attached
> > i don't start a new tread because the patches share common code
>
> This fails to apply per the CF bot.  Could you send a rebase?
> --
>


Attaches are rebased patches

regards
Surafel
diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index a31abce7c9..135b98da5d 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -3096,7 +3096,8 @@ estimate_path_cost_size(PlannerInfo *root,
 		if (fpextra && fpextra->has_limit)
 		{
 			adjust_limit_rows_costs(&rows, &startup_cost, &total_cost,
-	fpextra->offset_est, fpextra->count_est);
+	LIMIT_OPTION_COUNT, fpextra->offset_est,
+	fpextra->count_est);
 			retrieved_rows = rows;
 		}
 	}
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index b93e4ca208..9811380ec6 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } { ONLY | WITH TIES } ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } { ONLY | WITH TIES } ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1434,7 +1434,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } { ONLY | WITH TIES }
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } { ONLY | WITH TIES }
 
 In this syntax, the start
 or count value is required by
@@ -1444,6 +1444,9 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
+With PERCENT count specifies the maximum number of rows to return 
+in percentage round up to the nearest integer. Using PERCENT
+is best suited to returning single-digit percentages of the query's total row count.
 The WITH TIES option is used to return any additional
 rows that tie for the last place in the result set according to
 the ORDER BY clause; ORDER BY
diff --git a/src/backend/catalog/sql_features.txt b/src/backend/catalog/sql_features.txt
index b6e58e8493..68e0c5e2e1 100644
--- a/src/backend/catalog/sql_features.txt
+++ b/src/backend/catalog/sql_features.txt
@@ -344,7 +344,7 @@ F862	 in subqueries			YES
 F863	Nested  in 			YES	
 F864	Top-level  in views			YES	
 F865	 in 			YES	
-F866	FETCH FIRST clause: PERCENT option			NO	
+F866	FETCH FIRST clause: PERCENT option			YES	
 F867	FETCH FIRST clause: WITH TIES option			YES	
 R010	Row pattern recognition: FROM clause			NO	
 R020	Row pattern recognition: WINDOW clause			NO	
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index d85cf7d93e..cfb28a9fd4 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -21,6 +21,8 @@
 
 #include "postgres.h"
 
+#include 
+
 #include "executor/executor.h"
 #include "executor/nodeLimit.h"
 #include "miscadmin.h"
@@ -29,6 +31,8 @@
 static void recompute_limits(LimitState *node);
 static int64 compute_tuples_needed(LimitState *node);
 
+#define IsPercentOption(opt) \
+	(opt == LIMIT_OPTION_PERCENT || opt == LIMIT_OPTION_PER_WITH_TIES)
 
 /* 
  *		ExecLimit
@@ -53,6 +57,7 @@ ExecLimit(PlanState *pstate)
 	 */
 	direction = node->ps.state->es_direction;
 	outerPlan = outerPlanState(node);
+	slot = node->subSlot;
 
 	/*
 	 * The main logic is a simple state machine.
@@ -82,7 +87,15 @@ ExecLimit(PlanState *pstate)
 			/*
 			 * Check for empty window; if so, treat like empty subplan.
 			 */
-			if (node->count <= 0 && !node->noCount)
+			if (IsPercentOption(node->limitOption))
+			{
+if (node->percent == 0.0)
+{
+	node->lstate = LIMIT_EMPTY;
+	return NULL;
+}
+			}
+			else if (node->count <= 0 && !node->noCount)
 			{
 node->lstate = LIMIT_EMPTY;
 return NULL;
@@ -118,6 +131,16 @@ ExecLimit(PlanState *pstate)
 	break;
 			}
 
+			/*
+			 * We may needed this tuple in subsequent scan so put it into
+			 * tuplestore.
+			 */
+			if (IsPercentOption(node->limitOption))
+			{
+tuplestore_puttupleslot(node->tupleStore, slot);
+tuplestore_advance(node->tupleStore, true);
+			}
+
 			/*
 			 * Okay, we have the first tuple of the window.
 			 */
@@ -135,6 +158,85 @@ ExecLimit(PlanState *pstate)
 		case LIMIT_INWINDOW:
 			if (ScanDirectionIsForward(direction))
 			{
+/*
+ * In case of coming back from backward scan the tuple is
+ * already in tuple sto

Re: FETCH FIRST clause PERCENT option

[Michael Paquier]

On Mon, Aug 10, 2020 at 01:23:44PM +0300, Surafel Temesgen wrote:
> I also Implement PERCENT WITH TIES option. patch is attached
> i don't start a new tread because the patches share common code

This fails to apply per the CF bot.  Could you send a rebase?
--
Michael


signature.asc
Description: PGP signature

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

Hi


> PERCENT and WITH TIES can play together, per spec.
>

I also Implement PERCENT WITH TIES option. patch is attached
i don't start a new tread because the patches share common code

regards
Surafel
diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index 9fc53cad68..d512f4ddd0 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -3097,7 +3097,8 @@ estimate_path_cost_size(PlannerInfo *root,
 		if (fpextra && fpextra->has_limit)
 		{
 			adjust_limit_rows_costs(&rows, &startup_cost, &total_cost,
-	fpextra->offset_est, fpextra->count_est);
+	LIMIT_OPTION_COUNT, fpextra->offset_est,
+	fpextra->count_est);
 			retrieved_rows = rows;
 		}
 	}
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index b93e4ca208..9811380ec6 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } { ONLY | WITH TIES } ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } { ONLY | WITH TIES } ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1434,7 +1434,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } { ONLY | WITH TIES }
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } { ONLY | WITH TIES }
 
 In this syntax, the start
 or count value is required by
@@ -1444,6 +1444,9 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
+With PERCENT count specifies the maximum number of rows to return 
+in percentage round up to the nearest integer. Using PERCENT
+is best suited to returning single-digit percentages of the query's total row count.
 The WITH TIES option is used to return any additional
 rows that tie for the last place in the result set according to
 the ORDER BY clause; ORDER BY
diff --git a/src/backend/catalog/sql_features.txt b/src/backend/catalog/sql_features.txt
index b6e58e8493..68e0c5e2e1 100644
--- a/src/backend/catalog/sql_features.txt
+++ b/src/backend/catalog/sql_features.txt
@@ -344,7 +344,7 @@ F862	 in subqueries			YES
 F863	Nested  in 			YES	
 F864	Top-level  in views			YES	
 F865	 in 			YES	
-F866	FETCH FIRST clause: PERCENT option			NO	
+F866	FETCH FIRST clause: PERCENT option			YES	
 F867	FETCH FIRST clause: WITH TIES option			YES	
 R010	Row pattern recognition: FROM clause			NO	
 R020	Row pattern recognition: WINDOW clause			NO	
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index d85cf7d93e..cfb28a9fd4 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -21,6 +21,8 @@
 
 #include "postgres.h"
 
+#include 
+
 #include "executor/executor.h"
 #include "executor/nodeLimit.h"
 #include "miscadmin.h"
@@ -29,6 +31,8 @@
 static void recompute_limits(LimitState *node);
 static int64 compute_tuples_needed(LimitState *node);
 
+#define IsPercentOption(opt) \
+	(opt == LIMIT_OPTION_PERCENT || opt == LIMIT_OPTION_PER_WITH_TIES)
 
 /* 
  *		ExecLimit
@@ -53,6 +57,7 @@ ExecLimit(PlanState *pstate)
 	 */
 	direction = node->ps.state->es_direction;
 	outerPlan = outerPlanState(node);
+	slot = node->subSlot;
 
 	/*
 	 * The main logic is a simple state machine.
@@ -82,7 +87,15 @@ ExecLimit(PlanState *pstate)
 			/*
 			 * Check for empty window; if so, treat like empty subplan.
 			 */
-			if (node->count <= 0 && !node->noCount)
+			if (IsPercentOption(node->limitOption))
+			{
+if (node->percent == 0.0)
+{
+	node->lstate = LIMIT_EMPTY;
+	return NULL;
+}
+			}
+			else if (node->count <= 0 && !node->noCount)
 			{
 node->lstate = LIMIT_EMPTY;
 return NULL;
@@ -118,6 +131,16 @@ ExecLimit(PlanState *pstate)
 	break;
 			}
 
+			/*
+			 * We may needed this tuple in subsequent scan so put it into
+			 * tuplestore.
+			 */
+			if (IsPercentOption(node->limitOption))
+			{
+tuplestore_puttupleslot(node->tupleStore, slot);
+tuplestore_advance(node->tupleStore, true);
+			}
+
 			/*
 			 * Okay, we have the first tuple of the window.
 			 */
@@ -135,6 +158,85 @@ ExecLimit(PlanState *pstate)
 		case LIMIT_INWINDOW:
 			if (ScanDirectionIsForward(direction))
 			{
+/*
+ * In case of coming back from backward scan the tuple is
+ * already in tuple store.
+ */
+if (IsPercentOption(node->limitOption) && node->backwardPosition > 0)
+{
+	if (tuplestore_gettupleslot_heaptuple(node->tupleStore, true, true, slot))
+	{
+		

Re: FETCH FIRST clause PERCENT option

[Vik Fearing]

On 17/01/2020 07:20, Kyotaro Horiguchi wrote:
> Thank you for the notification, Tomas.
>
> At Thu, 16 Jan 2020 22:33:58 +0100, Tomas Vondra 
>  wrote in 
>> This patch is marked as RFC since September. Since then there was no
>> discussion on this thread, but Andrew proposed an alternative approach
>> based on window functions in a separate thread [1] (both for this and
>> for the WITH TIES case).
>>
>> I'll set this patch back to "needs review" - at this point we need to
>> decide which of the approaches is the right one.
> Even consdiering that we have only two usage , WITH TIES and PERCENT,
> of the mechanism for now, I like [1] for the generic nature,
> simpleness and smaller invasiveness to executor. The fact that cursor
> needs materialize to allow backward scan would not be a big problem.
> It seems that PERCENT will be easily implemented on that.
>
> However, isn't there a possibility that we allow FETCH FIRST x PERCENT
> WITH TIES, though I'm not sure what SQL spec tells about that?  I
> don't come up right now how to do that using window functions..


PERCENT and WITH TIES can play together, per spec.

-- 

Vik Fearing

Re: FETCH FIRST clause PERCENT option

[Kyotaro Horiguchi]

Thank you for the notification, Tomas.

At Thu, 16 Jan 2020 22:33:58 +0100, Tomas Vondra  
wrote in 
> This patch is marked as RFC since September. Since then there was no
> discussion on this thread, but Andrew proposed an alternative approach
> based on window functions in a separate thread [1] (both for this and
> for the WITH TIES case).
> 
> I'll set this patch back to "needs review" - at this point we need to
> decide which of the approaches is the right one.

Even consdiering that we have only two usage , WITH TIES and PERCENT,
of the mechanism for now, I like [1] for the generic nature,
simpleness and smaller invasiveness to executor. The fact that cursor
needs materialize to allow backward scan would not be a big problem.
It seems that PERCENT will be easily implemented on that.

However, isn't there a possibility that we allow FETCH FIRST x PERCENT
WITH TIES, though I'm not sure what SQL spec tells about that?  I
don't come up right now how to do that using window functions..

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

Hi,

This patch is marked as RFC since September. Since then there was no
discussion on this thread, but Andrew proposed an alternative approach
based on window functions in a separate thread [1] (both for this and
for the WITH TIES case).

I'll set this patch back to "needs review" - at this point we need to
decide which of the approaches is the right one.


regards

[1] 
https://www.postgresql.org/message-id/flat/87o8wvz253@news-spur.riddles.org.uk

--
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services 

Re: FETCH FIRST clause PERCENT option

[Ryan Lambert]

The following review has been posted through the commitfest application:
make installcheck-world:  tested, passed
Implements feature:   tested, passed
Spec compliant:   not tested
Documentation:not tested

The latest patch applies cleanly and passes all tests.  I believe all feedback 
has been addressed except for the one case that can be improved upon in later 
work.  Updating status as ready for committer.

Ryan Lambert

The new status of this patch is: Ready for Committer

Re: FETCH FIRST clause PERCENT option

[Kyotaro Horiguchi]

At Thu, 26 Sep 2019 15:13:42 -0600, Ryan Lambert  wrote 
in 
> On Thu, Sep 19, 2019 at 6:52 AM Surafel Temesgen 
> wrote:
> 
> > Hi Tom,
> > In the attached patch i include the comments given
> >
> > regards
> > Surafel
> >
> 
> Patch v9 applies and passes make installcheck-world.
> 
> > From: Tom Lane 
> > Date: 2019-09-05 22:26:29
> 
> > * I didn't really study the changes in nodeLimit.c, but doing
> > "tuplestore_rescan" in ExecReScanLimit is surely just wrong.  You
> > probably want to delete and recreate the tuplestore, instead, since
> > whatever data you already collected is of no further use.  Maybe, in
> > the case where no rescan of the child node is needed, you could re-use
> > the data already collected; but that would require a bunch of additional
> > logic.  I'm inclined to think that v1 of the patch shouldn't concern
> > itself with that sort of optimization.
> 
> I don't think this was addressed.

It seems to me that Tom is suggesting to conisider that kind of
optimization after once completing the patch without it. (That in
the first patch was just wrong, but more stuff is needed to
achieve that.)

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

Re: FETCH FIRST clause PERCENT option

[Ryan Lambert]

On Thu, Sep 19, 2019 at 6:52 AM Surafel Temesgen 
wrote:

> Hi Tom,
> In the attached patch i include the comments given
>
> regards
> Surafel
>

Patch v9 applies and passes make installcheck-world.

> From: Tom Lane 
> Date: 2019-09-05 22:26:29

> * I didn't really study the changes in nodeLimit.c, but doing
> "tuplestore_rescan" in ExecReScanLimit is surely just wrong.  You
> probably want to delete and recreate the tuplestore, instead, since
> whatever data you already collected is of no further use.  Maybe, in
> the case where no rescan of the child node is needed, you could re-use
> the data already collected; but that would require a bunch of additional
> logic.  I'm inclined to think that v1 of the patch shouldn't concern
> itself with that sort of optimization.

I don't think this was addressed.

Ryan Lambert


>

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

Hi Tom,
In the attached patch i include the comments given

regards
Surafel
diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index 82d8140ba2..692d6492bd 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -3035,7 +3035,8 @@ estimate_path_cost_size(PlannerInfo *root,
 		if (fpextra && fpextra->has_limit)
 		{
 			adjust_limit_rows_costs(&rows, &startup_cost, &total_cost,
-	fpextra->offset_est, fpextra->count_est);
+	fpextra->offset_est, fpextra->count_est,
+	LIMIT_OPTION_COUNT);
 			retrieved_rows = rows;
 		}
 	}
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 06d611b64c..4ef42df51d 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1430,7 +1430,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY
 
 In this syntax, the start
 or count value is required by
@@ -1440,8 +1440,10 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
-ROW
-and ROWS as well as FIRST
+With PERCENT count specifies the maximum number of rows to return 
+in percentage round up to the nearest integer. Using PERCENT
+is best suited to returning single-digit percentages of the query's total row count.
+ROW and ROWS as well as FIRST
 and NEXT are noise words that don't influence
 the effects of these clauses.
 According to the standard, the OFFSET clause must come
diff --git a/src/backend/catalog/sql_features.txt b/src/backend/catalog/sql_features.txt
index 059ec02cd0..1adc312f7a 100644
--- a/src/backend/catalog/sql_features.txt
+++ b/src/backend/catalog/sql_features.txt
@@ -348,7 +348,7 @@ F862	 in subqueries			YES
 F863	Nested  in 			YES	
 F864	Top-level  in views			YES	
 F865	 in 			YES	
-F866	FETCH FIRST clause: PERCENT option			NO	
+F866	FETCH FIRST clause: PERCENT option			YES	
 F867	FETCH FIRST clause: WITH TIES option			NO	
 R010	Row pattern recognition: FROM clause			NO	
 R020	Row pattern recognition: WINDOW clause			NO	
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index baa669abe8..c03bb2c971 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -21,6 +21,8 @@
 
 #include "postgres.h"
 
+#include 
+
 #include "executor/executor.h"
 #include "executor/nodeLimit.h"
 #include "miscadmin.h"
@@ -52,6 +54,7 @@ ExecLimit(PlanState *pstate)
 	 */
 	direction = node->ps.state->es_direction;
 	outerPlan = outerPlanState(node);
+	slot = node->subSlot;
 
 	/*
 	 * The main logic is a simple state machine.
@@ -81,7 +84,15 @@ ExecLimit(PlanState *pstate)
 			/*
 			 * Check for empty window; if so, treat like empty subplan.
 			 */
-			if (node->count <= 0 && !node->noCount)
+			if (node->limitOption == LIMIT_OPTION_PERCENT)
+			{
+if (node->percent == 0.0)
+{
+	node->lstate = LIMIT_EMPTY;
+	return NULL;
+}
+			}
+			else if (node->count <= 0 && !node->noCount)
 			{
 node->lstate = LIMIT_EMPTY;
 return NULL;
@@ -107,6 +118,16 @@ ExecLimit(PlanState *pstate)
 	break;
 			}
 
+			/*
+			 * We may needed this tuple in subsequent scan so put it into
+			 * tuplestore.
+			 */
+			if (node->limitOption == LIMIT_OPTION_PERCENT)
+			{
+tuplestore_puttupleslot(node->tupleStore, slot);
+tuplestore_advance(node->tupleStore, true);
+			}
+
 			/*
 			 * Okay, we have the first tuple of the window.
 			 */
@@ -124,6 +145,82 @@ ExecLimit(PlanState *pstate)
 		case LIMIT_INWINDOW:
 			if (ScanDirectionIsForward(direction))
 			{
+/*
+ * In case of coming back from backward scan the tuple is
+ * already in tuple store.
+ */
+if (node->limitOption == LIMIT_OPTION_PERCENT && node->backwardPosition > 0)
+{
+	if (tuplestore_gettupleslot_heaptuple(node->tupleStore, true, true, slot))
+	{
+		node->subSlot = slot;
+		node->position++;
+		node->backwardPosition--;
+		return slot;
+	}
+	else
+	{
+		node->lstate = LIMIT_SUBPLANEOF;
+		return NULL;
+	}
+}
+
+/*
+ * In LIMIT_OPTION_PERCENT case no need of executing outerPlan multiple
+ * times.
+ */
+if (node->limitOption == LIMIT_OPTION_PERCENT && node->reachEnd)
+{
+	node->lstate = LIMIT_WI

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

Hi Tom,
On Fri, Sep 6, 2019 at 1:26 AM Tom Lane  wrote:

> Surafel Temesgen  writes:
> > [ percent-incremental-v8.patch ]
>
> I took a quick look through this.
>
> * Why is this adding new functionality in tuplestore.c?  Especially
> functionality to get out a different tuple representation than what was
> put in?


If am not mistaken tuplestore store MinimalTuple(tuple without transaction
status information) for minimal memory usage. it change what we put in into
MinimalTuple so we need the same format for getting out. The other way I
can think of doing it is allocating new MinimalTuple slot every time to get
the tuple which have resource like. The operation is very similar to
RunFromStore without the ability of reusing once allocated slot.


> I can't see a reason why nodeLimit should need that, and for
> sure I don't see a reason why it would need it for this feature if it
> didn't before.
>

Only this feature needs tuple storage



> * The business with query.limitCount having different data types
> depending on LimitOption seems entirely bletcherous and bug-inducing.
> (There are live bugs of that sort in what you have now: notably, that
> kluge in adjust_limit_rows_costs will crash and burn if float8 is
> pass-by-reference.  Breaking the Datum abstraction is bad practice.)
> I wonder if we could get away with making limitCount be float8 all the
> time.  This would mean that you'd lose exactness for counts above 2^53
> (or so, depending on whether float8 is IEEE or not), but I doubt that
> anybody would ever notice.
>
> * Names like "PERCENTAGE" seem mighty generic to be exposing as globally
> known symbols; also you're disregarding a fairly widespread PG
> convention that members of an enum should have names derived from the
> enum type's name.  So I'd be inclined to make the values of enum
> LimitOption be LIMIT_OPTION_COUNT and LIMIT_OPTION_PERCENT.  (I don't
> especially like EXACT_NUMBER, first because it's going to be quite long
> with the prefix, and second because it suggests that maybe there's an
> INEXACT_NUMBER alternative.)
>
> * The "void *limitOption" business in gram.y also seems pretty ugly;
> it'd be better for that to be enum LimitOption.  I gather you want
> the null to indicate "no option given", but likely it'd be better
> to invent a LIMIT_OPTION_DEFAULT enum alternative for that purpose.
>
>
i like to fix the above three more because of scaling

* An alternative to the three points above is just to separate
> Query.limitCount (an int8) from Query.limitPercent (a float8), with the
> understanding that at most one of the two can be non-null, and use
> similar representations at other steps of the processing chain.  Then
> you don't need enum limitOption at all.  That might not scale especially
> nicely to additional variants, but trying to overload limitCount even
> further isn't going to be nice either.
>
> * The proposed change in grouping_planner() seems like a complete hack.
> Why would you not change the way that limit_tuples was computed earlier,
> instead?
>
That is, it seems like the part of the planner to teach about
> this feature is preprocess_limit (and limit_needed), not someplace else.
> It is surely not sane that only this one planner decision would react to
> a percentage-based limit.
>
> * I didn't really study the changes in nodeLimit.c, but doing
> "tuplestore_rescan" in ExecReScanLimit is surely just wrong.  You
> probably want to delete and recreate the tuplestore, instead, since
> whatever data you already collected is of no further use.  Maybe, in
> the case where no rescan of the child node is needed, you could re-use
> the data already collected; but that would require a bunch of additional
> logic.  I'm inclined to think that v1 of the patch shouldn't concern
> itself with that sort of optimization.
>
>
ok


> * I don't see any delta in ruleutils.c, but surely that needs to know
> about this feature so that it can correctly print views using it.
>

ok

regards
Surafel

Re: FETCH FIRST clause PERCENT option

[Tom Lane]

Surafel Temesgen  writes:
> [ percent-incremental-v8.patch ]

I took a quick look through this.

* Why is this adding new functionality in tuplestore.c?  Especially
functionality to get out a different tuple representation than what was
put in?  I can't see a reason why nodeLimit should need that, and for
sure I don't see a reason why it would need it for this feature if it
didn't before.

* The business with query.limitCount having different data types
depending on LimitOption seems entirely bletcherous and bug-inducing.
(There are live bugs of that sort in what you have now: notably, that
kluge in adjust_limit_rows_costs will crash and burn if float8 is
pass-by-reference.  Breaking the Datum abstraction is bad practice.)
I wonder if we could get away with making limitCount be float8 all the
time.  This would mean that you'd lose exactness for counts above 2^53
(or so, depending on whether float8 is IEEE or not), but I doubt that
anybody would ever notice.

* Names like "PERCENTAGE" seem mighty generic to be exposing as globally
known symbols; also you're disregarding a fairly widespread PG
convention that members of an enum should have names derived from the
enum type's name.  So I'd be inclined to make the values of enum
LimitOption be LIMIT_OPTION_COUNT and LIMIT_OPTION_PERCENT.  (I don't
especially like EXACT_NUMBER, first because it's going to be quite long
with the prefix, and second because it suggests that maybe there's an
INEXACT_NUMBER alternative.)

* The "void *limitOption" business in gram.y also seems pretty ugly;
it'd be better for that to be enum LimitOption.  I gather you want
the null to indicate "no option given", but likely it'd be better
to invent a LIMIT_OPTION_DEFAULT enum alternative for that purpose.

* An alternative to the three points above is just to separate
Query.limitCount (an int8) from Query.limitPercent (a float8), with the
understanding that at most one of the two can be non-null, and use
similar representations at other steps of the processing chain.  Then
you don't need enum limitOption at all.  That might not scale especially
nicely to additional variants, but trying to overload limitCount even
further isn't going to be nice either.

* The proposed change in grouping_planner() seems like a complete hack.
Why would you not change the way that limit_tuples was computed earlier,
instead?  That is, it seems like the part of the planner to teach about
this feature is preprocess_limit (and limit_needed), not someplace else.
It is surely not sane that only this one planner decision would react to
a percentage-based limit.

* I didn't really study the changes in nodeLimit.c, but doing
"tuplestore_rescan" in ExecReScanLimit is surely just wrong.  You
probably want to delete and recreate the tuplestore, instead, since
whatever data you already collected is of no further use.  Maybe, in
the case where no rescan of the child node is needed, you could re-use
the data already collected; but that would require a bunch of additional
logic.  I'm inclined to think that v1 of the patch shouldn't concern
itself with that sort of optimization.

* I don't see any delta in ruleutils.c, but surely that needs to know
about this feature so that it can correctly print views using it.

regards, tom lane

Re: FETCH FIRST clause PERCENT option

[Ryan Lambert]

The following review has been posted through the commitfest application:
make installcheck-world:  tested, passed
Implements feature:   tested, passed
Spec compliant:   tested, passed
Documentation:tested, passed

Hi everyone,

The v8 patch [1] applies cleanly and passes tests.  I reviewed the conversation 
to date and from what I can see, all identified bugs have been fixed and 
concerns either fixed or addressed.  Marking as ready for committer.

[1] 
https://www.postgresql.org/message-id/attachment/103486/percent-incremental-v8.patch

Ryan Lambert

The new status of this patch is: Ready for Committer

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Tue, Aug 20, 2019 at 9:10 AM Kyotaro Horiguchi 
wrote:

> Hi,
>
> At Wed, 7 Aug 2019 10:20:09 +0300, Surafel Temesgen 
> wrote in <
> calay4q98xbvhtz4yj9dccmg2-s1_jurr7fyanfw+bkmr22o...@mail.gmail.com>
> > Hi
> > On Wed, Aug 7, 2019 at 6:11 AM Kyotaro Horiguchi <
> horikyota@gmail.com>
> > wrote:
> >
> > >
> > > I have some comments.
> > >
> > > This patch uses distinct parameters for exact number and
> > > percentage. On the othe hand planner has a notion of
> > > tuple_fraction covering the both. The same handling is also used
> > > for tuple number estimation. Using the same scheme will make
> > > things far simpler. See the comment of grouping_planner().
> > >
> > >
> > Its because of data type difference .In planner the data type is the same
>
> I meant that, with the usage of tuple_fraction, one variable can
> represent both the absolute limit and relative limit. This
> simplifies parse structs.
>
In grouping_planner the patch set tuple bound to -1 in create_ordered_paths
because it iterate until the end in percentage. Did that answer your
question?

>
>
> > > In executor part, addition to LimiteState.position, this patch
> > > uses LimiteState.backwardPosition to count how many tuples we're
> > > back from the end of the current tuplestore. But things will get
> > > simpler by just asking the tuplestore for the number of holding
> > > tuples.
> > >
> > >
> > backwardPosition hold how many tuple returned in backward scan
>
> I meant that we don't need to hold the number in estate.
>

I did it this way because I didn't find an API in tuplestore to do this


>
> > > +slot = node->subSlot;
> > >
> > > Why is this needed? The variable is properly set before use and
> > > the assignment is bogus in the first place.
> > >
> > >
> > its because Tuplestore needs initialized slot.
>
> I meant that the initilized slot is overwritten before first use.
>
> > > The new code block in LIMIT_RESCAN in ExecLimit is useless since
> > > it is exatctly the same with existing code block. Why didn't you
> > > use the existing if block?
> > >
> >
> > But they test different scenario
>
> I meant that the two different scenario can share the code block.
>

Sorry for not clarifying will .One have to be check before offsetting and
the other is after offsetting


>
> > > >if (node->limitOption == PERCENTAGE)
> > > +{
> > > +node->perExactCount = ceil(node->percent *
> > > node->position / 100.0);
> > > +
> > > +
> > >
> > > node->position is the number of tuples returned to upper node so
> > > far (not the number of tuples this node has read from the lower
> > > node so far).  I don't understand what the expression means.
> > >
> >
> > node->position hold the number of tuples this node has read from the
> lower
> > node so far. see LIMIT_RESCAN state
>
> Reallly? node->position is incremented when
> tuplestore_gettupleslot_heaptuple() succeeded and reutnrs the
> tuple to the caller immediately...
>
> > > +if (node->perExactCount == node->perExactCount +
> 1)
> > > +node->perExactCount++;
> > >
> > > What? The condition never be true. As the result, the following
> > > if block below won't run.
> > >
> >
> > it became true according to the number of tuple returned in from the
> lower
> > node so far
> > and percentage specification.
>
> Mmm. How do you think X can be equal to  (X + 1)?
>

Oops my bad .The attached patch remove the need of doing that

>
> > > >/*
> > > + * Return the tuple up to the number of exact count in
> > > OFFSET
> > > + * clause without percentage value consideration.
> > > + */
> > > +if (node->perExactCount > 0)
> > > +{
> > > +
> > >
> > >
> > >
> > >
> > > +/*
> > > + * We may needed this tuple in backward scan so put it
> into
> > > + * tuplestore.
> > > + */
> > > +if (node->limitOption == PERCENTAGE)
> > > +{
> > > +tuplestore_puttupleslot(node->tupleStore, slot);
> > > +tuplestore_advance(node->tupleStore, true);
> > > +}
> > >
> > > "needed"->"need" ? The comment says that this is needed for
> > > backward scan, but it is actually required even in forward
> > > scan. More to the point, tuplestore_advance lacks comment.
> >
> >
> > ok
> >
> >
> > >
> > > Anyway, the code in LIMIT_RESCAN is broken in some ways. For
> > > example, if it is used as the inner scan of a NestLoop, the
> > > tuplestore accumulates tuples by every scan. You will see that
> > > the tuplestore stores up to 1000 tuples (10 times of the inner)
> > > by the following query.
> > >
> >
> > It this because in percentage we scan the whole table
>
> It's useless and rather harmful that the tuplestore holds
> indefinite number of duplicate set of the whole tuples from the
> lower node. We must reuse tuples already store

Re: FETCH FIRST clause PERCENT option

[Kyotaro Horiguchi]

Hi,

At Wed, 7 Aug 2019 10:20:09 +0300, Surafel Temesgen  
wrote in 
> Hi
> On Wed, Aug 7, 2019 at 6:11 AM Kyotaro Horiguchi 
> wrote:
> 
> >
> > I have some comments.
> >
> > This patch uses distinct parameters for exact number and
> > percentage. On the othe hand planner has a notion of
> > tuple_fraction covering the both. The same handling is also used
> > for tuple number estimation. Using the same scheme will make
> > things far simpler. See the comment of grouping_planner().
> >
> >
> Its because of data type difference .In planner the data type is the same

I meant that, with the usage of tuple_fraction, one variable can
represent both the absolute limit and relative limit. This
simplifies parse structs.

> > In executor part, addition to LimiteState.position, this patch
> > uses LimiteState.backwardPosition to count how many tuples we're
> > back from the end of the current tuplestore. But things will get
> > simpler by just asking the tuplestore for the number of holding
> > tuples.
> >
> >
> backwardPosition hold how many tuple returned in backward scan

I meant that we don't need to hold the number in estate.

> > +slot = node->subSlot;
> >
> > Why is this needed? The variable is properly set before use and
> > the assignment is bogus in the first place.
> >
> >
> its because Tuplestore needs initialized slot.

I meant that the initilized slot is overwritten before first use.

> > The new code block in LIMIT_RESCAN in ExecLimit is useless since
> > it is exatctly the same with existing code block. Why didn't you
> > use the existing if block?
> >
> 
> But they test different scenario

I meant that the two different scenario can share the code block.

> > >if (node->limitOption == PERCENTAGE)
> > +{
> > +node->perExactCount = ceil(node->percent *
> > node->position / 100.0);
> > +
> > +
> >
> > node->position is the number of tuples returned to upper node so
> > far (not the number of tuples this node has read from the lower
> > node so far).  I don't understand what the expression means.
> >
> 
> node->position hold the number of tuples this node has read from the lower
> node so far. see LIMIT_RESCAN state

Reallly? node->position is incremented when
tuplestore_gettupleslot_heaptuple() succeeded and reutnrs the
tuple to the caller immediately...

> > +if (node->perExactCount == node->perExactCount + 1)
> > +node->perExactCount++;
> >
> > What? The condition never be true. As the result, the following
> > if block below won't run.
> >
> 
> it became true according to the number of tuple returned in from the lower
> node so far
> and percentage specification.

Mmm. How do you think X can be equal to  (X + 1)?

> > >/*
> > + * Return the tuple up to the number of exact count in
> > OFFSET
> > + * clause without percentage value consideration.
> > + */
> > +if (node->perExactCount > 0)
> > +{
> > +
> >
> >
> >
> >
> > +/*
> > + * We may needed this tuple in backward scan so put it into
> > + * tuplestore.
> > + */
> > +if (node->limitOption == PERCENTAGE)
> > +{
> > +tuplestore_puttupleslot(node->tupleStore, slot);
> > +tuplestore_advance(node->tupleStore, true);
> > +}
> >
> > "needed"->"need" ? The comment says that this is needed for
> > backward scan, but it is actually required even in forward
> > scan. More to the point, tuplestore_advance lacks comment.
> 
> 
> ok
> 
> 
> >
> > Anyway, the code in LIMIT_RESCAN is broken in some ways. For
> > example, if it is used as the inner scan of a NestLoop, the
> > tuplestore accumulates tuples by every scan. You will see that
> > the tuplestore stores up to 1000 tuples (10 times of the inner)
> > by the following query.
> >
> 
> It this because in percentage we scan the whole table

It's useless and rather harmful that the tuplestore holds
indefinite number of duplicate set of the whole tuples from the
lower node. We must reuse tuples already stored in the tuplestore
or clear it before the next round.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Mon, Aug 19, 2019 at 1:55 PM Erik Rijkers  wrote:

> Another little thing, not sure it's a bug:
>
> limit interprets its argument by rounding up or down as one would
> expect:
>
> table onek limit 10.4;  --> gives 10 rows
> table onek limit 10.6;  --> gives 11 rows
>
> but  FETCH count PERCENT  does not do that; it rounds always up.
>
> select * from (table onek limit 100) f fetch first 10.4 percent rows
> only; --> gives 11 rows
> select * from (table onek limit 100) f fetch first 10.6 percent rows
> only; --> gives 11 rows
>
> I see that it's documented in the .sgml to behave as it does, but it
> seems wrong to me;
> shouldn't that 10.4-percent-query yield 10 rows instead of 11?
>
>

According to sql standard we have to use the result ceiling value

regards
Surafel

Re: FETCH FIRST clause PERCENT option

[Erik Rijkers]

On 2019-08-19 11:18, Surafel Temesgen wrote:


[..]

[percent-incremental-v7.patch]


Thanks.


Another little thing, not sure it's a bug:

limit interprets its argument by rounding up or down as one would 
expect:


table onek limit 10.4;  --> gives 10 rows
table onek limit 10.6;  --> gives 11 rows

but  FETCH count PERCENT  does not do that; it rounds always up.

select * from (table onek limit 100) f fetch first 10.4 percent rows 
only; --> gives 11 rows
select * from (table onek limit 100) f fetch first 10.6 percent rows 
only; --> gives 11 rows


I see that it's documented in the .sgml to behave as it does, but it 
seems wrong to me;

shouldn't that 10.4-percent-query yield 10 rows instead of 11?


thanks,

Erik Rijkers

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

Hi Erik
On Mon, Aug 19, 2019 at 10:47 AM Erik Rijkers  wrote:

> Hi,
>
> (running with those two patches applied)
>
>select * from onek where thousand < 5 order by thousand fetch first -1
> percent rows only
> is correctly caught (with "ERROR:  PERCENT must not be negative") but:
>
>select * from  onek where thousand < 5 order by thousand fetch first
> 101 percent rows only
> is not. It doesn't return, and cannot be Ctrl-C'ed out of, which I guess
> is another bug?
>

Fixed

regards
Surafel
diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index 1759b9e1b6..fb9c2f5319 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -3035,7 +3035,8 @@ estimate_path_cost_size(PlannerInfo *root,
 		if (fpextra && fpextra->has_limit)
 		{
 			adjust_limit_rows_costs(&rows, &startup_cost, &total_cost,
-	fpextra->offset_est, fpextra->count_est);
+	fpextra->offset_est, fpextra->count_est,
+	EXACT_NUMBER);
 			retrieved_rows = rows;
 		}
 	}
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 06d611b64c..4ef42df51d 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1430,7 +1430,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY
 
 In this syntax, the start
 or count value is required by
@@ -1440,8 +1440,10 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
-ROW
-and ROWS as well as FIRST
+With PERCENT count specifies the maximum number of rows to return 
+in percentage round up to the nearest integer. Using PERCENT
+is best suited to returning single-digit percentages of the query's total row count.
+ROW and ROWS as well as FIRST
 and NEXT are noise words that don't influence
 the effects of these clauses.
 According to the standard, the OFFSET clause must come
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index baa669abe8..7a9626d7f9 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -21,6 +21,8 @@
 
 #include "postgres.h"
 
+#include 
+
 #include "executor/executor.h"
 #include "executor/nodeLimit.h"
 #include "miscadmin.h"
@@ -52,6 +54,7 @@ ExecLimit(PlanState *pstate)
 	 */
 	direction = node->ps.state->es_direction;
 	outerPlan = outerPlanState(node);
+	slot = node->subSlot;
 
 	/*
 	 * The main logic is a simple state machine.
@@ -81,7 +84,15 @@ ExecLimit(PlanState *pstate)
 			/*
 			 * Check for empty window; if so, treat like empty subplan.
 			 */
-			if (node->count <= 0 && !node->noCount)
+			if (node->limitOption == PERCENTAGE)
+			{
+if (node->percent == 0.0)
+{
+	node->lstate = LIMIT_EMPTY;
+	return NULL;
+}
+			}
+			else if (node->count <= 0 && !node->noCount)
 			{
 node->lstate = LIMIT_EMPTY;
 return NULL;
@@ -102,11 +113,32 @@ ExecLimit(PlanState *pstate)
 	node->lstate = LIMIT_EMPTY;
 	return NULL;
 }
+
+/*
+ * Count the number of rows to return in exact number so far
+ */
+if (node->limitOption == PERCENTAGE)
+{
+	node->perExactCount = ceil(node->percent * node->position / 100.0);
+
+	if (node->perExactCount == node->perExactCount + 1)
+		node->perExactCount++;
+}
 node->subSlot = slot;
 if (++node->position > node->offset)
 	break;
 			}
 
+			/*
+			 * We may needed this tuple in subsequent scan so put it into
+			 * tuplestore.
+			 */
+			if (node->limitOption == PERCENTAGE)
+			{
+tuplestore_puttupleslot(node->tupleStore, slot);
+tuplestore_advance(node->tupleStore, true);
+			}
+
 			/*
 			 * Okay, we have the first tuple of the window.
 			 */
@@ -124,6 +156,106 @@ ExecLimit(PlanState *pstate)
 		case LIMIT_INWINDOW:
 			if (ScanDirectionIsForward(direction))
 			{
+/*
+ * In case of coming back from backward scan the tuple is
+ * already in tuple store.
+ */
+if (node->limitOption == PERCENTAGE && node->backwardPosition > 0)
+{
+	if (tuplestore_gettupleslot_heaptuple(node->tupleStore, true, true, slot))
+	{
+		node->subSlot = slot;
+		node->position++;
+		node->backwardPosition--;
+		return slot;
+	}
+	else
+	{
+		node->lstate = LIMIT_SUBPLANEOF;
+

Re: FETCH FIRST clause PERCENT option

[Erik Rijkers]

On 2019-08-19 01:33, Ryan Lambert wrote:
The following review has been posted through the commitfest 
application:

make installcheck-world:  tested, passed
Implements feature:   tested, passed
Spec compliant:   not tested
Documentation:tested, passed

The latest patch [1] and the cleanup patch [2] apply cleanly to the
latest master (5f110933).  I reviewed the conversation and don't see
any outstanding questions or concerns.  Updating status to ready for
committer.

[1] > 
https://www.postgresql.org/message-id/attachment/103028/percent-incremental-v6.patch
[2] > 
https://www.postgresql.org/message-id/attachment/103157/percent-incremental-v6-comment-cleanup.patch


Ryan Lambert

The new status of this patch is: Ready for Committer


Hi,

(running with those two patches applied)

  select * from onek where thousand < 5 order by thousand fetch first -1 
percent rows only

is correctly caught (with "ERROR:  PERCENT must not be negative") but:

  select * from  onek where thousand < 5 order by thousand fetch first 
101 percent rows only
is not. It doesn't return, and cannot be Ctrl-C'ed out of, which I guess 
is another bug?



thanks,

Erik Rijkers

Re: FETCH FIRST clause PERCENT option

[Ryan Lambert]

The following review has been posted through the commitfest application:
make installcheck-world:  tested, passed
Implements feature:   tested, passed
Spec compliant:   not tested
Documentation:tested, passed

The latest patch [1] and the cleanup patch [2] apply cleanly to the latest 
master (5f110933).  I reviewed the conversation and don't see any outstanding 
questions or concerns.  Updating status to ready for committer.

[1] 
https://www.postgresql.org/message-id/attachment/103028/percent-incremental-v6.patch
[2] 
https://www.postgresql.org/message-id/attachment/103157/percent-incremental-v6-comment-cleanup.patch

Ryan Lambert

The new status of this patch is: Ready for Committer

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

Hi
On Wed, Aug 7, 2019 at 6:11 AM Kyotaro Horiguchi 
wrote:

>
> I have some comments.
>
> This patch uses distinct parameters for exact number and
> percentage. On the othe hand planner has a notion of
> tuple_fraction covering the both. The same handling is also used
> for tuple number estimation. Using the same scheme will make
> things far simpler. See the comment of grouping_planner().
>
>
Its because of data type difference .In planner the data type is the same

In executor part, addition to LimiteState.position, this patch
> uses LimiteState.backwardPosition to count how many tuples we're
> back from the end of the current tuplestore. But things will get
> simpler by just asking the tuplestore for the number of holding
> tuples.
>
>
backwardPosition hold how many tuple returned in backward scan


>
> +slot = node->subSlot;
>
> Why is this needed? The variable is properly set before use and
> the assignment is bogus in the first place.
>
>
its because Tuplestore needs initialized slot.


> The new code block in LIMIT_RESCAN in ExecLimit is useless since
> it is exatctly the same with existing code block. Why didn't you
> use the existing if block?
>

But they test different scenario


>
> >if (node->limitOption == PERCENTAGE)
> +{
> +node->perExactCount = ceil(node->percent *
> node->position / 100.0);
> +
> +
>
> node->position is the number of tuples returned to upper node so
> far (not the number of tuples this node has read from the lower
> node so far).  I don't understand what the expression means.
>

node->position hold the number of tuples this node has read from the lower
node so far. see LIMIT_RESCAN state


>
>
> +if (node->perExactCount == node->perExactCount + 1)
> +node->perExactCount++;
>
> What? The condition never be true. As the result, the following
> if block below won't run.
>

it became true according to the number of tuple returned in from the lower
node so far
and percentage specification.


>
> >/*
> + * Return the tuple up to the number of exact count in
> OFFSET
> + * clause without percentage value consideration.
> + */
> +if (node->perExactCount > 0)
> +{
> +
>
>
>
>
> +/*
> + * We may needed this tuple in backward scan so put it into
> + * tuplestore.
> + */
> +if (node->limitOption == PERCENTAGE)
> +{
> +tuplestore_puttupleslot(node->tupleStore, slot);
> +tuplestore_advance(node->tupleStore, true);
> +}
>
> "needed"->"need" ? The comment says that this is needed for
> backward scan, but it is actually required even in forward
> scan. More to the point, tuplestore_advance lacks comment.


ok


>
> Anyway, the code in LIMIT_RESCAN is broken in some ways. For
> example, if it is used as the inner scan of a NestLoop, the
> tuplestore accumulates tuples by every scan. You will see that
> the tuplestore stores up to 1000 tuples (10 times of the inner)
> by the following query.
>

It this because in percentage we scan the whole table


regards
Surafel

Re: FETCH FIRST clause PERCENT option

[Kyotaro Horiguchi]

Hello.

At Thu, 1 Aug 2019 15:54:11 +0300, Surafel Temesgen  
wrote in 
> > Other than that, we can rip the clause if it is 100%
> >
> >
> > You mean if PERCENT=100 it should short circuit and run the query
> > normally?  I like that.
> >
> 
> The patch did not did it automatically. Its query writer obligation to do
> that currently

I have some comments.

This patch uses distinct parameters for exact number and
percentage. On the othe hand planner has a notion of
tuple_fraction covering the both. The same handling is also used
for tuple number estimation. Using the same scheme will make
things far simpler. See the comment of grouping_planner().

In executor part, addition to LimiteState.position, this patch
uses LimiteState.backwardPosition to count how many tuples we're
back from the end of the current tuplestore. But things will get
simpler by just asking the tuplestore for the number of holding
tuples.


+slot = node->subSlot;

Why is this needed? The variable is properly set before use and
the assignment is bogus in the first place.

The new code block in LIMIT_RESCAN in ExecLimit is useless since
it is exatctly the same with existing code block. Why didn't you
use the existing if block?

>if (node->limitOption == PERCENTAGE)
+{
+node->perExactCount = ceil(node->percent * node->position 
/ 100.0);
+
+

node->position is the number of tuples returned to upper node so
far (not the number of tuples this node has read from the lower
node so far).  I don't understand what the expression means.


+if (node->perExactCount == node->perExactCount + 1)
+node->perExactCount++;

What? The condition never be true. As the result, the following
if block below won't run.

>/*
+ * Return the tuple up to the number of exact count in OFFSET
+ * clause without percentage value consideration.
+ */
+if (node->perExactCount > 0)
+{
+




+/*
+ * We may needed this tuple in backward scan so put it into
+ * tuplestore.
+ */
+if (node->limitOption == PERCENTAGE)
+{
+tuplestore_puttupleslot(node->tupleStore, slot);
+tuplestore_advance(node->tupleStore, true);
+}

"needed"->"need" ? The comment says that this is needed for
backward scan, but it is actually required even in forward
scan. More to the point, tuplestore_advance lacks comment.

Anyway, the code in LIMIT_RESCAN is broken in some ways. For
example, if it is used as the inner scan of a NestLoop, the
tuplestore accumulates tuples by every scan. You will see that
the tuplestore stores up to 1000 tuples (10 times of the inner)
by the following query.

create table t0 as (select a from generate_series(0, 99) a);
create table t1 as (select a from generate_series(0, 9) a);
analyze;
set enable_hashjoin to false;
set enable_mergejoin to false;
set enable_material to false;
explain analyze select t.*, t1.* from (select a from t0 fetch first 10 percent 
rows only) as t join t1 on (t.a = t1.a);
   QUERY PLAN 
---
 Nested Loop  (cost=0.20..7.35 rows=10 width=8) (actual ...)
   ->  Seq Scan on t1  (cost=0.00..1.10 rows=10 width=4) (actual ...)
   ->  Limit  (cost=0.20..0.40 rows=10 width=4) (actual ...)
 ->  Seq Scan on t0  (cost=0.00..2.00 rows=100 width=4) (actual ...)

That's it for now.


regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

Re: FETCH FIRST clause PERCENT option

[Ryan Lambert]

Surafel,

The patch did not did it automatically. Its query writer obligation to do
> that currently


Ok.

Your latest patch [1] passes make installcheck-world, I didn't test the
actual functionality this round.

   plan = (Plan *) make_limit(plan,
> subparse->limitOffset,
> -   subparse->limitCount);
> +   subparse->limitCount,
> +   subparse->limitOption);
>


I assume the limit percentage number goes into subparse->limitCount?  If
so, I don't see that documented.  Or does this truly only store the count?

The remainder of the code seems to make sense.  I attached a patch with a
few minor changes in the comments.  I need to go back to my notes and see
if I covered all the testing I had thought of, I should get to that later
this week.

[1]
https://www.postgresql.org/message-id/attachment/103028/percent-incremental-v6.patch

*Ryan Lambert*


>


percent-incremental-v6-comment-cleanup.patch
Description: Binary data

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

Hi Ryan,
sorry for not be fast to replay


> I was suggesting a warning in the documentation so users aren't caught
> unaware about the performance characteristics.  My first version was very
> rough, how about adding this in doc/src/sgml/ref/select.sgml?
>
> "Using PERCENT is best suited to returning single-digit
> percentages of the query's total row count."
>
>
> The following paragraphs in that same section give suggestions and
> warnings regarding LIMIT and OFFSET usage, I think this is more in line
> with the wording of those existing warnings.
>
>
Agreed and done


> Other than that, we can rip the clause if it is 100%
>
>
> You mean if PERCENT=100 it should short circuit and run the query
> normally?  I like that.
>

The patch did not did it automatically. Its query writer obligation to do
that currently

regards
Surafel
diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index 1759b9e1b6..fb9c2f5319 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -3035,7 +3035,8 @@ estimate_path_cost_size(PlannerInfo *root,
 		if (fpextra && fpextra->has_limit)
 		{
 			adjust_limit_rows_costs(&rows, &startup_cost, &total_cost,
-	fpextra->offset_est, fpextra->count_est);
+	fpextra->offset_est, fpextra->count_est,
+	EXACT_NUMBER);
 			retrieved_rows = rows;
 		}
 	}
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 06d611b64c..4ef42df51d 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1430,7 +1430,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY
 
 In this syntax, the start
 or count value is required by
@@ -1440,8 +1440,10 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
-ROW
-and ROWS as well as FIRST
+With PERCENT count specifies the maximum number of rows to return 
+in percentage round up to the nearest integer. Using PERCENT
+is best suited to returning single-digit percentages of the query's total row count.
+ROW and ROWS as well as FIRST
 and NEXT are noise words that don't influence
 the effects of these clauses.
 According to the standard, the OFFSET clause must come
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index baa669abe8..c503512588 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -21,6 +21,8 @@
 
 #include "postgres.h"
 
+#include 
+
 #include "executor/executor.h"
 #include "executor/nodeLimit.h"
 #include "miscadmin.h"
@@ -52,6 +54,7 @@ ExecLimit(PlanState *pstate)
 	 */
 	direction = node->ps.state->es_direction;
 	outerPlan = outerPlanState(node);
+	slot = node->subSlot;
 
 	/*
 	 * The main logic is a simple state machine.
@@ -81,7 +84,15 @@ ExecLimit(PlanState *pstate)
 			/*
 			 * Check for empty window; if so, treat like empty subplan.
 			 */
-			if (node->count <= 0 && !node->noCount)
+			if (node->limitOption == PERCENTAGE)
+			{
+if (node->percent == 0.0)
+{
+	node->lstate = LIMIT_EMPTY;
+	return NULL;
+}
+			}
+			else if (node->count <= 0 && !node->noCount)
 			{
 node->lstate = LIMIT_EMPTY;
 return NULL;
@@ -102,11 +113,32 @@ ExecLimit(PlanState *pstate)
 	node->lstate = LIMIT_EMPTY;
 	return NULL;
 }
+
+/*
+ * Count the number of rows to return in exact number so far
+ */
+if (node->limitOption == PERCENTAGE)
+{
+	node->perExactCount = ceil(node->percent * node->position / 100.0);
+
+	if (node->perExactCount == node->perExactCount + 1)
+		node->perExactCount++;
+}
 node->subSlot = slot;
 if (++node->position > node->offset)
 	break;
 			}
 
+			/*
+			 * We may needed this tuple in backward scan so put it into
+			 * tuplestore.
+			 */
+			if (node->limitOption == PERCENTAGE)
+			{
+tuplestore_puttupleslot(node->tupleStore, slot);
+tuplestore_advance(node->tupleStore, true);
+			}
+
 			/*
 			 * Okay, we have the first tuple of the window.
 			 */
@@ -124,6 +156,106 @@ ExecLimit(PlanState *pstate)
 		case LIMIT_INWINDOW:
 			if (ScanDirectionIsForward(direction))
 			{
+/*
+ * In case of coming back from backward scan the tuple is
+ * already in tuple store.
+ */
+if 

Re: FETCH FIRST clause PERCENT option

[Ryan Lambert]

Surafel,

On Wed, Jul 17, 2019 at 3:45 AM Surafel Temesgen 
wrote:

>
> Hi Ryan,
> On Tue, Jul 9, 2019 at 4:13 PM Ryan Lambert 
> wrote:
>
>>
>> "It is possible for FETCH FIRST N PERCENT to create poorly performing
>> query plans when the N supplied exceeds 50 percent.  In these cases query
>> execution can take an order of magnitude longer to execute than simply
>> returning the full table.  If performance is critical using an explicit row
>> count for limiting is recommended."
>>
>
> I don’t understand how fetch first n percent functionality can be replaced
> with explicit row count limiting. There may be a way to do it in a client
> side but we can not be sure of its performance advantage
>
>
> regards
>
> Surafel
>
>

I was suggesting a warning in the documentation so users aren't caught
unaware about the performance characteristics.  My first version was very
rough, how about adding this in doc/src/sgml/ref/select.sgml?

"Using PERCENT is best suited to returning single-digit
percentages of the query's total row count."


The following paragraphs in that same section give suggestions and warnings
regarding LIMIT and OFFSET usage, I think this is more in line with the
wording of those existing warnings.

Other than that, we can rip the clause if it is 100%


You mean if PERCENT=100 it should short circuit and run the query
normally?  I like that.
That got me thinking, I didn't check what happens with PERCENT>100, I'll
try to test that soon.

Thanks,
Ryan


>

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

Hi Ryan,
On Tue, Jul 9, 2019 at 4:13 PM Ryan Lambert  wrote:

>
> "It is possible for FETCH FIRST N PERCENT to create poorly performing
> query plans when the N supplied exceeds 50 percent.  In these cases query
> execution can take an order of magnitude longer to execute than simply
> returning the full table.  If performance is critical using an explicit row
> count for limiting is recommended."
>

I don’t understand how fetch first n percent functionality can be replaced
with explicit row count limiting. There may be a way to do it in a client
side but we can not be sure of its performance advantage


regards

Surafel

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

Hi Alvaro,
On Wed, Jul 10, 2019 at 6:44 PM Alvaro Herrera 
wrote:

> What's with the new tuplestore function for getting heap tuples?  That
> looks really odd.
>


Previously I create new TTSOpsMinimalTuple type slots for every tuple
returned in order to fetch it from tuplestore because tuplestore store
tuple in MinimalTuple format and created slot format changes in subsequent
operation. This case resource leak as Andres mention and I create this
function to remove the need for creating new slot for every returned tuple
because of form difference


regards

Surafel

Re: FETCH FIRST clause PERCENT option

[Alvaro Herrera]

On 2019-Jul-08, Surafel Temesgen wrote:

> Hi Thomas,
> Thank you for informing me
> 
> Hi Surafel,
> >
> > There's a call to adjust_limit_rows_costs() hiding under
> > contrib/postgres_fdw, so this fails check-world.
> >
> 
> Fixed . I also include the review given by Ryan in attached patch

What's with the new tuplestore function for getting heap tuples?  That
looks really odd.

-- 
Álvaro Herrerahttps://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: FETCH FIRST clause PERCENT option

[Kyotaro Horiguchi]

Hello.

At Wed, 10 Jul 2019 15:02:57 +0900 (Tokyo Standard Time), Kyotaro Horiguchi 
 wrote in 
<20190710.150257.260806103.horikyota@gmail.com>
> It is seen by a simpler test.
> 
> create table t as select a from generate_series(0, 9) a;
> analyze t;
> explain analyze select * from t order by a desc;
>  Execution Time: 116.613 ms
> explain analyze select * from t order by a desc fetch first 1 percent rows 
> only;
>  Execution Time: 158.458 ms
> explain analyze select * from t order by a desc fetch first 100 percent rows 
> only;
>  Execution Time: 364.442 ms
> 
> I didn't looked closer to the version. Fetching from tuplestore
> and returning all tuples costs 206ms and it is exceeding the cost
> of fething of the whole table and returning all tuples. I don't
> believe tuplestore that isn't splling out to disk is so slower
> than (cached) table access.
> 
> Other than that, we can rip the clause if it is 100%

As a more significant point, I found that the first query in the
aboves runs faster by about 10-18% on master(unpatched).

explain analyze select * from t order by a desc;
 Execution Time: 96.690 ms

But perf didn't give me useful information.

patched:

1185711.7065  postgres qsort_ssup
9026  8.9114  postgres ApplySortComparator
6443  6.3612  [vdso] (tgid:8388 range:0x7ffed49ed000-0x7ffed49eefff) [vdso] 
(tgid:8388 range:0x7ffed49ed000-0x7ffed49eefff)
5826  5.7520  postgres btint4fastcmp
4699  4.6393  no-vmlinux   /no-vmlinux
3451  3.4072  libc-2.17.so __memcpy_ssse3_back
3270  3.2285  postgres LogicalTapeWrite
2972  2.9343  postgres copytup_heap
2961  2.9234  postgres readtup_heap
2769  2.7338  postgres LogicalTapeRead
2457  2.4258  postgres GetMemoryChunkContext
2147  2.1197  postgres InstrStopNode
2021  1.9953  postgres heapgettup_pagemode
1583  1.5629  postgres writetup_heap
1555  1.5353  postgres tuplesort_gettuple_common
1508  1.4889  postgres AllocSetAlloc
...

master:

1293212.0168  postgres qsort_ssup
9491  8.8193  postgres ApplySortComparator
6705  6.2305  postgres btint4fastcmp
6557  6.0930  [vdso] (tgid:6341 range:0x7ffdd0315000-0x7ffdd0316fff) [vdso] 
(tgid:6341 range:0x7ffdd0315000-0x7ffdd0316fff)
4874  4.5291  no-vmlinux   /no-vmlinux
4059  3.7717  postgres readtup_heap
3707  3.4447  libc-2.17.so __memcpy_ssse3_back
3583  3.3294  postgres LogicalTapeWrite
3382  3.1427  postgres LogicalTapeRead
3001  2.7886  postgres copytup_heap
2522  2.3435  postgres GetMemoryChunkContext
2464  2.2896  postgres heapgettup_pagemode
2115  1.9653  postgres InstrStopNode
1847  1.7163  postgres tuplesort_gettuple_common
1652  1.5351  postgres writetup_heap
1565  1.4542  postgres AllocSetAlloc

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

Re: FETCH FIRST clause PERCENT option

[Kyotaro Horiguchi]

Hello.

At Tue, 9 Jul 2019 21:56:32 -0600, Ryan Lambert  wrote 
in 
> I did some more testing.  I initialized a database with 1 million rows with
> indexes and joins to test against and ran pgbench with a few different
> settings for % to return.  I started with a base query not utilizing the
> new functionality. The queries used are similar to my prior examples, code
> at [1].
> 
> createdb bench_test
> psql -d bench_test -f init/reporting.sql -v scale=10
> 
> The following provided 3.21 TPS and an average latency of 623.  The
> "per_change_" columns in the table below use those values.
> 
> pgbench -c 2 -j 2 -T 600 -P 60 -s 10 \
>-f tests/reporting1.sql bench_test
> 
> The remainder of the tests use the following, only adjusting fetch_percent
> value:
> 
> pgbench -c 2 -j 2 -T 600 -P 60 -s 10 \
>--define=fetch_percent=1 \
>-f tests/reporting_fetch_percent.sql \
>bench_test
> 
> 
> Returning 1% it runs well.  By 10% the TPS drops by 30% while the average
> latency increases by 43%.  When returning 95% of the table latency has
> increased by 548%.
> 
> 
>  fetch_percent | tps  | latency_avg_ms | per_change_tps | per_change_latency
> ---+--+++
>  1 | 3.37 |593 |   0.05 |  -0.05
>  5 | 2.85 |700 |  -0.11 |   0.12
> 10 | 2.24 |891 |  -0.30 |   0.43
> 25 | 1.40 |   1423 |  -0.56 |   1.28
> 45 | 0.93 |   2147 |  -0.71 |   2.45
> 95 | 0.49 |   4035 |  -0.85 |   5.48
> 
> 
> I manually tested the inner select queries without the outer aggregation
> thinking it might be a different story with a simple select and no CTE.
> Unfortunately it showed the same overall characteristics.  1% returns in
> about 550 ms, 45% took 1950, and 95% took 4050.
> 
> [1] https://github.com/rustprooflabs/pgbench-tests

It is seen by a simpler test.

create table t as select a from generate_series(0, 9) a;
analyze t;
explain analyze select * from t order by a desc;
 Execution Time: 116.613 ms
explain analyze select * from t order by a desc fetch first 1 percent rows only;
 Execution Time: 158.458 ms
explain analyze select * from t order by a desc fetch first 100 percent rows 
only;
 Execution Time: 364.442 ms

I didn't looked closer to the version. Fetching from tuplestore
and returning all tuples costs 206ms and it is exceeding the cost
of fething of the whole table and returning all tuples. I don't
believe tuplestore that isn't splling out to disk is so slower
than (cached) table access.

Other than that, we can rip the clause if it is 100%

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

Re: FETCH FIRST clause PERCENT option

[Ryan Lambert]

I did some more testing.  I initialized a database with 1 million rows with
indexes and joins to test against and ran pgbench with a few different
settings for % to return.  I started with a base query not utilizing the
new functionality. The queries used are similar to my prior examples, code
at [1].

createdb bench_test
psql -d bench_test -f init/reporting.sql -v scale=10

The following provided 3.21 TPS and an average latency of 623.  The
"per_change_" columns in the table below use those values.

pgbench -c 2 -j 2 -T 600 -P 60 -s 10 \
   -f tests/reporting1.sql bench_test

The remainder of the tests use the following, only adjusting fetch_percent
value:

pgbench -c 2 -j 2 -T 600 -P 60 -s 10 \
   --define=fetch_percent=1 \
   -f tests/reporting_fetch_percent.sql \
   bench_test


Returning 1% it runs well.  By 10% the TPS drops by 30% while the average
latency increases by 43%.  When returning 95% of the table latency has
increased by 548%.


 fetch_percent | tps  | latency_avg_ms | per_change_tps | per_change_latency
---+--+++
 1 | 3.37 |593 |   0.05 |  -0.05
 5 | 2.85 |700 |  -0.11 |   0.12
10 | 2.24 |891 |  -0.30 |   0.43
25 | 1.40 |   1423 |  -0.56 |   1.28
45 | 0.93 |   2147 |  -0.71 |   2.45
95 | 0.49 |   4035 |  -0.85 |   5.48


I manually tested the inner select queries without the outer aggregation
thinking it might be a different story with a simple select and no CTE.
Unfortunately it showed the same overall characteristics.  1% returns in
about 550 ms, 45% took 1950, and 95% took 4050.

[1] https://github.com/rustprooflabs/pgbench-tests

Ryan


>>

Re: FETCH FIRST clause PERCENT option

[Ryan Lambert]

Surafel,

> The cost of FITCH FIRST N PERCENT execution in current implementation is
the cost of pulling the full table plus the cost of storing and fetching
the tuple from tuplestore so it can > not perform better than pulling the
full table in any case . This is because we can't determined the number of
rows to return without executing the plan until the end. We can find the >
estimation of rows that will be return in planner estimation but that is
not exact.

Ok, I can live with that for the normal use cases.  This example from the
end of my previous message using 95% seems like a problem still, I don't
like syntax that unexpectedly kills performance like this one.  If this
can't be improved in the initial release of the feature I'd suggest we at
least make a strong disclaimer in the docs, along the lines of:

"It is possible for FETCH FIRST N PERCENT to create poorly performing query
plans when the N supplied exceeds 50 percent.  In these cases query
execution can take an order of magnitude longer to execute than simply
returning the full table.  If performance is critical using an explicit row
count for limiting is recommended."

I'm not certain the 50 percent is the true threshold of where things start
to fall apart, I just used that as a likely guess for now.  I can do some
more testing this week to identify where things start falling apart
performance wise.  Thanks,

EXPLAIN (ANALYZE, COSTS)
WITH t AS (
SELECT id, v1, v2
FROM r10mwide
FETCH FIRST 95 PERCENT ROWS ONLY
) SELECT AVG(v1), MIN(v1), AVG(v1 + v2) FROM t
;
   QUERY PLAN

-
 Aggregate  (cost=651432.48..651432.49 rows=1 width=24) (actual
time=58981.043..58981.044 rows=1 loops=1)
   ->  Limit  (cost=230715.67..461431.34 rows=9500057 width=20) (actual
time=0.017..55799.389 rows=950 loops=1)
 ->  Seq Scan on r10mwide  (cost=0.00..242858.60 rows=1060
width=20) (actual time=0.014..3847.146 rows=1000 loops=1)
 Planning Time: 0.117 ms
 Execution Time: 59079.680 ms
(5 rows)

Ryan Lambert

>
>

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

Hi Ryan,
On Tue, Jul 9, 2019 at 1:27 AM Ryan Lambert  wrote:

> Hi Surafel,
>
> The v5 patch [1] applies cleanly and passes make installcheck-world.
>
> My concern with this is its performance.  As a user I would expect using
> this feature to enable queries to run faster than they would simply pulling
> the full table.  I tested on tables ranging from 10k rows up to 10 million
> with the same basic result that using FETCH FIRST N PERCENT is slower than
> using the full table.  At best it ran slightly slower than querying the
> full table, at worst it increased execution times by 1400% when using a
> large high percentage (95%).
>
>

The cost of FITCH FIRST N PERCENT execution in current implementation is
the cost of pulling the full table plus the cost of storing and fetching
the tuple from tuplestore so it can not perform better than pulling the
full table in any case . This is because we can't determined the number of
rows to return without executing the plan until the end. We can find the
estimation of rows that will be return in planner estimation but that is
not exact.


regards

Surafel

Re: FETCH FIRST clause PERCENT option

[Ryan Lambert]

Hi Surafel,

The v5 patch [1] applies cleanly and passes make installcheck-world.

My concern with this is its performance.  As a user I would expect using
this feature to enable queries to run faster than they would simply pulling
the full table.  I tested on tables ranging from 10k rows up to 10 million
with the same basic result that using FETCH FIRST N PERCENT is slower than
using the full table.  At best it ran slightly slower than querying the
full table, at worst it increased execution times by 1400% when using a
large high percentage (95%).

Testing was on a DigitalOcean 2 CPU, 2GB RAM droplet.  All queries were
executed multiple times (6+) with EXPLAIN (ANALYZE, COSTS) and /timing
enabled, query plans provided are from the faster end of each query.

Create the test table:

DROP TABLE IF EXISTS r10mwide;
CREATE TABLE r10mwide AS
SELECT generate_series(1, 1000)::INT AS id,
random() AS v1,
random() AS v2,
random() AS v3,
random() AS v4,
random() AS v5,
random() AS v6,
random() AS v7,
random() AS v8,
random() AS v9,
random() AS v10
;
VACUUM ANALYZE;


Start with a baseline query from the full table, the limiting will be done
within the CTE as I would typically do in a production query.  The outer
query does basic aggregates.  Below I provide each full query for easy
copy/paste but the only real change is how the inner results are limited.
This runs in 2.2s off the full table.

EXPLAIN (ANALYZE, COSTS)
WITH t AS (
SELECT id, v1, v2
FROM r10mwide
) SELECT AVG(v1), MIN(v1), AVG(v1 + v2) FROM t
;

   QUERY
PLAN
-
 Finalize Aggregate  (cost=227192.07..227192.08 rows=1 width=24) (actual
time=2230.419..2230.419 rows=1 loops=1)
   ->  Gather  (cost=227191.84..227192.05 rows=2 width=72) (actual
time=2228.321..2231.225 rows=3 loops=1)
 Workers Planned: 2
 Workers Launched: 2
 ->  Partial Aggregate  (cost=226191.84..226191.85 rows=1 width=72)
(actual time=2218.754..2218.754 rows=1 loops=3)
   ->  Parallel Seq Scan on r10mwide  (cost=0.00..184524.92
rows=4166692 width=16) (actual time=0.032..934.652 rows=333 loops=3)
 Planning Time: 0.116 ms
 Execution Time: 2231.935 ms
(8 rows)

Time: 2232.459 ms (00:02.232)


It did use parallel, since the FETCH FIRST queries apparently won't I
turned that off and ran it again.

SET max_parallel_workers_per_gather = 0;

New query plan for full table, no parallel, just over 4 seconds.

  QUERY PLAN

---
 Aggregate  (cost=342859.21..342859.22 rows=1 width=24) (actual
time=4253.861..4253.862 rows=1 loops=1)
   ->  Seq Scan on r10mwide  (cost=0.00..242858.60 rows=1060 width=16)
(actual time=0.062..1728.346 rows=1000 loops=1)
 Planning Time: 0.082 ms
 Execution Time: 4253.918 ms
(4 rows)


The following uses the explicit row count to pull 100k rows (1%) and gives
a massive improvement in speed, this query ranged from 55- 120ms.  This is
the type of speedup I would expect, and it beats the full-table query hands
down, regardless of parallel query.

EXPLAIN (ANALYZE, COSTS)
WITH t AS (
SELECT id, v1, v2
FROM r10mwide
FETCH FIRST 10 ROWS ONLY
) SELECT AVG(v1), MIN(v1), AVG(v1 + v2) FROM t
;

   QUERY PLAN

-
 Aggregate  (cost=4428.58..4428.59 rows=1 width=24) (actual
time=55.963..55.963 rows=1 loops=1)
   ->  Limit  (cost=0.00..2428.57 rows=10 width=20) (actual
time=0.041..35.725 rows=10 loops=1)
 ->  Seq Scan on r10mwide  (cost=0.00..242858.60 rows=1060
width=20) (actual time=0.039..24.796 rows=10 loops=1)
 Planning Time: 0.134 ms
 Execution Time: 56.032 ms
(5 rows)

Time: 57.262 ms



Using FETCH FIRST 1 PERCENT it takes over 7 seconds, 71% slower than
querying the full table.

EXPLAIN (ANALYZE, COSTS)
WITH t AS (
SELECT id, v1, v2
FROM r10mwide
FETCH FIRST 1 PERCENT ROWS ONLY
) SELECT AVG(v1), MIN(v1), AVG(v1 + v2) FROM t
;
 QUERY PLAN

-
 Aggregate  (cost=6857.22..6857.23 rows=1 width=24) (actual
time=7112.205..7112.206 rows=1 loops=1)
   ->  Limit  (cost=2428.60..4857.19 rows=11 width=20) (actual
time=0.036..7047.394 rows=10 loops=1)
 ->  Seq Scan on r10mwide  (cost=0.00..242858.60 rows=1060
width=20) (actual time=0.033..3072.881 rows=1000 loops=1)
 Planning Time: 0.

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

Hi Thomas,
Thank you for informing me

Hi Surafel,
>
> There's a call to adjust_limit_rows_costs() hiding under
> contrib/postgres_fdw, so this fails check-world.
>

Fixed . I also include the review given by Ryan in attached patch

regards
Surafel
diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index 1759b9e1b6..fb9c2f5319 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -3035,7 +3035,8 @@ estimate_path_cost_size(PlannerInfo *root,
 		if (fpextra && fpextra->has_limit)
 		{
 			adjust_limit_rows_costs(&rows, &startup_cost, &total_cost,
-	fpextra->offset_est, fpextra->count_est);
+	fpextra->offset_est, fpextra->count_est,
+	EXACT_NUMBER);
 			retrieved_rows = rows;
 		}
 	}
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 06d611b64c..1c240a3dab 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1430,7 +1430,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY
 
 In this syntax, the start
 or count value is required by
@@ -1440,7 +1440,8 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
-ROW
+With PERCENT count specifies the maximum number of rows to return
+in percentage round up to the nearest integer.ROW
 and ROWS as well as FIRST
 and NEXT are noise words that don't influence
 the effects of these clauses.
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index baa669abe8..c503512588 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -21,6 +21,8 @@
 
 #include "postgres.h"
 
+#include 
+
 #include "executor/executor.h"
 #include "executor/nodeLimit.h"
 #include "miscadmin.h"
@@ -52,6 +54,7 @@ ExecLimit(PlanState *pstate)
 	 */
 	direction = node->ps.state->es_direction;
 	outerPlan = outerPlanState(node);
+	slot = node->subSlot;
 
 	/*
 	 * The main logic is a simple state machine.
@@ -81,7 +84,15 @@ ExecLimit(PlanState *pstate)
 			/*
 			 * Check for empty window; if so, treat like empty subplan.
 			 */
-			if (node->count <= 0 && !node->noCount)
+			if (node->limitOption == PERCENTAGE)
+			{
+if (node->percent == 0.0)
+{
+	node->lstate = LIMIT_EMPTY;
+	return NULL;
+}
+			}
+			else if (node->count <= 0 && !node->noCount)
 			{
 node->lstate = LIMIT_EMPTY;
 return NULL;
@@ -102,11 +113,32 @@ ExecLimit(PlanState *pstate)
 	node->lstate = LIMIT_EMPTY;
 	return NULL;
 }
+
+/*
+ * Count the number of rows to return in exact number so far
+ */
+if (node->limitOption == PERCENTAGE)
+{
+	node->perExactCount = ceil(node->percent * node->position / 100.0);
+
+	if (node->perExactCount == node->perExactCount + 1)
+		node->perExactCount++;
+}
 node->subSlot = slot;
 if (++node->position > node->offset)
 	break;
 			}
 
+			/*
+			 * We may needed this tuple in backward scan so put it into
+			 * tuplestore.
+			 */
+			if (node->limitOption == PERCENTAGE)
+			{
+tuplestore_puttupleslot(node->tupleStore, slot);
+tuplestore_advance(node->tupleStore, true);
+			}
+
 			/*
 			 * Okay, we have the first tuple of the window.
 			 */
@@ -124,6 +156,106 @@ ExecLimit(PlanState *pstate)
 		case LIMIT_INWINDOW:
 			if (ScanDirectionIsForward(direction))
 			{
+/*
+ * In case of coming back from backward scan the tuple is
+ * already in tuple store.
+ */
+if (node->limitOption == PERCENTAGE && node->backwardPosition > 0)
+{
+	if (tuplestore_gettupleslot_heaptuple(node->tupleStore, true, true, slot))
+	{
+		node->subSlot = slot;
+		node->position++;
+		node->backwardPosition--;
+		return slot;
+	}
+	else
+	{
+		node->lstate = LIMIT_SUBPLANEOF;
+		return NULL;
+	}
+}
+
+/*
+ * In PERCENTAGE case no need of executing outerPlan multiple
+ * times.
+ */
+if (node->limitOption == PERCENTAGE && node->reachEnd)
+{
+	node->lstate = LIMIT_WINDOWEND;
+
+	/*
+	 * If we know we won't need to back up, we can release
+	 * resources at this point.
+	 */
+	if (!(node->ps.state->es_top_eflags & EXEC_FLAG_BACKWARD))
+		(void) ExecShutdownNode(out

Re: FETCH FIRST clause PERCENT option

[Ryan Lambert]

The following review has been posted through the commitfest application:
make installcheck-world:  not tested
Implements feature:   tested, passed
Spec compliant:   not tested
Documentation:not tested

The basic functionality works as I expect.  In the following example I would 
have guessed it would return 4 rows instead of 5.  I don't mind that it uses 
ceil here, but think that deserves a mention in the documentation.  

CREATE TABLE r100 (id INT);
INSERT INTO r100 SELECT generate_series(1, 100);

SELECT * FROM r100 FETCH FIRST 4.01 PERCENT ROWS ONLY;
 id

  1
  2
  3
  4
  5
(5 rows)

There's a missing space between the period and following sentence in 
src\backend\executor\nodeLimit.c
"previous time we got a different result.In PERCENTAGE option there are"


There's a missing space and the beginning "w" should be capitalized in 
doc\src\sgml\ref\select.sgml
with PERCENT count specifies the maximum number of rows 
to return 
in percentage.ROW

Another missing space after the period.

previous time we got a different result.In PERCENTAGE option there are"


Ryan Lambert

The new status of this patch is: Waiting on Author

Re: FETCH FIRST clause PERCENT option

[Thomas Munro]

On Thu, Jun 27, 2019 at 9:06 PM Surafel Temesgen  wrote:
> The attached patch include the fix for all the comment given

Hi Surafel,

There's a call to adjust_limit_rows_costs() hiding under
contrib/postgres_fdw, so this fails check-world.

-- 
Thomas Munro
https://enterprisedb.com

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

Hello,
The attached patch include the fix for all the comment given
regards
Surafel

On Mon, Apr 1, 2019 at 12:34 AM Andres Freund  wrote:

> Hi,
>
> On 2019-03-29 12:04:50 +0300, Surafel Temesgen wrote:
>
> > + if (node->limitOption == PERCENTAGE)
> > + {
> > + while (node->position -
> node->offset < node->count)
> > + {
> > + slot =
> MakeSingleTupleTableSlot(tupleDescriptor, &TTSOpsMinimalTuple);
> > + if
> (tuplestore_gettupleslot(node->tupleStore, true, true, slot))
>
> There's several blocks like this - how's this not going to be a resource
> leak? As far as I can tell you're just going to create new slots, and
> their previous contents aren't going to be cleared?   I think there very
> rarely are cases where an executor node's *Next function (or its
> subsidiaries) creates slots. Normally you ought to create them *once* on
> the *Init function.
>
> You might not directly leak memory, because this is probably running in
> a short lived context, but you'll leak tuple desc references etc. (Or if
> it were a heap buffer slot, buffer pins).
>
>
> > + /* In PERCENTAGE case the result is already in
> tuplestore */
> > + if (node->limitOption == PERCENTAGE)
> > + {
> > + slot =
> MakeSingleTupleTableSlot(tupleDescriptor, &TTSOpsMinimalTuple);
> > + if
> (tuplestore_gettupleslot(node->tupleStore, false, false, slot))
> > + {
> > + node->subSlot = slot;
> > + node->lstate = LIMIT_INWINDOW;
> > + }
> > + else
> > + elog(ERROR, "LIMIT subplan failed
> to run backwards");
> > + }
> > + else if (node->limitOption == EXACT_NUMBER)
> > + {
> >   /*
> >* Backing up from subplan EOF, so re-fetch
> previous tuple; there
> >* should be one!  Note previous tuple must be in
> window.
> > @@ -194,6 +329,7 @@ ExecLimit(PlanState *pstate)
> >   node->subSlot = slot;
> >   node->lstate = LIMIT_INWINDOW;
> >   /* position does not change 'cause we didn't
> advance it before */
> > + }
>
> The indentation here looks wrong...
>
> >   break;
> >
> >   case LIMIT_WINDOWEND:
> > @@ -278,17 +414,29 @@ recompute_limits(LimitState *node)
> >   /* Interpret NULL count as no count (LIMIT ALL) */
> >   if (isNull)
> >   {
> > - node->count = 0;
> > + node->count = 1;
> >   node->noCount = true;
>
> Huh?
>
>
> >   }
> >   else
> >   {
> > - node->count = DatumGetInt64(val);
> > - if (node->count < 0)
> > - ereport(ERROR,
> > -
>  (errcode(ERRCODE_INVALID_ROW_COUNT_IN_LIMIT_CLAUSE),
> > -  errmsg("LIMIT must not be
> negative")));
> > - node->noCount = false;
> > + if (node->limitOption == PERCENTAGE)
> > + {
> > + /*
> > +  * We expect to return at least one row
> (unless there
> > +  * are no rows in the subplan), and we'll
> update this
> > +  * count later as we go.
> > +  */
> > + node->count = 0;
> > + node->percent = DatumGetFloat8(val);
> > + }
> > + else
> > + {
> > + node->count = DatumGetInt64(val);
> > + if (node->count < 0)
> > + ereport(ERROR,
> > +
>  (errcode(ERRCODE_INVALID_ROW_COUNT_IN_LIMIT_CLAUSE),
> > +  errmsg("LIMIT
> must not be negative")));
> > + }
>
> Shouldn't we error out with a negative count, regardless of PERCENT? Or
> is that prevented elsewhere?
>
> >   }
> >   }
> >   else
> > @@ -299,8 +447,10 @@ recompute_limits(LimitState *node)
> >   }
> >
> >   /* Reset position to start-of-scan */
> > - node->position = 0;
> > + node->position = 0;;
>
> unnecessary
>
>
> >   if (parse->sortClause)
> >   {
> > - current_rel = create_ordered_paths(root,
> > -
> current_rel,
> > -
> fina

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

On Fri, Apr 05, 2019 at 03:14:56PM +0300, Surafel Temesgen wrote:

On Tue, Mar 26, 2019 at 5:46 PM Tomas Vondra 
wrote:


On Tue, Mar 26, 2019 at 03:06:52PM +0300, Surafel Temesgen wrote:
>On Thu, Feb 28, 2019 at 11:16 PM Tomas Vondra <
tomas.von...@2ndquadrant.com>
>wrote:
>
>>
>> To give you a (admittedly, somewhat contrived and artificial example):
>>
>> SELECT * FROM t1 WHERE id IN (
>>   SELECT id FROM t2 ORDER BY x FETCH FIRST 10 PERCENT ROWS ONLY
>> );
>>
>> Maybe this example is bogus and/or does not really matter in practice. I
>> don't know, but I've been unable to convince myself that's the case.
>
>
>does this means we abandon incremental approach? and am not sure of
>calculating
>percentage after OFFSET clause is acceptable or not
>

I don't follow. I don't think I've suggested to abandon the incremental
approach - I've explained why I think it's what the patch should be doing,
and illustrated that with an example.



but it is more complex than the previous approach and it will be more
complex
in starting calculating percentage before offset row count. Doesn't
simplicity prefer?



Sure, simplicity has it's value. And often the simplest solution is the
best one. But in But in plenty of cases it's a tradeoff between simplicity
and efficiency, i.e. the simplest solution may perform poorly. I've tried
to explain why I think the incremental solution is the right approach
here - not having to always execute the subplan to completion. I still
haven't heard reasons why that argument is irrelevant (it may be).

regards

--
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Tue, Mar 26, 2019 at 5:46 PM Tomas Vondra 
wrote:

> On Tue, Mar 26, 2019 at 03:06:52PM +0300, Surafel Temesgen wrote:
> >On Thu, Feb 28, 2019 at 11:16 PM Tomas Vondra <
> tomas.von...@2ndquadrant.com>
> >wrote:
> >
> >>
> >> To give you a (admittedly, somewhat contrived and artificial example):
> >>
> >> SELECT * FROM t1 WHERE id IN (
> >>   SELECT id FROM t2 ORDER BY x FETCH FIRST 10 PERCENT ROWS ONLY
> >> );
> >>
> >> Maybe this example is bogus and/or does not really matter in practice. I
> >> don't know, but I've been unable to convince myself that's the case.
> >
> >
> >does this means we abandon incremental approach? and am not sure of
> >calculating
> >percentage after OFFSET clause is acceptable or not
> >
>
> I don't follow. I don't think I've suggested to abandon the incremental
> approach - I've explained why I think it's what the patch should be doing,
> and illustrated that with an example.
>

but it is more complex than the previous approach and it will be more
complex
in starting calculating percentage before offset row count. Doesn't
simplicity prefer?

regards
Surafel

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

Thank you for looking at it
On Mon, Apr 1, 2019 at 12:34 AM Andres Freund  wrote:

> Hi,
>
> On 2019-03-29 12:04:50 +0300, Surafel Temesgen wrote:
>
> > + if (node->limitOption == PERCENTAGE)
> > + {
> > + while (node->position -
> node->offset < node->count)
> > + {
> > + slot =
> MakeSingleTupleTableSlot(tupleDescriptor, &TTSOpsMinimalTuple);
> > + if
> (tuplestore_gettupleslot(node->tupleStore, true, true, slot))
>
> There's several blocks like this - how's this not going to be a resource
> leak? As far as I can tell you're just going to create new slots, and
> their previous contents aren't going to be cleared?   I think there very
> rarely are cases where an executor node's *Next function (or its
> subsidiaries) creates slots. Normally you ought to create them *once* on
> the *Init function.
>
>
create it in init stage is good idea. i make it this way because
tuplestore_gettupleslot
work with TTSOpsMinimalTuple


> You might not directly leak memory, because this is probably running in
> a short lived context, but you'll leak tuple desc references etc. (Or if
> it were a heap buffer slot, buffer pins).
>
>
> > + /* In PERCENTAGE case the result is already in
> tuplestore */
> > + if (node->limitOption == PERCENTAGE)
> > + {
> > + slot =
> MakeSingleTupleTableSlot(tupleDescriptor, &TTSOpsMinimalTuple);
> > + if
> (tuplestore_gettupleslot(node->tupleStore, false, false, slot))
> > + {
> > + node->subSlot = slot;
> > + node->lstate = LIMIT_INWINDOW;
> > + }
> > + else
> > + elog(ERROR, "LIMIT subplan failed
> to run backwards");
> > + }
> > + else if (node->limitOption == EXACT_NUMBER)
> > + {
> >   /*
> >* Backing up from subplan EOF, so re-fetch
> previous tuple; there
> >* should be one!  Note previous tuple must be in
> window.
> > @@ -194,6 +329,7 @@ ExecLimit(PlanState *pstate)
> >   node->subSlot = slot;
> >   node->lstate = LIMIT_INWINDOW;
> >   /* position does not change 'cause we didn't
> advance it before */
> > + }
>
> The indentation here looks wrong...
>
> >   break;
> >
> >   case LIMIT_WINDOWEND:
> > @@ -278,17 +414,29 @@ recompute_limits(LimitState *node)
> >   /* Interpret NULL count as no count (LIMIT ALL) */
> >   if (isNull)
> >   {
> > - node->count = 0;
> > + node->count = 1;
> >   node->noCount = true;
>
> Huh?
>
>
> >   }
> >   else
> >   {
> > - node->count = DatumGetInt64(val);
> > - if (node->count < 0)
> > - ereport(ERROR,
> > -
>  (errcode(ERRCODE_INVALID_ROW_COUNT_IN_LIMIT_CLAUSE),
> > -  errmsg("LIMIT must not be
> negative")));
> > - node->noCount = false;
> > + if (node->limitOption == PERCENTAGE)
> > + {
> > + /*
> > +  * We expect to return at least one row
> (unless there
> > +  * are no rows in the subplan), and we'll
> update this
> > +  * count later as we go.
> > +  */
> > + node->count = 0;
> > + node->percent = DatumGetFloat8(val);
> > + }
> > + else
> > + {
> > + node->count = DatumGetInt64(val);
> > + if (node->count < 0)
> > + ereport(ERROR,
> > +
>  (errcode(ERRCODE_INVALID_ROW_COUNT_IN_LIMIT_CLAUSE),
> > +  errmsg("LIMIT
> must not be negative")));
> > + }
>
> Shouldn't we error out with a negative count, regardless of PERCENT? Or
> is that prevented elsewhere?
>

yes it should error out .will fix it in next patch


> >   }
> >   }
> >   else
> > @@ -299,8 +447,10 @@ recompute_limits(LimitState *node)
> >   }
> >
> >   /* Reset position to start-of-scan */
> > - node->position = 0;
> > + node->position = 0;;
>
> unnecessary
>
>
> >   if (parse->sortClause)
>

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Tue, Mar 26, 2019 at 5:46 PM Tomas Vondra 
wrote:

> On Tue, Mar 26, 2019 at 03:06:52PM +0300, Surafel Temesgen wrote:
>
> As for the OFFSET, I don't see why that would be incompatible with PERCENT
> clause. I'm not sure if it should compute the percentage from the total
> number of rows (including those skipped by the OFFSET) or the rest, but I
> assume SQL standard says something about that.
>
>
I check SQL standard and it is different in PERCENT cause. It state that
the percentage is computed using the number of rows before removing the
rows specified by offset row count

regards
Surafel

Re: FETCH FIRST clause PERCENT option

[Tom Lane]

Andres Freund  writes:
>> offset_clause:
>> @@ -15435,6 +15442,7 @@ reserved_keyword:
>>  | ONLY
>>  | OR
>>  | ORDER
>> +| PERCENT
>>  | PLACING
>>  | PRIMARY
>>  | REFERENCES

> Are we really ok with adding a new reserved keyword 'PERCENT' for this?

I'm not.  It doesn't look like it ought to be necessary to reserve it,
either, given that we don't have to reduce the production right there.

(If that doesn't work right away, try getting rid of row_or_rows
in favor of spelling out those alternatives in separate productions.)

More generally: using an undocumented list as the data structure for
select_limit's result was already a pretty bad idea, I think, and
this patch certainly makes it totally unreadable.  Probably ought
to refactor that to use some sort of struct.

regards, tom lane

Re: FETCH FIRST clause PERCENT option

[Andres Freund]

Hi,

On 2019-03-29 12:04:50 +0300, Surafel Temesgen wrote:

> + if (node->limitOption == PERCENTAGE)
> + {
> + while (node->position - node->offset < 
> node->count)
> + {
> + slot = 
> MakeSingleTupleTableSlot(tupleDescriptor, &TTSOpsMinimalTuple);
> + if 
> (tuplestore_gettupleslot(node->tupleStore, true, true, slot))

There's several blocks like this - how's this not going to be a resource
leak? As far as I can tell you're just going to create new slots, and
their previous contents aren't going to be cleared?   I think there very
rarely are cases where an executor node's *Next function (or its
subsidiaries) creates slots. Normally you ought to create them *once* on
the *Init function.

You might not directly leak memory, because this is probably running in
a short lived context, but you'll leak tuple desc references etc. (Or if
it were a heap buffer slot, buffer pins).


> + /* In PERCENTAGE case the result is already in 
> tuplestore */
> + if (node->limitOption == PERCENTAGE)
> + {
> + slot = 
> MakeSingleTupleTableSlot(tupleDescriptor, &TTSOpsMinimalTuple);
> + if (tuplestore_gettupleslot(node->tupleStore, 
> false, false, slot))
> + {
> + node->subSlot = slot;
> + node->lstate = LIMIT_INWINDOW;
> + }
> + else
> + elog(ERROR, "LIMIT subplan failed to 
> run backwards");
> + }
> + else if (node->limitOption == EXACT_NUMBER)
> + {
>   /*
>* Backing up from subplan EOF, so re-fetch previous 
> tuple; there
>* should be one!  Note previous tuple must be in 
> window.
> @@ -194,6 +329,7 @@ ExecLimit(PlanState *pstate)
>   node->subSlot = slot;
>   node->lstate = LIMIT_INWINDOW;
>   /* position does not change 'cause we didn't advance it 
> before */
> + }

The indentation here looks wrong...

>   break;
>  
>   case LIMIT_WINDOWEND:
> @@ -278,17 +414,29 @@ recompute_limits(LimitState *node)
>   /* Interpret NULL count as no count (LIMIT ALL) */
>   if (isNull)
>   {
> - node->count = 0;
> + node->count = 1;
>   node->noCount = true;

Huh?


>   }
>   else
>   {
> - node->count = DatumGetInt64(val);
> - if (node->count < 0)
> - ereport(ERROR,
> - 
> (errcode(ERRCODE_INVALID_ROW_COUNT_IN_LIMIT_CLAUSE),
> -  errmsg("LIMIT must not be 
> negative")));
> - node->noCount = false;
> + if (node->limitOption == PERCENTAGE)
> + {
> + /*
> +  * We expect to return at least one row (unless 
> there
> +  * are no rows in the subplan), and we'll 
> update this
> +  * count later as we go.
> +  */
> + node->count = 0;
> + node->percent = DatumGetFloat8(val);
> + }
> + else
> + {
> + node->count = DatumGetInt64(val);
> + if (node->count < 0)
> + ereport(ERROR,
> + 
> (errcode(ERRCODE_INVALID_ROW_COUNT_IN_LIMIT_CLAUSE),
> +  errmsg("LIMIT must not 
> be negative")));
> + }

Shouldn't we error out with a negative count, regardless of PERCENT? Or
is that prevented elsewhere?

>   }
>   }
>   else
> @@ -299,8 +447,10 @@ recompute_limits(LimitState *node)
>   }
>  
>   /* Reset position to start-of-scan */
> - node->position = 0;
> + node->position = 0;;

unnecessary


>   if (parse->sortClause)
>   {
> - current_rel = create_ordered_paths(root,
> - 
>current_rel,
> - 
>final_target,
> -   

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

Hi,
On Thu, Feb 28, 2019 at 2:50 PM Kyotaro HORIGUCHI <
horiguchi.kyot...@lab.ntt.co.jp> wrote:

> 
> -* previous time we got a different result.
> +* previous time we got a different result.In PERCENTAGE option
> there are
> +* no bound on the number of output tuples */
>  */
> 
>
>
 Thank you for testing .Fixed
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 06d611b64c..e3ce4d7e36 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1430,7 +1430,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY
 
 In this syntax, the start
 or count value is required by
@@ -1440,7 +1440,8 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
-ROW
+with PERCENT count specifies the maximum number of rows to return 
+in percentage.ROW
 and ROWS as well as FIRST
 and NEXT are noise words that don't influence
 the effects of these clauses.
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index baa669abe8..7bfd7fd034 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -21,6 +21,8 @@
 
 #include "postgres.h"
 
+#include 
+
 #include "executor/executor.h"
 #include "executor/nodeLimit.h"
 #include "miscadmin.h"
@@ -44,6 +46,7 @@ ExecLimit(PlanState *pstate)
 	ScanDirection direction;
 	TupleTableSlot *slot;
 	PlanState  *outerPlan;
+	TupleDesc   tupleDescriptor;
 
 	CHECK_FOR_INTERRUPTS();
 
@@ -52,6 +55,8 @@ ExecLimit(PlanState *pstate)
 	 */
 	direction = node->ps.state->es_direction;
 	outerPlan = outerPlanState(node);
+	slot = node->subSlot;
+	tupleDescriptor = node->ps.ps_ResultTupleDesc;
 
 	/*
 	 * The main logic is a simple state machine.
@@ -81,7 +86,15 @@ ExecLimit(PlanState *pstate)
 			/*
 			 * Check for empty window; if so, treat like empty subplan.
 			 */
-			if (node->count <= 0 && !node->noCount)
+			if (node->limitOption == PERCENTAGE)
+			{
+if (node->percent == 0.0)
+{
+	node->lstate = LIMIT_EMPTY;
+	return NULL;
+}
+			}
+			else if (node->count <= 0 && !node->noCount)
 			{
 node->lstate = LIMIT_EMPTY;
 return NULL;
@@ -107,6 +120,15 @@ ExecLimit(PlanState *pstate)
 	break;
 			}
 
+			/*
+			 * We may needed this tuple in backward scan so put it into tuplestore.
+			 */
+			if (node->limitOption == PERCENTAGE)
+			{
+tuplestore_puttupleslot(node->tupleStore, slot);
+tuplestore_advance(node->tupleStore, true);
+			}
+
 			/*
 			 * Okay, we have the first tuple of the window.
 			 */
@@ -124,6 +146,72 @@ ExecLimit(PlanState *pstate)
 		case LIMIT_INWINDOW:
 			if (ScanDirectionIsForward(direction))
 			{
+
+/*
+ * In case of coming back from backward scan the tuple is already
+ * in tuple store.
+ */
+if (node->limitOption == PERCENTAGE && node->backwardPosition > 0)
+{
+	slot = MakeSingleTupleTableSlot(tupleDescriptor, &TTSOpsMinimalTuple);
+	if (tuplestore_gettupleslot(node->tupleStore, true, false, slot))
+	{
+		node->subSlot = slot;
+		node->position++;
+		node->backwardPosition--;
+		return slot;
+	}
+	else
+	{
+		node->lstate = LIMIT_SUBPLANEOF;
+		return NULL;
+	}
+}
+
+/*
+ * In PERCENTAGE case no need of executing outerPlan multiple times.
+ */
+if (node->limitOption ==  PERCENTAGE && node->reachEnd)
+{
+	node->lstate = LIMIT_WINDOWEND;
+
+	/*
+	 * If we know we won't need to back up, we can release
+	 * resources at this point.
+	 */
+	if (!(node->ps.state->es_top_eflags & EXEC_FLAG_BACKWARD))
+		(void) ExecShutdownNode(outerPlan);
+
+	return NULL;
+}
+
+/*
+ * When in percentage mode, we need to see if we can get any
+ * additional rows from the subplan (enough to increase the
+ * node->count value).
+ */
+if (node->limitOption == PERCENTAGE)
+{
+	/* loop until the node->count increments */
+	while (node->position - node->offset >= node->count)
+	{
+		int64	cnt;
+
+		slot = ExecProcNode(outerPlan);
+		if (TupIsNull(slot))
+		{
+			node->reachEnd = true;
+			break;
+		}
+
+		tuplestore_puttupleslot(node->tupleStore, slot);
+
+		cnt = tuple

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

On Tue, Mar 26, 2019 at 03:06:52PM +0300, Surafel Temesgen wrote:

On Thu, Feb 28, 2019 at 11:16 PM Tomas Vondra 
wrote:



To give you a (admittedly, somewhat contrived and artificial example):

SELECT * FROM t1 WHERE id IN (
  SELECT id FROM t2 ORDER BY x FETCH FIRST 10 PERCENT ROWS ONLY
);

Maybe this example is bogus and/or does not really matter in practice. I
don't know, but I've been unable to convince myself that's the case.



does this means we abandon incremental approach? and am not sure of
calculating
percentage after OFFSET clause is acceptable or not



I don't follow. I don't think I've suggested to abandon the incremental
approach - I've explained why I think it's what the patch should be doing,
and illustrated that with an example. I admit the example may be too
artificial and we never (or very infrequently) generate such plans. But
no such argument was presented here.

As for the OFFSET, I don't see why that would be incompatible with PERCENT
clause. I'm not sure if it should compute the percentage from the total
number of rows (including those skipped by the OFFSET) or the rest, but I
assume SQL standard says something about that.

cheers


--
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Thu, Feb 28, 2019 at 11:16 PM Tomas Vondra 
wrote:

>
> To give you a (admittedly, somewhat contrived and artificial example):
>
> SELECT * FROM t1 WHERE id IN (
>   SELECT id FROM t2 ORDER BY x FETCH FIRST 10 PERCENT ROWS ONLY
> );
>
> Maybe this example is bogus and/or does not really matter in practice. I
> don't know, but I've been unable to convince myself that's the case.


does this means we abandon incremental approach? and am not sure of
calculating
percentage after OFFSET clause is acceptable or not

regards
Surafel

Re: Re: FETCH FIRST clause PERCENT option

[David Steele]

Hi Surafel,

On 3/1/19 8:18 PM, Tomas Vondra wrote:




I'm not sure hat E-Rows and A-Rows are? I suppose that's estimated and
actual - but as I explained above, that's not what I propose the
incremental approach to use.


Marked as Waiting for Author since it appears Tomas is waiting for a 
response from you.


Regards,
--
-David
da...@pgmasters.net

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

On 3/1/19 2:31 AM, Kyotaro HORIGUCHI wrote:
> Hello.
> 
> At Thu, 28 Feb 2019 21:16:25 +0100, Tomas Vondra 
>  wrote in 
> 
>>> One biggest issue seems to be we don't know the total number of
> 
> # One *of* the biggest *issues*?
> 
>>> outer tuples before actually reading a null tuple. I doubt of
>>> general shortcut for that. It also seems preventing limit node
>>> from just using materialized outer.
>>>
>>
>> Sure, if you actually want all tuples, you'll have to execute the outer
>> plan till completion. But that's not what I'm talking about - what if we
>> only ever need to read one row from the limit?
> 
> We have no choice than once reading all tuples just to find we
> are to return just one row, since estimator is not guaranteed to
> be exact as required for this purpose.
> 

I don't follow - I'm not suggesting to base this on row estimates at
all. I'm saying that we can read in rows, and decide how many rows we
are expected to produce.

For example, with 1% sample, we'll produce about 1 row for each 100 rows
read from the outer plan. So we read the first row, and compute

  rows_to_produce = ceil(0.01 * rows_read)

and every time this increases, we emit another row from a tuplestore.


>> To give you a (admittedly, somewhat contrived and artificial example):
>>
>> SELECT * FROM t1 WHERE id IN (
>>   SELECT id FROM t2 ORDER BY x FETCH FIRST 10 PERCENT ROWS ONLY
>> );
>>
>> Maybe this example is bogus and/or does not really matter in practice. I
>> don't know, but I've been unable to convince myself that's the case.
> 
> I see such kind of idiom common. Even in the quite simple example
> above, *we* cannot tell how many tuples the inner should return
> unless we actually fetch all tuples in t2. This is the same
> problem with count(*).
> 
> The query is equivalent to the folloing one.
> 
>  SELECT * FROM t1 WHERE id IN (
>SELECT id FROM t2 ORDER BY x
>  FETCH FIRST (SELECT ceil(count(*) * 0.1) FROM t2) ROWS ONLY
>  );
> 
> This scans t2 twice, but this patch does only one full scan
> moving another partial scan to tuplestore. We would win if the
> outer is complex enough.
> 

But what if all t1 rows match the very first row in the subselect? In
that case we don't need to read all rows from the subplan - we only need
to read the first chunk (until we emit the first row).

> Anyway, even excluding the count(*) issue, it seems that we are
> not alone in that point. (That is, at least Oracle shows
> different E-Rows and A-Rows for PERCENT).
> 

I'm not sure hat E-Rows and A-Rows are? I suppose that's estimated and
actual - but as I explained above, that's not what I propose the
incremental approach to use.

regards

-- 
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Fri, Mar 1, 2019 at 4:33 AM Kyotaro HORIGUCHI <
horiguchi.kyot...@lab.ntt.co.jp> wrote:

> Hello.
>
> At Thu, 28 Feb 2019 21:16:25 +0100, Tomas Vondra <
> tomas.von...@2ndquadrant.com> wrote in <
> fbd08ad3-5dd8-3169-6cba-38d610d7b...@2ndquadrant.com>
> > > One biggest issue seems to be we don't know the total number of
>
> # One *of* the biggest *issues*?
>
> > > outer tuples before actually reading a null tuple. I doubt of
> > > general shortcut for that. It also seems preventing limit node
> > > from just using materialized outer.
> > >
> >
> > Sure, if you actually want all tuples, you'll have to execute the outer
> > plan till completion. But that's not what I'm talking about - what if we
> > only ever need to read one row from the limit?
>
> We have no choice than once reading all tuples just to find we
> are to return just one row, since estimator is not guaranteed to
> be exact as required for this purpose.
>
> > To give you a (admittedly, somewhat contrived and artificial example):
> >
> > SELECT * FROM t1 WHERE id IN (
> >   SELECT id FROM t2 ORDER BY x FETCH FIRST 10 PERCENT ROWS ONLY
> > );
> >
> > Maybe this example is bogus and/or does not really matter in practice. I
> > don't know, but I've been unable to convince myself that's the case.
>
> I see such kind of idiom common. Even in the quite simple example
> above, *we* cannot tell how many tuples the inner should return
> unless we actually fetch all tuples in t2. This is the same
> problem with count(*).
>
> The query is equivalent to the folloing one.
>
>  SELECT * FROM t1 WHERE id IN (
>SELECT id FROM t2 ORDER BY x
>  FETCH FIRST (SELECT ceil(count(*) * 0.1) FROM t2) ROWS ONLY
>  );
>
> This scans t2 twice, but this patch does only one full scan
> moving another partial scan to tuplestore. We would win if the
> outer is complex enough.
>

Okay here is the previous implementation with uptread review comment
included and it also consider OFFSET clause in percentage calculation

regards
Surafel
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 06d611b64c..e3ce4d7e36 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1430,7 +1430,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY
 
 In this syntax, the start
 or count value is required by
@@ -1440,7 +1440,8 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
-ROW
+with PERCENT count specifies the maximum number of rows to return 
+in percentage.ROW
 and ROWS as well as FIRST
 and NEXT are noise words that don't influence
 the effects of these clauses.
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index baa669abe8..2be6f655fb 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -43,6 +43,7 @@ ExecLimit(PlanState *pstate)
 	LimitState *node = castNode(LimitState, pstate);
 	ScanDirection direction;
 	TupleTableSlot *slot;
+	TupleDesc   tupleDescriptor;
 	PlanState  *outerPlan;
 
 	CHECK_FOR_INTERRUPTS();
@@ -52,6 +53,8 @@ ExecLimit(PlanState *pstate)
 	 */
 	direction = node->ps.state->es_direction;
 	outerPlan = outerPlanState(node);
+	slot = node->subSlot;
+	tupleDescriptor = node->ps.ps_ResultTupleDesc;
 
 	/*
 	 * The main logic is a simple state machine.
@@ -60,6 +63,26 @@ ExecLimit(PlanState *pstate)
 	{
 		case LIMIT_INITIAL:
 
+			if (node->limitOption == PERCENTAGE)
+			{
+
+/*
+ * In PERCENTAGE option the number of tuple to return can't
+ * be determine before receiving the last tuple. so execute
+ * outerPlan until the end and store the result tuple to avoid
+ * executing twice
+ */
+for (;;)
+{
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		break;
+	}
+	tuplestore_puttupleslot(node->tuple_store, slot);
+}
+			}
+
 			/*
 			 * First call for this node, so compute limit/offset. (We can't do
 			 * this any earlier, because parameters from upper nodes will not
@@ -87,24 +110,47 @@ ExecLimit(PlanState *pstate)
 return NULL;
 			}
 
-			/*
-			 * Fetch rows from subplan until we reach position > offset.
-			 */
-			for (;;)
+			if (node->limitOption == PERCENTAGE)
 			{
-slot = ExecProcNode(outerPlan);
-	

Re: FETCH FIRST clause PERCENT option

[Kyotaro HORIGUCHI]

Hello.

At Thu, 28 Feb 2019 21:16:25 +0100, Tomas Vondra  
wrote in 
> > One biggest issue seems to be we don't know the total number of

# One *of* the biggest *issues*?

> > outer tuples before actually reading a null tuple. I doubt of
> > general shortcut for that. It also seems preventing limit node
> > from just using materialized outer.
> > 
> 
> Sure, if you actually want all tuples, you'll have to execute the outer
> plan till completion. But that's not what I'm talking about - what if we
> only ever need to read one row from the limit?

We have no choice than once reading all tuples just to find we
are to return just one row, since estimator is not guaranteed to
be exact as required for this purpose.

> To give you a (admittedly, somewhat contrived and artificial example):
> 
> SELECT * FROM t1 WHERE id IN (
>   SELECT id FROM t2 ORDER BY x FETCH FIRST 10 PERCENT ROWS ONLY
> );
> 
> Maybe this example is bogus and/or does not really matter in practice. I
> don't know, but I've been unable to convince myself that's the case.

I see such kind of idiom common. Even in the quite simple example
above, *we* cannot tell how many tuples the inner should return
unless we actually fetch all tuples in t2. This is the same
problem with count(*).

The query is equivalent to the folloing one.

 SELECT * FROM t1 WHERE id IN (
   SELECT id FROM t2 ORDER BY x
 FETCH FIRST (SELECT ceil(count(*) * 0.1) FROM t2) ROWS ONLY
 );

This scans t2 twice, but this patch does only one full scan
moving another partial scan to tuplestore. We would win if the
outer is complex enough.

Anyway, even excluding the count(*) issue, it seems that we are
not alone in that point. (That is, at least Oracle shows
different E-Rows and A-Rows for PERCENT).

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

On 2/28/19 12:26 PM, Kyotaro HORIGUCHI wrote:
> Hello.
> 
> At Sat, 23 Feb 2019 22:27:44 +0100, Tomas Vondra 
>  wrote in 
> <81a5c0e9-c17d-28f3-4647-8a4659cdf...@2ndquadrant.com>
>>
>>
>> On 2/23/19 8:53 AM, Surafel Temesgen wrote:
>>>
>>>
>>> On Sun, Feb 10, 2019 at 2:22 AM Tomas Vondra
>>> mailto:tomas.von...@2ndquadrant.com>> wrote:
>>>  
>>>
>>>
>>> I'm not sure I understand - are you saying every time the user does a
>>> FETCH, we have to run the outer plan from scratch? I don't see why would
>>> that be necessary? And if it is, how come there's no noticeable
>>> performance difference?
>>>
>>> Can you share a patch implementing the incremental approach, and a query
>>> demonstrating the issue?
>>>
>>>
>>> I didn't implement it but its obvious that it doesn't work similarly
>>> with previous approach.
>>>
>>
>> Sure, but that's hardly a sufficient argument for the current approach.
>>
>>> We need different implementation and my plan was to use tuplestore per
>>> call and clear
>>>
>>> it after returning tuple but I see that the plan will not go far because
>>> mainly the last returned
>>>
>>> slot is not the last slot we get from outerPlan execution
>>>
>>
>> I'm sorry, I still don't understand what the supposed problem is. I
>> don't think it's all that different from what nodeMaterial.c does, for
>> example.
>>
>> As I explained before, having to execute the outer plan till completion
>> before returning any tuples is an issue. So either it needs fixing or an
>> explanation why it's not an issue.
> 
> One biggest issue seems to be we don't know the total number of
> outer tuples before actually reading a null tuple. I doubt of
> general shortcut for that. It also seems preventing limit node
> from just using materialized outer.
> 

Sure, if you actually want all tuples, you'll have to execute the outer
plan till completion. But that's not what I'm talking about - what if we
only ever need to read one row from the limit?

To give you a (admittedly, somewhat contrived and artificial example):

SELECT * FROM t1 WHERE id IN (
  SELECT id FROM t2 ORDER BY x FETCH FIRST 10 PERCENT ROWS ONLY
);

Maybe this example is bogus and/or does not really matter in practice. I
don't know, but I've been unable to convince myself that's the case.


regards


-- 
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: FETCH FIRST clause PERCENT option

[Kyotaro HORIGUCHI]

At Thu, 28 Feb 2019 13:32:17 +0300, Surafel Temesgen  
wrote in 
> On Sun, Feb 24, 2019 at 12:27 AM Tomas Vondra 
> wrote:
> 
> >
> > I'm sorry, I still don't understand what the supposed problem is. I
> > don't think it's all that different from what nodeMaterial.c does, for
> > example.
> >
> >
> sorry for the noise .Attache is complete patch for incremental approach

Mmm. It seems just moving
reading-all-before-return-the-first-tuple from LIMIT_INITIAL to
LIMIT_RESCAN. If I read it correctly node->conut is not correct
since it is calculated as "the percentage of the number of tuples
read *so far* excluding the offset portion.).


|  while (node->position - node->offset >= node->count)
|  {
| slot = ExecProcNode(outerPlan);
| (if Tupleis Null then break;)
| tuplestore_puttupleslot(node->tuple_store, slot);
| cnt = tuplestore_tuple_count(node->tuple_store);
| node->count = ceil(node->percent * cnt / 100.0);
|  }

If I'm missing nothing, this seems almost same with the follows.

  while ((position - offset) * percent / 100 >=
 (position - offset) * percent / 100) {}


In the first place, the patch breaks nodeLimit.c and build fails.


-* previous time we got a different result.
+* previous time we got a different result.In PERCENTAGE option there 
are 
+* no bound on the number of output tuples */
 */


regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

Re: FETCH FIRST clause PERCENT option

[Kyotaro HORIGUCHI]

Hello.

At Sat, 23 Feb 2019 22:27:44 +0100, Tomas Vondra  
wrote in <81a5c0e9-c17d-28f3-4647-8a4659cdf...@2ndquadrant.com>
> 
> 
> On 2/23/19 8:53 AM, Surafel Temesgen wrote:
> > 
> > 
> > On Sun, Feb 10, 2019 at 2:22 AM Tomas Vondra
> > mailto:tomas.von...@2ndquadrant.com>> wrote:
> >  
> > 
> > 
> > I'm not sure I understand - are you saying every time the user does a
> > FETCH, we have to run the outer plan from scratch? I don't see why would
> > that be necessary? And if it is, how come there's no noticeable
> > performance difference?
> > 
> > Can you share a patch implementing the incremental approach, and a query
> > demonstrating the issue?
> > 
> > 
> > I didn't implement it but its obvious that it doesn't work similarly
> > with previous approach.
> > 
> 
> Sure, but that's hardly a sufficient argument for the current approach.
> 
> > We need different implementation and my plan was to use tuplestore per
> > call and clear
> > 
> > it after returning tuple but I see that the plan will not go far because
> > mainly the last returned
> > 
> > slot is not the last slot we get from outerPlan execution
> > 
> 
> I'm sorry, I still don't understand what the supposed problem is. I
> don't think it's all that different from what nodeMaterial.c does, for
> example.
> 
> As I explained before, having to execute the outer plan till completion
> before returning any tuples is an issue. So either it needs fixing or an
> explanation why it's not an issue.

One biggest issue seems to be we don't know the total number of
outer tuples before actually reading a null tuple. I doubt of
general shortcut for that. It also seems preventing limit node
from just using materialized outer.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Sun, Feb 24, 2019 at 12:27 AM Tomas Vondra 
wrote:

>
> I'm sorry, I still don't understand what the supposed problem is. I
> don't think it's all that different from what nodeMaterial.c does, for
> example.
>
>
sorry for the noise .Attache is complete patch for incremental approach

regards
Surafel
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 06d611b64c..e3ce4d7e36 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1430,7 +1430,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY
 
 In this syntax, the start
 or count value is required by
@@ -1440,7 +1440,8 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
-ROW
+with PERCENT count specifies the maximum number of rows to return 
+in percentage.ROW
 and ROWS as well as FIRST
 and NEXT are noise words that don't influence
 the effects of these clauses.
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index baa669abe8..ca957901e3 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -21,6 +21,8 @@
 
 #include "postgres.h"
 
+#include 
+
 #include "executor/executor.h"
 #include "executor/nodeLimit.h"
 #include "miscadmin.h"
@@ -44,6 +46,9 @@ ExecLimit(PlanState *pstate)
 	ScanDirection direction;
 	TupleTableSlot *slot;
 	PlanState  *outerPlan;
+	TupleDesc   tupleDescriptor;
+	slot = node->subSlot;
+	tupleDescriptor = node->ps.ps_ResultTupleDesc;
 
 	CHECK_FOR_INTERRUPTS();
 
@@ -81,7 +86,15 @@ ExecLimit(PlanState *pstate)
 			/*
 			 * Check for empty window; if so, treat like empty subplan.
 			 */
-			if (node->count <= 0 && !node->noCount)
+			if (node->limitOption == PERCENTAGE)
+			{
+if (node->percent == 0.0)
+{
+	node->lstate = LIMIT_EMPTY;
+	return NULL;
+}
+			}
+			else if (node->count <= 0 && !node->noCount)
 			{
 node->lstate = LIMIT_EMPTY;
 return NULL;
@@ -107,6 +120,15 @@ ExecLimit(PlanState *pstate)
 	break;
 			}
 
+			/*
+			 * We may needed this tuple in backward scan so put it into tuplestore.
+			 */
+			if (node->limitOption == PERCENTAGE)
+			{
+tuplestore_puttupleslot(node->tuple_store, slot);
+tuplestore_advance(node->tuple_store, true);
+			}
+
 			/*
 			 * Okay, we have the first tuple of the window.
 			 */
@@ -130,6 +152,72 @@ ExecLimit(PlanState *pstate)
  * advancing the subplan or the position variable; but change
  * the state machine state to record having done so.
  */
+
+/*
+ * In case of coming back from backward scan the tuple is already
+ * in tuple store.
+ */
+if (node->limitOption == PERCENTAGE && node->backwardPosition > 0)
+{
+	slot = MakeSingleTupleTableSlot(tupleDescriptor, &TTSOpsMinimalTuple);
+	if (tuplestore_gettupleslot(node->tuple_store, true, false, slot))
+	{
+		node->subSlot = slot;
+		node->position++;
+		node->backwardPosition--;
+		return slot;
+	}
+	else
+	{
+		node->lstate = LIMIT_SUBPLANEOF;
+		return NULL;
+	}
+}
+
+/*
+ * In PERCENTAGE case no need of executing outerPlan multiple times.
+ */
+if (node->limitOption ==  PERCENTAGE && node->reachEnd)
+{
+	node->lstate = LIMIT_WINDOWEND;
+
+	/*
+	 * If we know we won't need to back up, we can release
+	 * resources at this point.
+	 */
+	if (!(node->ps.state->es_top_eflags & EXEC_FLAG_BACKWARD))
+		(void) ExecShutdownNode(outerPlan);
+
+	return NULL;
+}
+
+/*
+ * When in percentage mode, we need to see if we can get any
+ * additional rows from the subplan (enough to increase the
+ * node->count value).
+ */
+if (node->limitOption == PERCENTAGE)
+{
+	/* loop until the node->count increments */
+	while (node->position - node->offset >= node->count)
+	{
+		int64	cnt;
+
+		slot = ExecProcNode(outerPlan);
+		if (TupIsNull(slot))
+		{
+			node->reachEnd = true;
+			break;
+		}
+
+		tuplestore_puttupleslot(node->tuple_store, slot);
+
+		cnt = tuplestore_tuple_count(node->tuple_store);
+
+		node->count = ceil(node->percent * cnt / 100.0);
+	}
+}
+
 if (!node->noCount &&
 	node->position - node->offs

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

On 2/23/19 8:53 AM, Surafel Temesgen wrote:
> 
> 
> On Sun, Feb 10, 2019 at 2:22 AM Tomas Vondra
> mailto:tomas.von...@2ndquadrant.com>> wrote:
>  
> 
> 
> I'm not sure I understand - are you saying every time the user does a
> FETCH, we have to run the outer plan from scratch? I don't see why would
> that be necessary? And if it is, how come there's no noticeable
> performance difference?
> 
> Can you share a patch implementing the incremental approach, and a query
> demonstrating the issue?
> 
> 
> I didn't implement it but its obvious that it doesn't work similarly
> with previous approach.
> 

Sure, but that's hardly a sufficient argument for the current approach.

> We need different implementation and my plan was to use tuplestore per
> call and clear
> 
> it after returning tuple but I see that the plan will not go far because
> mainly the last returned
> 
> slot is not the last slot we get from outerPlan execution
> 

I'm sorry, I still don't understand what the supposed problem is. I
don't think it's all that different from what nodeMaterial.c does, for
example.

As I explained before, having to execute the outer plan till completion
before returning any tuples is an issue. So either it needs fixing or an
explanation why it's not an issue.

regards

-- 
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Sun, Feb 10, 2019 at 2:22 AM Tomas Vondra 
wrote:


>
> I'm not sure I understand - are you saying every time the user does a
> FETCH, we have to run the outer plan from scratch? I don't see why would
> that be necessary? And if it is, how come there's no noticeable
> performance difference?
>
> Can you share a patch implementing the incremental approach, and a query
> demonstrating the issue?
>
>
I didn't implement it but its obvious that it doesn't work similarly with
previous approach.

We need different implementation and my plan was to use tuplestore per call
and clear

it after returning tuple but I see that the plan will not go far because
mainly the last returned

slot is not the last slot we get from outerPlan execution


regards

Surafel

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

On 2/1/19 12:10 PM, Surafel Temesgen wrote:
> here is a rebased version of  previous  patch with planner
> comment included
> 

It's really hard to say which comment is that ..

FWIW I find the changes in nodeLimit lacking sufficient comments. The
comments present are somewhat obvious - what we need are comments
explaining why things happen. For example the LIMIT_INITIAL now includes
this chunk of code:

case LIMIT_INITIAL:

if (node->limitOption == PERCENTAGE)
{

/*
 * Find all rows the plan will return.
 */
for (;;)
{
slot = ExecProcNode(outerPlan);
if (TupIsNull(slot))
{
break;
}
tuplestore_puttupleslot(node->totalTuple, slot);
}
}

Ignoring the fact that the comment is incorrectly indented, it states a
rather obvious fact - that we fetch all rows from a plan and stash them
into a tuplestore. What is missing is comment explaining why we need to
do that.

This applies to other changes in nodeLimit too, and elsewhere.

Another detail is that we generally leave a free line before "if", i.e.
instead of

}
if (node->limitOption == PERCENTAGE)
{

it should be

}

if (node->limitOption == PERCENTAGE)
{

because it's fairly easy to misread as "else if". Even better, there
should be a comment before the "if" explaining what the branch does.

regards

-- 
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

On 1/30/19 7:07 AM, Surafel Temesgen wrote:
> 
> 
> On Mon, Jan 28, 2019 at 1:28 AM Tomas Vondra
> mailto:tomas.von...@2ndquadrant.com>> wrote:
> 
> 
> 
> OK. Does that mean you agree the incremental approach is reasonable?
> 
> 
> there are no noticeable performance difference but i love previous
> approach more regarding cursor operation it fetch tuple forward and
> backward from tuplestore only but in incremental approach we have to 
> re execute outer node in every forward and backward fetching operation
> 

I'm not sure I understand - are you saying every time the user does a
FETCH, we have to run the outer plan from scratch? I don't see why would
that be necessary? And if it is, how come there's no noticeable
performance difference?

Can you share a patch implementing the incremental approach, and a query
demonstrating the issue?

regards

-- 
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

here is a rebased version of  previous  patch with planner
comment included

regards
Surafel


On Wed, Jan 30, 2019 at 9:07 AM Surafel Temesgen 
wrote:

>
>
> On Mon, Jan 28, 2019 at 1:28 AM Tomas Vondra 
> wrote:
>
>>
>>
>> OK. Does that mean you agree the incremental approach is reasonable?
>>
>
> there are no noticeable performance difference but i love previous
> approach more regarding cursor operation it fetch tuple forward and
> backward from tuplestore only but in incremental approach we have to
> re execute outer node in every forward and backward fetching operation
>
> regards
> Surafel
>
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 4db8142afa..8491b7831a 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1397,7 +1397,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY
 
 In this syntax, the start
 or count value is required by
@@ -1407,7 +1407,8 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
-ROW
+with PERCENT count specifies the maximum number of rows to return 
+in percentage.ROW
 and ROWS as well as FIRST
 and NEXT are noise words that don't influence
 the effects of these clauses.
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index baa669abe8..5ba895362d 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -43,6 +43,7 @@ ExecLimit(PlanState *pstate)
 	LimitState *node = castNode(LimitState, pstate);
 	ScanDirection direction;
 	TupleTableSlot *slot;
+	TupleDesc   tupleDescriptor;
 	PlanState  *outerPlan;
 
 	CHECK_FOR_INTERRUPTS();
@@ -52,6 +53,8 @@ ExecLimit(PlanState *pstate)
 	 */
 	direction = node->ps.state->es_direction;
 	outerPlan = outerPlanState(node);
+	slot = node->subSlot;
+	tupleDescriptor = node->ps.ps_ResultTupleDesc;
 
 	/*
 	 * The main logic is a simple state machine.
@@ -60,6 +63,23 @@ ExecLimit(PlanState *pstate)
 	{
 		case LIMIT_INITIAL:
 
+			if (node->limitOption == PERCENTAGE)
+			{
+
+			/*
+			 * Find all rows the plan will return.
+			 */
+for (;;)
+{
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		break;
+	}
+	tuplestore_puttupleslot(node->totalTuple, slot);
+}
+			}
+
 			/*
 			 * First call for this node, so compute limit/offset. (We can't do
 			 * this any earlier, because parameters from upper nodes will not
@@ -87,24 +107,46 @@ ExecLimit(PlanState *pstate)
 return NULL;
 			}
 
-			/*
-			 * Fetch rows from subplan until we reach position > offset.
-			 */
-			for (;;)
+			if (node->limitOption == PERCENTAGE)
 			{
-slot = ExecProcNode(outerPlan);
-if (TupIsNull(slot))
+for (;;)
 {
-	/*
-	 * The subplan returns too few tuples for us to produce
-	 * any output at all.
-	 */
-	node->lstate = LIMIT_EMPTY;
-	return NULL;
+	slot = MakeSingleTupleTableSlot(tupleDescriptor, &TTSOpsMinimalTuple);
+	if (!tuplestore_gettupleslot(node->totalTuple, true, true, slot))
+	{
+		node->lstate = LIMIT_EMPTY;
+		return NULL;
+	}
+	else
+	{
+		node->subSlot = slot;
+		if (++node->position > node->offset)
+		break;
+	}
+}
+			}
+			else if (node->limitOption == EXACT_NUMBER)
+			{
+
+/*
+ * Fetch rows from subplan until we reach position > offset.
+ */
+for (;;)
+{
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		/*
+		 * The subplan returns too few tuples for us to produce
+		 * any output at all.
+		 */
+		node->lstate = LIMIT_EMPTY;
+		return NULL;
+	}
+	node->subSlot = slot;
+	if (++node->position > node->offset)
+		break;
 }
-node->subSlot = slot;
-if (++node->position > node->offset)
-	break;
 			}
 
 			/*
@@ -144,18 +186,35 @@ ExecLimit(PlanState *pstate)
 
 	return NULL;
 }
+if (node->limitOption == PERCENTAGE)
+{
+	slot = MakeSingleTupleTableSlot(tupleDescriptor, &TTSOpsMinimalTuple);
+	if (tuplestore_gettupleslot(node->totalTuple, true, false, slot))
+	{
+		node->subSlot = slot;
+		node->position++;
+	}
+	else
+	{
+		node->lstate = LIMIT_SUBPLANEOF;
+		return NULL;
+	}
+}
+	

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Mon, Jan 28, 2019 at 1:28 AM Tomas Vondra 
wrote:

>
>
> OK. Does that mean you agree the incremental approach is reasonable?
>

there are no noticeable performance difference but i love previous
approach more regarding cursor operation it fetch tuple forward and
backward from tuplestore only but in incremental approach we have to
re execute outer node in every forward and backward fetching operation

regards
Surafel

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

On 1/24/19 10:57 AM, Surafel Temesgen wrote:
> 
> 
> On Wed, Jan 9, 2019 at 8:18 PM Tomas Vondra
> mailto:tomas.von...@2ndquadrant.com>> wrote:
>  
> 
> 
> See the attached patch, which recomputes the count regularly. I don't
> claim the patch is committable or that it has no other bugs, but
> hopefully it shows what I meant.
> 
> 
> 
> I got only one issue it is not work well with cursor
> 
> postgres=# START TRANSACTION;
> 
> START TRANSACTION
> 
> postgres=# create table t as select i from generate_series(1,1000) s(i);
> 
> SELECT 1000
> 
> postgres=# declare c cursor for select * from t fetch first 5 percent
> rows only;
> 
> DECLARE CURSOR
> 
> postgres=# fetch all in c;
> 
> ERROR: trying to store a minimal tuple into wrong type of slot
> 
> 
> I am looking at it .
> 

OK. Does that mean you agree the incremental approach is reasonable?

> meanwhile i fix row estimation and cost and make create_ordered_paths
> creation with no LIMIT consideration in PERCENTAGE case
> 

I haven't reviewed the costing code yet, but linear interpolation seems
reasonable in principle. I find the create_limit_path changes somewhat
sloppy and difficult to comprehend (particularly without comments).

For example, the fact that the code computes the total cost based on
count_rows, which may be tweaked later seems suspicious. I mean, the doe
now does this:

if (limitOption = PERCENTAGE)
{
... update count_rows
... compute total_cost
}

if (count_rows > pathnode->path.rows)
count_rows = pathnode->path.rows;

... compute total_cost for the EXACT_NUMBER case

pathnode->path.rows = count_rows;

But I need to do think about this a bit more.

FWIW I see there are no regression tests for the PERCENT option.


regards

-- 
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Wed, Jan 9, 2019 at 8:18 PM Tomas Vondra 
wrote:


>
> See the attached patch, which recomputes the count regularly. I don't
> claim the patch is committable or that it has no other bugs, but
> hopefully it shows what I meant.
>
>

I got only one issue it is not work well with cursor

postgres=# START TRANSACTION;

START TRANSACTION

postgres=# create table t as select i from generate_series(1,1000) s(i);

SELECT 1000

postgres=# declare c cursor for select * from t fetch first 5 percent rows
only;

DECLARE CURSOR

postgres=# fetch all in c;

ERROR: trying to store a minimal tuple into wrong type of slot


I am looking at it .

meanwhile i fix row estimation and cost and make create_ordered_paths
creation with no LIMIT consideration in PERCENTAGE case


regards

Surafel
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 4db8142afa..8491b7831a 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1397,7 +1397,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY
 
 In this syntax, the start
 or count value is required by
@@ -1407,7 +1407,8 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
-ROW
+with PERCENT count specifies the maximum number of rows to return 
+in percentage.ROW
 and ROWS as well as FIRST
 and NEXT are noise words that don't influence
 the effects of these clauses.
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index baa669abe8..8fc70b1b5f 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -21,6 +21,8 @@
 
 #include "postgres.h"
 
+#include 
+
 #include "executor/executor.h"
 #include "executor/nodeLimit.h"
 #include "miscadmin.h"
@@ -44,6 +46,7 @@ ExecLimit(PlanState *pstate)
 	ScanDirection direction;
 	TupleTableSlot *slot;
 	PlanState  *outerPlan;
+	slot = node->subSlot;
 
 	CHECK_FOR_INTERRUPTS();
 
@@ -81,7 +84,15 @@ ExecLimit(PlanState *pstate)
 			/*
 			 * Check for empty window; if so, treat like empty subplan.
 			 */
-			if (node->count <= 0 && !node->noCount)
+			if (node->limitOption == PERCENTAGE)
+			{
+if (node->percent == 0.0)
+{
+	node->lstate = LIMIT_EMPTY;
+	return NULL;
+}
+			}
+			else if (node->count <= 0 && !node->noCount)
 			{
 node->lstate = LIMIT_EMPTY;
 return NULL;
@@ -122,6 +133,7 @@ ExecLimit(PlanState *pstate)
 			return NULL;
 
 		case LIMIT_INWINDOW:
+
 			if (ScanDirectionIsForward(direction))
 			{
 /*
@@ -130,6 +142,32 @@ ExecLimit(PlanState *pstate)
  * advancing the subplan or the position variable; but change
  * the state machine state to record having done so.
  */
+
+/*
+ * When in percentage mode, we need to see if we can get any
+ * additional rows from the subplan (enough to increase the
+ * node->count value).
+ */
+if (node->limitOption == PERCENTAGE)
+{
+	/* loop until the node->count increments */
+	while (node->position - node->offset >= node->count)
+	{
+		int64	cnt;
+
+		slot = ExecProcNode(outerPlan);
+		if (TupIsNull(slot))
+			break;
+
+		tuplestore_puttupleslot(node->totalTuple, slot);
+
+		/* plus 1, because the first tuple is returned from LIMIT_RESCAN */
+		cnt = 1 + tuplestore_tuple_count(node->totalTuple);
+
+		node->count = ceil(node->percent * cnt / 100.0);
+	}
+}
+
 if (!node->noCount &&
 	node->position - node->offset >= node->count)
 {
@@ -145,6 +183,20 @@ ExecLimit(PlanState *pstate)
 	return NULL;
 }
 
+if (node->limitOption == PERCENTAGE)
+{
+	while (node->position - node->offset < node->count)
+	{
+		if (tuplestore_gettupleslot(node->totalTuple, true, true, slot))
+		{
+			node->subSlot = slot;
+			node->position++;
+		}
+	}
+}
+else if (node->limitOption == EXACT_NUMBER)
+{
+
 /*
  * Get next tuple from subplan, if any.
  */
@@ -156,6 +208,7 @@ ExecLimit(PlanState *pstate)
 }
 node->subSlot = slot;
 node->position++;
+}
 			}
 			else
 			{
@@ -278,17 +331,29 @@ recompute_limits(LimitState *node)
 		/* Interpret NULL count as no count (LIMIT ALL) */
 		if (isNull)
 		{
-			node->count = 0;
+			node->count = 1;
 			node->noCount 

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

On 1/9/19 4:43 PM, Surafel Temesgen wrote:
> 
> 
> On Wed, Jan 9, 2019 at 5:38 PM Tomas Vondra
> mailto:tomas.von...@2ndquadrant.com>> wrote:
> 
> 
> It's hard to say what exactly are you doing, because you haven't shared
> any code nor examples. 
> 
> 
> okay i attach in progress patch
> 

Yeah, that's what I thought - the patch computes

node->count = DatumGetInt64(100 / DatumGetFloat8(val));

and then always fetches this number of records before emitting the next
row from the tuplestore. That's wrong, as I explained before, because
the distance does change, due to rounding.

See the attached patch, which recomputes the count regularly. I don't
claim the patch is committable or that it has no other bugs, but
hopefully it shows what I meant.

FWIW you don't need to create any slots - the two already created are
enough. You certainly don't need to create the slots within a loop.

regards

-- 
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 4db8142afa..8491b7831a 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1397,7 +1397,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY
 
 In this syntax, the start
 or count value is required by
@@ -1407,7 +1407,8 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
-ROW
+with PERCENT count specifies the maximum number of rows to return 
+in percentage.ROW
 and ROWS as well as FIRST
 and NEXT are noise words that don't influence
 the effects of these clauses.
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index baa669abe8..87d751bdbe 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -21,6 +21,8 @@
 
 #include "postgres.h"
 
+#include 
+
 #include "executor/executor.h"
 #include "executor/nodeLimit.h"
 #include "miscadmin.h"
@@ -44,6 +46,7 @@ ExecLimit(PlanState *pstate)
 	ScanDirection direction;
 	TupleTableSlot *slot;
 	PlanState  *outerPlan;
+	slot = node->subSlot;
 
 	CHECK_FOR_INTERRUPTS();
 
@@ -81,7 +84,15 @@ ExecLimit(PlanState *pstate)
 			/*
 			 * Check for empty window; if so, treat like empty subplan.
 			 */
-			if (node->count <= 0 && !node->noCount)
+			if (node->limitOption == PERCENTAGE)
+			{
+if (node->percent == 0.0)
+{
+	node->lstate = LIMIT_EMPTY;
+	return NULL;
+}
+			}
+			else if (node->count <= 0 && !node->noCount)
 			{
 node->lstate = LIMIT_EMPTY;
 return NULL;
@@ -122,6 +133,7 @@ ExecLimit(PlanState *pstate)
 			return NULL;
 
 		case LIMIT_INWINDOW:
+
 			if (ScanDirectionIsForward(direction))
 			{
 /*
@@ -130,6 +142,32 @@ ExecLimit(PlanState *pstate)
  * advancing the subplan or the position variable; but change
  * the state machine state to record having done so.
  */
+
+/*
+ * When in percentage mode, we need to see if we can get any
+ * additional rows from the subplan (enough to increase the
+ * node->count value).
+ */
+if (node->limitOption == PERCENTAGE)
+{
+	/* loop until the node->count increments */
+	while (node->position - node->offset >= node->count)
+	{
+		int64	cnt;
+
+		slot = ExecProcNode(outerPlan);
+		if (TupIsNull(slot))
+			break;
+
+		tuplestore_puttupleslot(node->totalTuple, slot);
+
+		/* plus 1, because the first tuple is returned from LIMIT_RESCAN */
+		cnt = 1 + tuplestore_tuple_count(node->totalTuple);
+
+		node->count = ceil(node->percent * cnt / 100.0);
+	}
+}
+
 if (!node->noCount &&
 	node->position - node->offset >= node->count)
 {
@@ -145,6 +183,20 @@ ExecLimit(PlanState *pstate)
 	return NULL;
 }
 
+if (node->limitOption == PERCENTAGE)
+{
+	while (node->position - node->offset < node->count)
+	{
+		if (tuplestore_gettupleslot(node->totalTuple, true, true, slot))
+		{
+			node->subSlot = slot;
+			node->position++;
+		}
+	}
+}
+else if (node->limitOption == EXACT_NUMBER)
+{
+
 /*
  * Get next tuple from subplan, if any.
  */
@@ -156,6 +208,7 @@ ExecLimit(PlanState *pstate)
 }
 no

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Wed, Jan 9, 2019 at 5:38 PM Tomas Vondra 
wrote:

>
> It's hard to say what exactly are you doing, because you haven't shared
> any code nor examples.


okay i attach in progress patch

regards
Surafel
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 4db8142afa..8491b7831a 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1397,7 +1397,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY
 
 In this syntax, the start
 or count value is required by
@@ -1407,7 +1407,8 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
-ROW
+with PERCENT count specifies the maximum number of rows to return 
+in percentage.ROW
 and ROWS as well as FIRST
 and NEXT are noise words that don't influence
 the effects of these clauses.
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index baa669abe8..ee04992c9f 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -44,6 +44,9 @@ ExecLimit(PlanState *pstate)
 	ScanDirection direction;
 	TupleTableSlot *slot;
 	PlanState  *outerPlan;
+	TupleDesc   tupleDescriptor;
+	slot = node->subSlot;
+	tupleDescriptor = node->ps.ps_ResultTupleDesc;
 
 	CHECK_FOR_INTERRUPTS();
 
@@ -131,7 +134,7 @@ ExecLimit(PlanState *pstate)
  * the state machine state to record having done so.
  */
 if (!node->noCount &&
-	node->position - node->offset >= node->count)
+	node->position - node->offset >= node->count && node->limitOption != PERCENTAGE)
 {
 	node->lstate = LIMIT_WINDOWEND;
 
@@ -144,6 +147,42 @@ ExecLimit(PlanState *pstate)
 
 	return NULL;
 }
+if (node->limitOption == PERCENTAGE)
+{
+
+	for (int i=1; node->count >= i; i++)
+	{
+
+		slot = MakeSingleTupleTableSlot(tupleDescriptor, &TTSOpsMinimalTuple);
+
+		slot = ExecProcNode(outerPlan);
+		if (TupIsNull(slot))
+		{
+			node->lstate = LIMIT_WINDOWEND;
+
+			/*
+			 * If we know we won't need to back up, we can release
+			 * resources at this point.
+			 */
+			if (!(node->ps.state->es_top_eflags & EXEC_FLAG_BACKWARD))
+(void) ExecShutdownNode(outerPlan);
+
+			return NULL;
+		}
+		tuplestore_puttupleslot(node->totalTuple, slot);
+	}
+	slot = MakeSingleTupleTableSlot(tupleDescriptor, &TTSOpsMinimalTuple);
+	if (tuplestore_gettupleslot(node->totalTuple, true, true, slot))
+	{
+		node->subSlot = slot;
+		node->position++;
+	} else {
+		node->lstate = LIMIT_SUBPLANEOF;
+		return NULL;
+	}
+}
+else if (node->limitOption == EXACT_NUMBER)
+{
 
 /*
  * Get next tuple from subplan, if any.
@@ -156,6 +195,7 @@ ExecLimit(PlanState *pstate)
 }
 node->subSlot = slot;
 node->position++;
+}
 			}
 			else
 			{
@@ -283,12 +323,16 @@ recompute_limits(LimitState *node)
 		}
 		else
 		{
-			node->count = DatumGetInt64(val);
-			if (node->count < 0)
-ereport(ERROR,
-		(errcode(ERRCODE_INVALID_ROW_COUNT_IN_LIMIT_CLAUSE),
-		 errmsg("LIMIT must not be negative")));
-			node->noCount = false;
+			if (node->limitOption == PERCENTAGE)
+node->count = DatumGetInt64(100 / DatumGetFloat8(val));
+			else
+			{
+node->count = DatumGetInt64(val);
+if (node->count < 0)
+	ereport(ERROR,
+			(errcode(ERRCODE_INVALID_ROW_COUNT_IN_LIMIT_CLAUSE),
+			 errmsg("LIMIT must not be negative")));
+			}
 		}
 	}
 	else
@@ -374,6 +418,9 @@ ExecInitLimit(Limit *node, EState *estate, int eflags)
 		   (PlanState *) limitstate);
 	limitstate->limitCount = ExecInitExpr((Expr *) node->limitCount,
 		  (PlanState *) limitstate);
+	limitstate->limitOption = node->limitOption;
+	if (node->limitOption == PERCENTAGE)
+		limitstate->totalTuple= tuplestore_begin_heap(true, false, work_mem);
 
 	/*
 	 * Initialize result type.
@@ -405,6 +452,8 @@ ExecEndLimit(LimitState *node)
 {
 	ExecFreeExprContext(&node->ps);
 	ExecEndNode(outerPlanState(node));
+	if (node->totalTuple!= NULL)
+		tuplestore_end(node->totalTuple);
 }
 
 
@@ -424,4 +473,6 @@ ExecReScanLimit(LimitState *node)
 	 */
 	if (node->ps.lefttree->chgParam == NULL)
 		ExecReScan(node->ps.lefttree);
+	if (node->totalTuple!= NULL)
+		tupl

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

On 1/9/19 7:09 AM, Surafel Temesgen wrote:
> 
> 
> On Sun, Jan 6, 2019 at 5:51 PM Tomas Vondra
> mailto:tomas.von...@2ndquadrant.com>> wrote:
> 
> 
> On 1/6/19 12:40 PM, Surafel Temesgen wrote:
> >
> >
> > On Fri, Jan 4, 2019 at 5:27 PM Tomas Vondra
> >  
>  >> wrote:
> >
> >
> >     What formula? All the math remains exactly the same, you just
> need to
> >     update the number of rows to return and track how many rows
> are already
> >     returned.
> >
> >     I haven't tried doing that, but AFAICS you'd need to tweak
> how/when
> >     node->count is computed - instead of computing it only once it
> needs to
> >     be updated after fetching each row from the subplan.
> >
> >     Furthermore, you'll need to stash the subplan rows somewhere
> (into a
> >     tuplestore probably), and whenever the node->count value
> increments,
> >     you'll need to grab a row from the tuplestore and return that
> (i.e.
> >     tweak node->position and set node->subSlot).
> >
> >     I hope that makes sense. The one thing I'm not quite sure about is
> >     whether tuplestore allows adding and getting rows at the same
> time.
> >
> >     Does that make sense
> >
> >
> >
> > In current implementation in LIMIT_INITIAL state we execute outer
> > plan to the end , store the resulting tuples in tuplestore and
> > calculate limitCount in number .We are in this state only once and
> > did not return any tuple. Once we are at LIMIT_INWINDOW state and
> > inner node execution asking for tuple it return from tuple store
> > immediately.
> >
> 
> Right.
> 
> > Inorder to do fast startup what I was thinking was dividing the work
> > done at LIMIT_INITIAL state in to limitCount. For example we want
> > 0.5 percent of the result which means in LIMIT_INWINDOW state we
> > execute outer node 200 times ,store the result in tuplestore and
> > return the first tuple. if we don’t get that much tuple that means we
> > reach end of the limit.
> Yes, pretty much.
> 
> 
> 
> While working on this method i found an issue that did not work will
> on percent with fractional part like 2.6 percent which means we have
> to emit per every 38.4615 tuple so it have to be round up to 39 or
> round down to 38 either way it leads to incorrect result  .Even with
> integer percentage sometimes the result came out with fractional
> part and needs rounding
> 

It's hard to say what exactly are you doing, because you haven't shared
any code nor examples. But it sounds as if you're computing how often to
produce an additional row at the beginning, and then simply multiplying
it. I'm not sure that's quite possible, because there will be issues due
to rounding.

Firstly, I see your last patch (from 30/11) does this:

node->count = DatumGetInt64((DatumGetFloat8(val) *
  tuplestore_tuple_count(node->totalTuple)) / 100);

but the SQL standard I have says this in the  section:

If  is specified, then let FFP be the
 simply contained in , and let LOCT be a  whose value is OCT.
Let FFRC be the value of

CEILING ( FFP * LOCT / 100.0E0 )

That's not what you patch does, though, because it relies on automatic
float->int conversion, which does floor() and not ceil(). FWIW the final
DatumGetInt64 seems unnecessary, because the expression is float8 and
not Datum.

The patch should do something like this instead:

node->count = ceil((DatumGetFloat8(val) *
  tuplestore_tuple_count(node->totalTuple)) / 100);

Now, back to the row count issue - let's assume the user requests

FETCH FIRST 3 PERCENT ROWS ONLY

This means we should produce 1 rows every 33 rows, but only very
roughly. In reality it's a bit less, because 3% is not 1/33 exactly.
Consider this query, which computes number of rows for

select sample_cnt, l - lag(l) over (order by sample_cnt) AS d from (
select sample_cnt, min(nrows) AS l from (
  select
  nrows,
  (nrows * 3.0 / 100.0) AS raw_sample_cnt,
  ceil(nrows * 3.0 / 100.0) AS sample_cnt
  from generate_series(1,1) s(nrows)
) foo group by sample_cnt order by sample_cnt
) bar;

The inner-most query simply computes how many rows we're supposed to
return based on certain row count. Then we compute points at which the
sample_cnt actually changes. And finally we compute the distance between
those points. You should get something like

 sample_cnt | d
+
  1 |
  2 | 33
  3 | 33
  4 | 34
  5 | 33
  6 | 33
  7 | 34
  8 | 33
  9 | 33
 10 | 34
 11 | 33
 ...

This shows that the distance oscillates be

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Sun, Jan 6, 2019 at 5:51 PM Tomas Vondra 
wrote:

>
> On 1/6/19 12:40 PM, Surafel Temesgen wrote:
> >
> >
> > On Fri, Jan 4, 2019 at 5:27 PM Tomas Vondra
> > mailto:tomas.von...@2ndquadrant.com>>
> wrote:
> >
> >
> > What formula? All the math remains exactly the same, you just need to
> > update the number of rows to return and track how many rows are
> already
> > returned.
> >
> > I haven't tried doing that, but AFAICS you'd need to tweak how/when
> > node->count is computed - instead of computing it only once it needs
> to
> > be updated after fetching each row from the subplan.
> >
> > Furthermore, you'll need to stash the subplan rows somewhere (into a
> > tuplestore probably), and whenever the node->count value increments,
> > you'll need to grab a row from the tuplestore and return that (i.e.
> > tweak node->position and set node->subSlot).
> >
> > I hope that makes sense. The one thing I'm not quite sure about is
> > whether tuplestore allows adding and getting rows at the same time.
> >
> > Does that make sense
> >
> >
> >
> > In current implementation in LIMIT_INITIAL state we execute outer
> > plan to the end , store the resulting tuples in tuplestore and
> > calculate limitCount in number .We are in this state only once and
> > did not return any tuple. Once we are at LIMIT_INWINDOW state and
> > inner node execution asking for tuple it return from tuple store
> > immediately.
> >
>
> Right.
>
> > Inorder to do fast startup what I was thinking was dividing the work
> > done at LIMIT_INITIAL state in to limitCount. For example we want
> > 0.5 percent of the result which means in LIMIT_INWINDOW state we
> > execute outer node 200 times ,store the result in tuplestore and
> > return the first tuple. if we don’t get that much tuple that means we
> > reach end of the limit.
> Yes, pretty much.
>


While working on this method i found an issue that did not work will on
percent
with fractional part like 2.6 percent which means we have to emit per every
38.4615
tuple so it have to be round up to 39 or round down to 38 either way it
leads to incorrect
result  .Even with integer percentage sometimes the result came out with
fractional part
and needs rounding

regards
Surafel

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

On 1/6/19 12:40 PM, Surafel Temesgen wrote:
> 
> 
> On Fri, Jan 4, 2019 at 5:27 PM Tomas Vondra
> mailto:tomas.von...@2ndquadrant.com>> wrote:
> 
> 
> What formula? All the math remains exactly the same, you just need to
> update the number of rows to return and track how many rows are already
> returned.
> 
> I haven't tried doing that, but AFAICS you'd need to tweak how/when
> node->count is computed - instead of computing it only once it needs to
> be updated after fetching each row from the subplan.
> 
> Furthermore, you'll need to stash the subplan rows somewhere (into a
> tuplestore probably), and whenever the node->count value increments,
> you'll need to grab a row from the tuplestore and return that (i.e.
> tweak node->position and set node->subSlot).
> 
> I hope that makes sense. The one thing I'm not quite sure about is
> whether tuplestore allows adding and getting rows at the same time.
> 
> Does that make sense
> 
> 
> 
> In current implementation in LIMIT_INITIAL state we execute outer
> plan to the end , store the resulting tuples in tuplestore and
> calculate limitCount in number .We are in this state only once and
> did not return any tuple. Once we are at LIMIT_INWINDOW state and
> inner node execution asking for tuple it return from tuple store
> immediately.
> 

Right.

> Inorder to do fast startup what I was thinking was dividing the work 
> done at LIMIT_INITIAL state in to limitCount. For example we want
> 0.5 percent of the result which means in LIMIT_INWINDOW state we
> execute outer node 200 times ,store the result in tuplestore and
> return the first tuple. if we don’t get that much tuple that means we
> reach end of the limit.
Yes, pretty much.

> But I think i can’t do it in this way because percent have meaning
> only with total number so LIMIT_INITIAL work can’t be avoid.
> 

I'm not sure I understand what you mean by "percent would have meaning
only with the total number". The important thing is that you can track
how many tuples you're supposed to return (based on the current number
of rows from the outer plan) and how many you've produced so far. And
those numbers are only growing.


regards

-- 
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Fri, Jan 4, 2019 at 5:27 PM Tomas Vondra 
wrote:

>
> What formula? All the math remains exactly the same, you just need to
> update the number of rows to return and track how many rows are already
> returned.
>
> I haven't tried doing that, but AFAICS you'd need to tweak how/when
> node->count is computed - instead of computing it only once it needs to
> be updated after fetching each row from the subplan.
>
> Furthermore, you'll need to stash the subplan rows somewhere (into a
> tuplestore probably), and whenever the node->count value increments,
> you'll need to grab a row from the tuplestore and return that (i.e.
> tweak node->position and set node->subSlot).
>
> I hope that makes sense. The one thing I'm not quite sure about is
> whether tuplestore allows adding and getting rows at the same time.
>
> Does that make sense



In current implementation in LIMIT_INITIAL state we execute outer plan to
the end , store the resulting tuples in tuplestore and calculate limitCount
in number .We are in this state only once and did not return any tuple.
Once we are at LIMIT_INWINDOW state and inner node execution asking for
tuple it return from tuple store immediately.

Inorder to do fast startup what I was thinking was dividing the work done
at LIMIT_INITIAL state in to limitCount. For example we want 0.5 percent of
the result which means in LIMIT_INWINDOW state we execute outer node 200
times ,store the result in tuplestore and return the first tuple. if we
don’t get that much tuple that means we reach end of the limit . But I
think i can’t do it in this way because percent have meaning only with
total number so LIMIT_INITIAL work can’t be avoid

regards

Surafel

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

On 1/4/19 8:08 AM, Surafel Temesgen wrote:
> 
> 
> 
> On Tue, Jan 1, 2019 at 10:08 PM Tomas Vondra
> mailto:tomas.von...@2ndquadrant.com>> wrote:
> 
> The execution part of the patch seems to be working correctly, but I
> think there's an improvement - we don't need to execute the outer plan
> to completion before emitting the first row. For example, let's say the
> outer plan produces 1 rows in total and we're supposed to return the
> first 1% of those rows. We can emit the first row after fetching the
> first 100 rows, we don't have to wait for fetching all 10k rows.
> 
> 
> my other concern is in this case we are going to determine limit and
> safe row to return on the fly
> which have additional computation and i fair that it will not perform
> better on most of the case
> 

I very much doubt recomputing the number of rows to return will be
measurable at all, considering those are int64 values. It's likely
negligible compared to all the other overhead (reading rows from the
subplan, etc.).

And even if it turns out expensive, there are ways to make it less
expensive (e.g. not computing it every time, but only when it actually
can increment the value - for example with 1% rows to return, it's
enough to recompute it every 100 rows).

It also very much depends on which use cases you consider important.
Clearly, you're only thinking about the case that actually requires
fetching everything. But there's also the use case where you have
another limit node somewhere above.

So let's do a simple "cost" analysis of these two use cases:

1) case #1 (no LIMIT above, requires fetching everything)

Let's assume the "cost" of the current approach (fetching everything in
advance, computing node->count once) is the baseline here.

With the incremental approach, there likely is some extra overhead.
Let's overshoot it a bit - with 5% overhead, it's 1.05 of the baseline.

2) case #2 (LIMIT/EXISTS above, only needs to fetch the first few rows,
perhaps just a single one)

Here, we take the incremental approach as the baseline. With the "fetch
everything first" approach we need to evaluate the whole subplan and
fetch all the results, instead of just some small fraction. But the
exact ratio is pretty much arbitrary - I can easily construct queries
that need to fetch 100x or 10x more data from the subplan.

So essentially using the "incremental fetch" approach means we may pay
5% overhead in cases where we need to fetch everything, while the "fetch
everything first" approach means we'll add arbitrary amount of overhead
to cases where we don't need to fetch everything.

Of course, you one way to deal with this would be to argue the cases
benefiting from incremental approach are very rare / non-existent.
Perhaps that's the case, although I don't see why it would be.

regards

-- 
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

On 1/4/19 7:40 AM, Surafel Temesgen wrote:
> 
> 
> On Thu, Jan 3, 2019 at 4:51 PM Tomas Vondra
> mailto:tomas.von...@2ndquadrant.com>> wrote:
> 
> 
> On 1/3/19 1:00 PM, Surafel Temesgen wrote:
> > Hi
> >
> > On Tue, Jan 1, 2019 at 10:08 PM Tomas Vondra
> >  
>  >> wrote:
> 
> >     The execution part of the patch seems to be working correctly,
> but I
> >     think there's an improvement - we don't need to execute the
> outer plan
> >     to completion before emitting the first row. For example,
> let's say the
> >     outer plan produces 1 rows in total and we're supposed to
> return the
> >     first 1% of those rows. We can emit the first row after
> fetching the
> >     first 100 rows, we don't have to wait for fetching all 10k rows.
> >
> >
> > but total rows count is not given how can we determine safe to
> return row
> >
> 
> But you know how many rows were fetched from the outer plan, and this
> number only grows grows. So the number of rows returned by FETCH FIRST
> ... PERCENT also only grows. For example with 10% of rows, you know that
> once you reach 100 rows you should emit ~10 rows, with 200 rows you know
> you should emit ~20 rows, etc. So you may track how many rows we're
> supposed to return / returned so far, and emit them early.
> 
> 
> 
> 
> yes that is clear but i don't find it easy to put that in formula. may
> be someone with good mathematics will help
> 

What formula? All the math remains exactly the same, you just need to
update the number of rows to return and track how many rows are already
returned.

I haven't tried doing that, but AFAICS you'd need to tweak how/when
node->count is computed - instead of computing it only once it needs to
be updated after fetching each row from the subplan.

Furthermore, you'll need to stash the subplan rows somewhere (into a
tuplestore probably), and whenever the node->count value increments,
you'll need to grab a row from the tuplestore and return that (i.e.
tweak node->position and set node->subSlot).

I hope that makes sense. The one thing I'm not quite sure about is
whether tuplestore allows adding and getting rows at the same time.

Does that make sense?

-- 
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Tue, Jan 1, 2019 at 10:08 PM Tomas Vondra 
wrote:

> The execution part of the patch seems to be working correctly, but I
> think there's an improvement - we don't need to execute the outer plan
> to completion before emitting the first row. For example, let's say the
> outer plan produces 1 rows in total and we're supposed to return the
> first 1% of those rows. We can emit the first row after fetching the
> first 100 rows, we don't have to wait for fetching all 10k rows.
>
>
my other concern is in this case we are going to determine limit and safe
row to return on the fly
which have additional computation and i fair that it will not perform
better on most of the case

regards
Surafel

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Thu, Jan 3, 2019 at 4:51 PM Tomas Vondra 
wrote:

>
> On 1/3/19 1:00 PM, Surafel Temesgen wrote:
> > Hi
> >
> > On Tue, Jan 1, 2019 at 10:08 PM Tomas Vondra
> > mailto:tomas.von...@2ndquadrant.com>>
> wrote:
>
> > The execution part of the patch seems to be working correctly, but I
> > think there's an improvement - we don't need to execute the outer
> plan
> > to completion before emitting the first row. For example, let's say
> the
> > outer plan produces 1 rows in total and we're supposed to return
> the
> > first 1% of those rows. We can emit the first row after fetching the
> > first 100 rows, we don't have to wait for fetching all 10k rows.
> >
> >
> > but total rows count is not given how can we determine safe to return row
> >
>
> But you know how many rows were fetched from the outer plan, and this
> number only grows grows. So the number of rows returned by FETCH FIRST
> ... PERCENT also only grows. For example with 10% of rows, you know that
> once you reach 100 rows you should emit ~10 rows, with 200 rows you know
> you should emit ~20 rows, etc. So you may track how many rows we're
> supposed to return / returned so far, and emit them early.
>
>
>

yes that is clear but i don't find it easy to put that in formula. may be
someone with good mathematics will help

regards
Surafel

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

On 1/3/19 1:00 PM, Surafel Temesgen wrote:
> Hi
> 
> On Tue, Jan 1, 2019 at 10:08 PM Tomas Vondra
> mailto:tomas.von...@2ndquadrant.com>> wrote:
> 
> ...
> 
> Firstly, the estimated number of rows should be about the same (10k) in
> both cases, but the "percent" case incorrectly uses the total row count
> (i.e. as if 100% rows were returned).
> 
> Ok I will correct it
> 
> 
> It's correct that the total cost for the "percent" case is based on 100%
> of rows, of course, because the code has to fetch everything and stash
> it into the tuplestore in LIMIT_INITIAL phase.
> 
> But that implies the startup cost for the "percent" case can't be 0,
> because all this work has to be done before emitting the first row.
> 
> 
> Correct, startup cost must be equal to total cost of source data path
> and total cost have to be slightly greater than startup cost . I am
> planing to increase the total cost by limitCount * 0.1(similar to the
> parallel_tuple_cost) because getting tuple from tuplestore are almost
> similar operation to passing a tuple from worker to master backend.
> 

OK, sounds good in principle.

> 
> So these costing aspects need fixing, IMHO.
> 
> 
> The execution part of the patch seems to be working correctly, but I
> think there's an improvement - we don't need to execute the outer plan
> to completion before emitting the first row. For example, let's say the
> outer plan produces 1 rows in total and we're supposed to return the
> first 1% of those rows. We can emit the first row after fetching the
> first 100 rows, we don't have to wait for fetching all 10k rows.
> 
> 
> but total rows count is not given how can we determine safe to return row
> 

But you know how many rows were fetched from the outer plan, and this
number only grows grows. So the number of rows returned by FETCH FIRST
... PERCENT also only grows. For example with 10% of rows, you know that
once you reach 100 rows you should emit ~10 rows, with 200 rows you know
you should emit ~20 rows, etc. So you may track how many rows we're
supposed to return / returned so far, and emit them early.

regards

-- 
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

Hi

On Tue, Jan 1, 2019 at 10:08 PM Tomas Vondra 
wrote:

> Hi,
>
> I've been looking at this patch today, and I think there's a couple of
> issues with the costing and execution. Consider a simple table with 1M
> rows:
>
>   create table t as select i from generate_series(1,100) s(i);
>
> and these two queries, that both select 1% of rows
>
>   select * from t fetch first 1 rows only;
>
>   select * from t fetch first 1 percent rows only;
>
> which are costed like this
>
>   test=# explain select * from t fetch first 1 rows only;
>  QUERY PLAN
>   -
>Limit  (cost=0.00..144.25 rows=1 width=4)
>  ->  Seq Scan on t  (cost=0.00..14425.00 rows=100 width=4)
>   (2 rows)
>
>   test=# explain select * from t fetch first 1 percent rows only;
>  QUERY PLAN
>   -
>Limit  (cost=0.00..14425.00 rows=100 width=4)
>  ->  Seq Scan on t  (cost=0.00..14425.00 rows=100 width=4)
>   (2 rows)
>
> There are two issues with the costs in the "percent" case.
>
> Firstly, the estimated number of rows should be about the same (10k) in
> both cases, but the "percent" case incorrectly uses the total row count
> (i.e. as if 100% rows were returned).
>
> Ok I will correct it


It's correct that the total cost for the "percent" case is based on 100%
> of rows, of course, because the code has to fetch everything and stash
> it into the tuplestore in LIMIT_INITIAL phase.
>
> But that implies the startup cost for the "percent" case can't be 0,
> because all this work has to be done before emitting the first row.
>
>
Correct, startup cost must be equal to total cost of source data path and
total cost have to be slightly greater than startup cost . I am planing to
increase the total cost by limitCount * 0.1(similar to the
parallel_tuple_cost) because getting tuple from tuplestore are almost
similar operation to passing a tuple from worker to master backend.


> So these costing aspects need fixing, IMHO.
>
>
> The execution part of the patch seems to be working correctly, but I
> think there's an improvement - we don't need to execute the outer plan
> to completion before emitting the first row. For example, let's say the
> outer plan produces 1 rows in total and we're supposed to return the
> first 1% of those rows. We can emit the first row after fetching the
> first 100 rows, we don't have to wait for fetching all 10k rows.
>
>
but total rows count is not given how can we determine safe to return row

Regards
Surafel

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

Hi,

I've been looking at this patch today, and I think there's a couple of
issues with the costing and execution. Consider a simple table with 1M rows:

  create table t as select i from generate_series(1,100) s(i);

and these two queries, that both select 1% of rows

  select * from t fetch first 1 rows only;

  select * from t fetch first 1 percent rows only;

which are costed like this

  test=# explain select * from t fetch first 1 rows only;
 QUERY PLAN
  -
   Limit  (cost=0.00..144.25 rows=1 width=4)
 ->  Seq Scan on t  (cost=0.00..14425.00 rows=100 width=4)
  (2 rows)

  test=# explain select * from t fetch first 1 percent rows only;
 QUERY PLAN
  -
   Limit  (cost=0.00..14425.00 rows=100 width=4)
 ->  Seq Scan on t  (cost=0.00..14425.00 rows=100 width=4)
  (2 rows)

There are two issues with the costs in the "percent" case.

Firstly, the estimated number of rows should be about the same (10k) in
both cases, but the "percent" case incorrectly uses the total row count
(i.e. as if 100% rows were returned).

It's correct that the total cost for the "percent" case is based on 100%
of rows, of course, because the code has to fetch everything and stash
it into the tuplestore in LIMIT_INITIAL phase.

But that implies the startup cost for the "percent" case can't be 0,
because all this work has to be done before emitting the first row.

So these costing aspects need fixing, IMHO.


The execution part of the patch seems to be working correctly, but I
think there's an improvement - we don't need to execute the outer plan
to completion before emitting the first row. For example, let's say the
outer plan produces 1 rows in total and we're supposed to return the
first 1% of those rows. We can emit the first row after fetching the
first 100 rows, we don't have to wait for fetching all 10k rows.

This may seem pointless at first because we eventually have to scan all
the rows anyway (as we don't know we're done otherwise). But it may be
nested in another early-terminating node in which case it will matter,
e.g. like this:

  SELECT * FROM t1 WHERE EXISTS (
SELECT * FROM t2 FETCH FIRST 1 PERCENT ROWS ONLY
  ) foo;

I admit this is example is somewhat artificial, but it's pretty easy to
end with queries like this with views etc. Overall the fast startup
seems like a quite valuable feature and we should try keeping it.

regards

-- 
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Fri, Nov 30, 2018 at 10:23 AM Surafel Temesgen 
wrote:


> Attach is a patch that include a fix for it
>
>
By mistake the patch coerce OFFSET clause to float8 too which is not
necessary .
The attached patch correct it
regards
Surafel
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 4db8142afa..8491b7831a 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1397,7 +1397,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY
 
 In this syntax, the start
 or count value is required by
@@ -1407,7 +1407,8 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
-ROW
+with PERCENT count specifies the maximum number of rows to return 
+in percentage.ROW
 and ROWS as well as FIRST
 and NEXT are noise words that don't influence
 the effects of these clauses.
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index 6792f9e86c..ddc11bf6ec 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -43,6 +43,7 @@ ExecLimit(PlanState *pstate)
 	LimitState *node = castNode(LimitState, pstate);
 	ScanDirection direction;
 	TupleTableSlot *slot;
+	TupleDesc   tupleDescriptor;
 	PlanState  *outerPlan;
 
 	CHECK_FOR_INTERRUPTS();
@@ -52,6 +53,8 @@ ExecLimit(PlanState *pstate)
 	 */
 	direction = node->ps.state->es_direction;
 	outerPlan = outerPlanState(node);
+	slot = node->subSlot;
+	tupleDescriptor = node->ps.ps_ResultTupleDesc;
 
 	/*
 	 * The main logic is a simple state machine.
@@ -60,6 +63,23 @@ ExecLimit(PlanState *pstate)
 	{
 		case LIMIT_INITIAL:
 
+			if (node->limitOption == PERCENTAGE)
+			{
+
+			/*
+			 * Find all rows the plan will return.
+			 */
+for (;;)
+{
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		break;
+	}
+	tuplestore_puttupleslot(node->totalTuple, slot);
+}
+			}
+
 			/*
 			 * First call for this node, so compute limit/offset. (We can't do
 			 * this any earlier, because parameters from upper nodes will not
@@ -87,24 +107,46 @@ ExecLimit(PlanState *pstate)
 return NULL;
 			}
 
-			/*
-			 * Fetch rows from subplan until we reach position > offset.
-			 */
-			for (;;)
+			if (node->limitOption == PERCENTAGE)
 			{
-slot = ExecProcNode(outerPlan);
-if (TupIsNull(slot))
+for (;;)
 {
-	/*
-	 * The subplan returns too few tuples for us to produce
-	 * any output at all.
-	 */
-	node->lstate = LIMIT_EMPTY;
-	return NULL;
+	slot = MakeSingleTupleTableSlot(tupleDescriptor, &TTSOpsMinimalTuple);
+	if (!tuplestore_gettupleslot(node->totalTuple, true, true, slot))
+	{
+		node->lstate = LIMIT_EMPTY;
+		return NULL;
+	}
+	else
+	{
+		node->subSlot = slot;
+		if (++node->position > node->offset)
+		break;
+	}
+}
+			}
+			else if (node->limitOption == EXACT_NUMBER)
+			{
+
+/*
+ * Fetch rows from subplan until we reach position > offset.
+ */
+for (;;)
+{
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		/*
+		 * The subplan returns too few tuples for us to produce
+		 * any output at all.
+		 */
+		node->lstate = LIMIT_EMPTY;
+		return NULL;
+	}
+	node->subSlot = slot;
+	if (++node->position > node->offset)
+		break;
 }
-node->subSlot = slot;
-if (++node->position > node->offset)
-	break;
 			}
 
 			/*
@@ -144,18 +186,34 @@ ExecLimit(PlanState *pstate)
 
 	return NULL;
 }
+if (node->limitOption == PERCENTAGE)
+{
+	if (tuplestore_gettupleslot(node->totalTuple, true, false, slot))
+	{
+		node->subSlot = slot;
+		node->position++;
+	}
+	else
+	{
+		node->lstate = LIMIT_SUBPLANEOF;
+		return NULL;
+	}
+}
+else if (node->limitOption == EXACT_NUMBER)
+{
 
-/*
- * Get next tuple from subplan, if any.
- */
-slot = ExecProcNode(outerPlan);
-if (TupIsNull(slot))
-{
-	node->lstate = LIMIT_SUBPLANEOF;
-	return NULL;
+	 /*
+	 * Get next tuple from subplan, if any.
+	 */
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		node->lstate = LIMIT_SUBPLANEOF;
+		return NULL;
+

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Sun, Nov 25, 2018 at 3:05 PM Vik Fearing 
wrote:

>
> I assume you meant 200 rows there, but the correct number of rows to
> return is 203 for that query.  Please fix it.
>

Attach is a patch that include a fix for it
regards
Surafel
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 4db8142afa..8491b7831a 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1397,7 +1397,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY
 
 In this syntax, the start
 or count value is required by
@@ -1407,7 +1407,8 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
-ROW
+with PERCENT count specifies the maximum number of rows to return 
+in percentage.ROW
 and ROWS as well as FIRST
 and NEXT are noise words that don't influence
 the effects of these clauses.
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index 6792f9e86c..ddc11bf6ec 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -43,6 +43,7 @@ ExecLimit(PlanState *pstate)
 	LimitState *node = castNode(LimitState, pstate);
 	ScanDirection direction;
 	TupleTableSlot *slot;
+	TupleDesc   tupleDescriptor;
 	PlanState  *outerPlan;
 
 	CHECK_FOR_INTERRUPTS();
@@ -52,6 +53,8 @@ ExecLimit(PlanState *pstate)
 	 */
 	direction = node->ps.state->es_direction;
 	outerPlan = outerPlanState(node);
+	slot = node->subSlot;
+	tupleDescriptor = node->ps.ps_ResultTupleDesc;
 
 	/*
 	 * The main logic is a simple state machine.
@@ -60,6 +63,23 @@ ExecLimit(PlanState *pstate)
 	{
 		case LIMIT_INITIAL:
 
+			if (node->limitOption == PERCENTAGE)
+			{
+
+			/*
+			 * Find all rows the plan will return.
+			 */
+for (;;)
+{
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		break;
+	}
+	tuplestore_puttupleslot(node->totalTuple, slot);
+}
+			}
+
 			/*
 			 * First call for this node, so compute limit/offset. (We can't do
 			 * this any earlier, because parameters from upper nodes will not
@@ -87,24 +107,46 @@ ExecLimit(PlanState *pstate)
 return NULL;
 			}
 
-			/*
-			 * Fetch rows from subplan until we reach position > offset.
-			 */
-			for (;;)
+			if (node->limitOption == PERCENTAGE)
 			{
-slot = ExecProcNode(outerPlan);
-if (TupIsNull(slot))
+for (;;)
 {
-	/*
-	 * The subplan returns too few tuples for us to produce
-	 * any output at all.
-	 */
-	node->lstate = LIMIT_EMPTY;
-	return NULL;
+	slot = MakeSingleTupleTableSlot(tupleDescriptor, &TTSOpsMinimalTuple);
+	if (!tuplestore_gettupleslot(node->totalTuple, true, true, slot))
+	{
+		node->lstate = LIMIT_EMPTY;
+		return NULL;
+	}
+	else
+	{
+		node->subSlot = slot;
+		if (++node->position > node->offset)
+		break;
+	}
+}
+			}
+			else if (node->limitOption == EXACT_NUMBER)
+			{
+
+/*
+ * Fetch rows from subplan until we reach position > offset.
+ */
+for (;;)
+{
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		/*
+		 * The subplan returns too few tuples for us to produce
+		 * any output at all.
+		 */
+		node->lstate = LIMIT_EMPTY;
+		return NULL;
+	}
+	node->subSlot = slot;
+	if (++node->position > node->offset)
+		break;
 }
-node->subSlot = slot;
-if (++node->position > node->offset)
-	break;
 			}
 
 			/*
@@ -144,18 +186,34 @@ ExecLimit(PlanState *pstate)
 
 	return NULL;
 }
+if (node->limitOption == PERCENTAGE)
+{
+	if (tuplestore_gettupleslot(node->totalTuple, true, false, slot))
+	{
+		node->subSlot = slot;
+		node->position++;
+	}
+	else
+	{
+		node->lstate = LIMIT_SUBPLANEOF;
+		return NULL;
+	}
+}
+else if (node->limitOption == EXACT_NUMBER)
+{
 
-/*
- * Get next tuple from subplan, if any.
- */
-slot = ExecProcNode(outerPlan);
-if (TupIsNull(slot))
-{
-	node->lstate = LIMIT_SUBPLANEOF;
-	return NULL;
+	 /*
+	 * Get next tuple from subplan, if any.
+	 */
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		node->lstate = LIMIT_SUBPLANEOF;
+		return NULL;
+

Re: FETCH FIRST clause PERCENT option

[Tom Lane]

Tomas Vondra  writes:
> On 11/25/18 1:05 PM, Vik Fearing wrote:
>> I don't see how this behavior is justified by reading the SQL standard.
>> Obviously only an integer number of rows is going to be returned, but
>> the percentage should be calculated correctly.

> Right. My draft of SQL standard says this:
> 31) The declared type of the  simply
> contained in  shall be numeric.

> So the standard pretty much requires treating the value as numeric.

Note that this should not be read as "it must be of the type that PG
calls numeric".  I'd read it as requiring a value that's in the numeric
type hierarchy.  float8 would be a reasonable implementation choice
if we're going to coerce to a specific type.

regards, tom lane

Re: FETCH FIRST clause PERCENT option

[Tomas Vondra]

On 11/25/18 1:05 PM, Vik Fearing wrote:
> On 25/11/2018 12:49, Surafel Temesgen wrote:
>>
>>
>> On Sun, Nov 25, 2018 at 1:24 PM Vik Fearing > > wrote:
>>
>>
>> Also, this query returns 210 rows instead of the expected 208:
>>
>>     select *
>>     from generate_series(1, 1000)
>>     fetch first 20.8 percent rows only
>>
>> this is because fetch first values work with integer and it change
>> fractional number to nearest integer number like select * from
>> generate_series(1, 1000) fetch first 20.3 rows only; is not an error
>> rather it return 20 rows.
> 
> I don't see how this behavior is justified by reading the SQL standard.
>  Obviously only an integer number of rows is going to be returned, but
> the percentage should be calculated correctly.
> 

Right. My draft of SQL standard says this:

 ::=
  FETCH { FIRST | NEXT } [  ] { ROW | ROWS }
  { ONLY | WITH TIES }

 ::= 
   | 

 ::=  PERCENT

and then

30) The declared type of  shall be an exact
numeric with scale 0 (zero).

31) The declared type of the  simply
contained in  shall
be numeric.

So the standard pretty much requires treating the value as numeric.

regards

-- 
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: FETCH FIRST clause PERCENT option

[Andrew Gierth]

> "Surafel" == Surafel Temesgen  writes:

 Surafel> this is because fetch first values work with integer and it
 Surafel> change fractional number to nearest integer number like select
 Surafel> * from generate_series(1, 1000) fetch first 20.3 rows only; is
 Surafel> not an error rather it return 20 rows.

 31) The declared type of the  simply
 contained in  shall be numeric.

Note that unlike  there is no requirement for the
type to be _exact_ numeric or to have scale 0.

later:

ii) If  is specified, then let FFP be the
 simply contained in , and let LOCT be a  whose value is OCT. Let
FFRC be the value of

  CEILING ( FFP * LOCT / 100.0E0 )

NOTE 333 -- The percentage is computed using the number
of rows before removing the rows specified by .

ceil(20.3 * 1000 / 100.0E0) is definitely 203, not 200.

-- 
Andrew.

Re: FETCH FIRST clause PERCENT option

[Vik Fearing]

On 25/11/2018 12:49, Surafel Temesgen wrote:
> 
> 
> On Sun, Nov 25, 2018 at 1:24 PM Vik Fearing  > wrote:
> 
> 
> Also, this query returns 210 rows instead of the expected 208:
> 
>     select *
>     from generate_series(1, 1000)
>     fetch first 20.8 percent rows only
> 
> this is because fetch first values work with integer and it change
> fractional number to nearest integer number like select * from
> generate_series(1, 1000) fetch first 20.3 rows only; is not an error
> rather it return 20 rows.

I don't see how this behavior is justified by reading the SQL standard.
 Obviously only an integer number of rows is going to be returned, but
the percentage should be calculated correctly.

I assume you meant 200 rows there, but the correct number of rows to
return is 203 for that query.  Please fix it.
-- 
Vik Fearing  +33 6 46 75 15 36
http://2ndQuadrant.fr PostgreSQL : Expertise, Formation et Support

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Sun, Nov 25, 2018 at 1:24 PM Vik Fearing 
wrote:

>
> Also, this query returns 210 rows instead of the expected 208:
>
> select *
> from generate_series(1, 1000)
> fetch first 20.8 percent rows only
>
>

this is because fetch first values work with integer and it change
fractional number to nearest integer number like select * from
generate_series(1, 1000) fetch first 20.3 rows only; is not an error rather
it return 20 rows.


> As for the design, I think you should put some serious consideration
> into Andrew Gierth's approach.  I would rather see something like that.
>
>

I didn't go in that direction because I don’t understand why this approach
is inferior to using window function and I am not fully understand on how
to implement it.

regards

Surafel

Re: FETCH FIRST clause PERCENT option

[Vik Fearing]

On 25/11/2018 10:00, Surafel Temesgen wrote:
> 
> 
> 
> 
> I messed around with your changes to the grammar and it seems you don't
> need to add PERCENT as a reserved keyword.  Moving this to the
> unreserved
> keyword section does not cause any shift/reduce errors, and the
> regression
> tests still pass.  Relative to your patch v4, these changes help:
> 
> 
> In sql standard PERCENT list as reserved world so i don't think it is a
> thing that can change by me.

Yes it absolutely is.  In PostgreSQL we only make words as reserved as
they need to be, and PERCENT doesn't need to be reserved at all.

Also, this query returns 210 rows instead of the expected 208:

select *
from generate_series(1, 1000)
fetch first 20.8 percent rows only

As for the design, I think you should put some serious consideration
into Andrew Gierth's approach.  I would rather see something like that.
-- 
Vik Fearing  +33 6 46 75 15 36
http://2ndQuadrant.fr PostgreSQL : Expertise, Formation et Support

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

> I messed around with your changes to the grammar and it seems you don't
> need to add PERCENT as a reserved keyword.  Moving this to the unreserved
> keyword section does not cause any shift/reduce errors, and the regression
> tests still pass.  Relative to your patch v4, these changes help:
>
>
In sql standard PERCENT list as reserved world so i don't think it is a
thing that can change by me.
I think the main reason create_table fail in  test_ddl_deparse  is because
i don't surround PERCENT key word in
/src/test/regress/expected/create_table.out too

regards
Surafel
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 4db8142afa..8491b7831a 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1397,7 +1397,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY
 
 In this syntax, the start
 or count value is required by
@@ -1407,7 +1407,8 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
-ROW
+with PERCENT count specifies the maximum number of rows to return 
+in percentage.ROW
 and ROWS as well as FIRST
 and NEXT are noise words that don't influence
 the effects of these clauses.
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index 6792f9e86c..762024be61 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -43,6 +43,7 @@ ExecLimit(PlanState *pstate)
 	LimitState *node = castNode(LimitState, pstate);
 	ScanDirection direction;
 	TupleTableSlot *slot;
+	TupleDesc   tupleDescriptor;
 	PlanState  *outerPlan;
 
 	CHECK_FOR_INTERRUPTS();
@@ -52,6 +53,8 @@ ExecLimit(PlanState *pstate)
 	 */
 	direction = node->ps.state->es_direction;
 	outerPlan = outerPlanState(node);
+	slot = node->subSlot;
+	tupleDescriptor = node->ps.ps_ResultTupleDesc;
 
 	/*
 	 * The main logic is a simple state machine.
@@ -60,6 +63,23 @@ ExecLimit(PlanState *pstate)
 	{
 		case LIMIT_INITIAL:
 
+			if (node->limitOption == PERCENTAGE)
+			{
+
+			/*
+			 * Find all rows the plan will return.
+			 */
+for (;;)
+{
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		break;
+	}
+	tuplestore_puttupleslot(node->totalTuple, slot);
+}
+			}
+
 			/*
 			 * First call for this node, so compute limit/offset. (We can't do
 			 * this any earlier, because parameters from upper nodes will not
@@ -87,24 +107,46 @@ ExecLimit(PlanState *pstate)
 return NULL;
 			}
 
-			/*
-			 * Fetch rows from subplan until we reach position > offset.
-			 */
-			for (;;)
+			if (node->limitOption == PERCENTAGE)
 			{
-slot = ExecProcNode(outerPlan);
-if (TupIsNull(slot))
+for (;;)
 {
-	/*
-	 * The subplan returns too few tuples for us to produce
-	 * any output at all.
-	 */
-	node->lstate = LIMIT_EMPTY;
-	return NULL;
+	slot = MakeSingleTupleTableSlot(tupleDescriptor, &TTSOpsMinimalTuple);
+	if (!tuplestore_gettupleslot(node->totalTuple, true, true, slot))
+	{
+		node->lstate = LIMIT_EMPTY;
+		return NULL;
+	}
+	else
+	{
+		node->subSlot = slot;
+		if (++node->position > node->offset)
+		break;
+	}
+}
+			}
+			else if (node->limitOption == EXACT_NUMBER)
+			{
+
+/*
+ * Fetch rows from subplan until we reach position > offset.
+ */
+for (;;)
+{
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		/*
+		 * The subplan returns too few tuples for us to produce
+		 * any output at all.
+		 */
+		node->lstate = LIMIT_EMPTY;
+		return NULL;
+	}
+	node->subSlot = slot;
+	if (++node->position > node->offset)
+		break;
 }
-node->subSlot = slot;
-if (++node->position > node->offset)
-	break;
 			}
 
 			/*
@@ -144,18 +186,34 @@ ExecLimit(PlanState *pstate)
 
 	return NULL;
 }
+if (node->limitOption == PERCENTAGE)
+{
+	if (tuplestore_gettupleslot(node->totalTuple, true, false, slot))
+	{
+		node->subSlot = slot;
+		node->position++;
+	}
+	else
+	{
+		node->lstate = LIMIT_SUBPLANEOF;
+		return NULL;
+	}
+}
+else if (node->limitOption == EXACT_NUMBER)
+{
 
-/*
- * Get next tuple from subplan, if an

Re: FETCH FIRST clause PERCENT option

[Mark Dilger]

> On Sep 25, 2018, at 8:08 AM, Mark Dilger  wrote:
> 
> 
> 
>> On Sep 25, 2018, at 5:07 AM, Surafel Temesgen  wrote:
>> 
>> hey
>> 
>> On 9/21/18, Mark Dilger  wrote:
>> 
>>> Surafel, there are no regression tests that I can see in your patch.  It
>>> would help if you added some, as then I could precisely what behavior you
>>> are expecting.
>> thank you for looking at it .the attach patch add regression tests
>> 
> 
> Surafel,
> 
> I messed around with your changes to the grammar and it seems you don't
> need to add PERCENT as a reserved keyword.  Moving this to the unreserved
> keyword section does not cause any shift/reduce errors, and the regression
> tests still pass.  Relative to your patch v4, these changes help:

I spoke too soon.  The main regression tests pass, but your change to
src/test/modules/test_ddl_deparse/sql/create_table.sql per Thomas's
suggestion is no longer needed, since PERCENT no longer needs to be
quoted.

I recommend you also apply the following to your v4 patch, which just
rolls back that one change you made, and at least for me, is enough
to get `make check-world` to pass:

diff --git a/src/test/modules/test_ddl_deparse/sql/create_table.sql 
b/src/test/modules/test_ddl_deparse/sql/create_table.sql
index 4325de2d04..5e78452729 100644
--- a/src/test/modules/test_ddl_deparse/sql/create_table.sql
+++ b/src/test/modules/test_ddl_deparse/sql/create_table.sql
@@ -94,7 +94,7 @@ CREATE TABLE student (
 ) INHERITS (person);
 
 CREATE TABLE stud_emp (
-   "percent"   int4
+   percent int4
 ) INHERITS (emp, student);

Re: FETCH FIRST clause PERCENT option

[Mark Dilger]

> On Sep 25, 2018, at 5:07 AM, Surafel Temesgen  wrote:
> 
> hey
> 
> On 9/21/18, Mark Dilger  wrote:
> 
>> Surafel, there are no regression tests that I can see in your patch.  It
>> would help if you added some, as then I could precisely what behavior you
>> are expecting.
> thank you for looking at it .the attach patch add regression tests
> 

Surafel,

I messed around with your changes to the grammar and it seems you don't
need to add PERCENT as a reserved keyword.  Moving this to the unreserved
keyword section does not cause any shift/reduce errors, and the regression
tests still pass.  Relative to your patch v4, these changes help:

diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index a97ee30924..9872cf7257 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -15186,6 +15186,7 @@ unreserved_keyword:
| PARTITION
| PASSING
| PASSWORD
+   | PERCENT
| PLANS
| POLICY
| PRECEDING
@@ -15465,7 +15466,6 @@ reserved_keyword:
| ONLY
| OR
| ORDER
-   | PERCENT
| PLACING
| PRIMARY
| REFERENCES
diff --git a/src/include/parser/kwlist.h b/src/include/parser/kwlist.h
index 150d51df8f..f23fee0653 100644
--- a/src/include/parser/kwlist.h
+++ b/src/include/parser/kwlist.h
@@ -300,7 +300,7 @@ PG_KEYWORD("partial", PARTIAL, UNRESERVED_KEYWORD)
 PG_KEYWORD("partition", PARTITION, UNRESERVED_KEYWORD)
 PG_KEYWORD("passing", PASSING, UNRESERVED_KEYWORD)
 PG_KEYWORD("password", PASSWORD, UNRESERVED_KEYWORD)
-PG_KEYWORD("percent", PERCENT, RESERVED_KEYWORD)
+PG_KEYWORD("percent", PERCENT, UNRESERVED_KEYWORD)
 PG_KEYWORD("placing", PLACING, RESERVED_KEYWORD)
 PG_KEYWORD("plans", PLANS, UNRESERVED_KEYWORD)
 PG_KEYWORD("policy", POLICY, UNRESERVED_KEYWORD)

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

hey

On 9/21/18, Mark Dilger  wrote:

> Surafel, there are no regression tests that I can see in your patch.  It
> would help if you added some, as then I could precisely what behavior you
> are expecting.
thank you for looking at it .the attach patch add regression tests
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 4db8142afa..8491b7831a 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1397,7 +1397,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY
 
 In this syntax, the start
 or count value is required by
@@ -1407,7 +1407,8 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
-ROW
+with PERCENT count specifies the maximum number of rows to return 
+in percentage.ROW
 and ROWS as well as FIRST
 and NEXT are noise words that don't influence
 the effects of these clauses.
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index bb28cf7d1d..1d5b48bbe9 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -43,6 +43,7 @@ ExecLimit(PlanState *pstate)
 	LimitState *node = castNode(LimitState, pstate);
 	ScanDirection direction;
 	TupleTableSlot *slot;
+	TupleDesc   tupleDescriptor;
 	PlanState  *outerPlan;
 
 	CHECK_FOR_INTERRUPTS();
@@ -52,6 +53,8 @@ ExecLimit(PlanState *pstate)
 	 */
 	direction = node->ps.state->es_direction;
 	outerPlan = outerPlanState(node);
+	slot = node->ps.ps_ResultTupleSlot;
+	tupleDescriptor = node->ps.ps_ResultTupleSlot->tts_tupleDescriptor;
 
 	/*
 	 * The main logic is a simple state machine.
@@ -60,6 +63,23 @@ ExecLimit(PlanState *pstate)
 	{
 		case LIMIT_INITIAL:
 
+			if (node->limitOption == PERCENTAGE)
+			{
+
+			/*
+			 * Find all rows the plan will return.
+			 */
+for (;;)
+{
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		break;
+	}
+	tuplestore_puttupleslot(node->totalTuple, slot);
+}
+			}
+
 			/*
 			 * First call for this node, so compute limit/offset. (We can't do
 			 * this any earlier, because parameters from upper nodes will not
@@ -87,24 +107,46 @@ ExecLimit(PlanState *pstate)
 return NULL;
 			}
 
-			/*
-			 * Fetch rows from subplan until we reach position > offset.
-			 */
-			for (;;)
+			if (node->limitOption == PERCENTAGE)
 			{
-slot = ExecProcNode(outerPlan);
-if (TupIsNull(slot))
+for (;;)
 {
-	/*
-	 * The subplan returns too few tuples for us to produce
-	 * any output at all.
-	 */
-	node->lstate = LIMIT_EMPTY;
-	return NULL;
+	slot = MakeSingleTupleTableSlot(tupleDescriptor);
+	if (!tuplestore_gettupleslot(node->totalTuple, true, true, slot))
+	{
+		node->lstate = LIMIT_EMPTY;
+		return NULL;
+	}
+	else
+	{
+		node->subSlot = slot;
+		if (++node->position > node->offset)
+		break;
+	}
+}
+			}
+			else if (node->limitOption == EXACT_NUMBER)
+			{
+
+/*
+ * Fetch rows from subplan until we reach position > offset.
+ */
+for (;;)
+{
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		/*
+		 * The subplan returns too few tuples for us to produce
+		 * any output at all.
+		 */
+		node->lstate = LIMIT_EMPTY;
+		return NULL;
+	}
+	node->subSlot = slot;
+	if (++node->position > node->offset)
+		break;
 }
-node->subSlot = slot;
-if (++node->position > node->offset)
-	break;
 			}
 
 			/*
@@ -144,18 +186,34 @@ ExecLimit(PlanState *pstate)
 
 	return NULL;
 }
+if (node->limitOption == PERCENTAGE)
+{
+	if (tuplestore_gettupleslot(node->totalTuple, true, false, slot))
+	{
+		node->subSlot = slot;
+		node->position++;
+	}
+	else
+	{
+		node->lstate = LIMIT_SUBPLANEOF;
+		return NULL;
+	}
+}
+else if (node->limitOption == EXACT_NUMBER)
+{
 
-/*
- * Get next tuple from subplan, if any.
- */
-slot = ExecProcNode(outerPlan);
-if (TupIsNull(slot))
-{
-	node->lstate = LIMIT_SUBPLANEOF;
-	return NULL;
+	 /*
+	 * Get next tuple from subplan, if any.
+	 */
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		node

Re: FETCH FIRST clause PERCENT option

[Mark Dilger]

> On Sep 20, 2018, at 5:29 PM, Andres Freund  wrote:
> 
> Hi,
> 
> On 2018-09-20 17:06:36 -0700, Mark Dilger wrote:
>> I should think that spilling anything to a tuplestore would only be needed
>> if the query contains an ORDER BY expression.  If you query
>> 
>>  FETCH FIRST 50 PERCENT * FROM foo;
>> 
>> you should just return every other row, discarding the rest, right?  It's
>> only when an explicit ordering is given that the need to store the results
>> arises.  Even with
>> 
>>  FETCH FIRST 50 PERCENT name FROM foo ORDER BY name;
>> 
>> you can return one row for every two rows that you get back from the
>> sort node, reducing the maximum number you need to store at any time to
>> no more than 25% of all rows.
> 
> I'm doubtful about the validity of these optimizations, particularly
> around being surprising. But I think more importantly, we should focus
> on the basic implementation that's needed anyway.

You may be right that getting the basic implementation finished first
is better than optimizing at this stage.  So the rest of what I'm going
to say is just in defense of the optimization, and not an argument for
needing to optimize right away.

As for reducing the surprise factor, I think that it would be surprising
if I ask for a smallish percentage of rows and it takes significantly longer
and significantly more memory or disk than asking for all the rows takes.
If I'm including an explicit ORDER BY, then that explains it, but otherwise,
I'd be surprised.  Note that I'm not saying I'd be surprised by it taking
roughly the same length of time / memory / disk.  I'd only be surprised if
it took a lot more.

There are plenty of SQL generation engines that people put in their software.
I'd expect something like

sprintf("FETCH FIRST %d PERCENT %s FROM %s", percentage, columns, 
tablename)

to show up in such engines, and percentage to sometimes be 100.  At least
in that case you should just return all rows rather than dumping them into
a tuplestore.  Likewise, if the percentage is 0, you'll want to finish quickly.
Actually, I don't know if the SQL spec would require side effects to still
happen, in which case you'd still have to generate all rows for their side
effects to happen, and then just not return them.  But still, no tuplestore.
So the implementation of FETCH FIRST would at least need to think about what
percentage is being requested, rather than just mindlessly adding a node to
the tree for storing everything, then computing the LIMIT based on the number
of rows stored, and then returning that number of rows.

mark

Re: FETCH FIRST clause PERCENT option

[Andres Freund]

Hi,

On 2018-09-20 17:06:36 -0700, Mark Dilger wrote:
> I should think that spilling anything to a tuplestore would only be needed
> if the query contains an ORDER BY expression.  If you query
> 
>   FETCH FIRST 50 PERCENT * FROM foo;
> 
> you should just return every other row, discarding the rest, right?  It's
> only when an explicit ordering is given that the need to store the results
> arises.  Even with
> 
>   FETCH FIRST 50 PERCENT name FROM foo ORDER BY name;
> 
> you can return one row for every two rows that you get back from the
> sort node, reducing the maximum number you need to store at any time to
> no more than 25% of all rows.

I'm doubtful about the validity of these optimizations, particularly
around being surprising. But I think more importantly, we should focus
on the basic implementation that's needed anyway.

Greetings,

Andres Freund

Re: FETCH FIRST clause PERCENT option

[Mark Dilger]

> On Aug 16, 2018, at 7:34 AM, Andres Freund  wrote:
> 
> Hi,
> 
> On 2018-08-16 17:27:45 +0300, Surafel Temesgen wrote:
>> FETCH FIRST with PERCENT option is SQL standard that state limit count to
>> be specified in a percentage in addition to specify it in exact count and
>> listed as one of Major features simply not implemented yet in recent wiki
>> page [1].
>> 
>> I implemented it by executing the query plan twice. One without limit
>> filter to find the total number of row that will be feed to limit node so
>> the exact limit count can be calculated and the query plan execute again
>> with limit filter with newly calculated exact count .

Surafel, there are no regression tests that I can see in your patch.  It
would help if you added some, as then I could precisely what behavior you
are expecting.  As it is, I'm just guessing here, but here goes


> Won't that have rather massive issues with multiple evaluations of
> clauses in the query?  Besides being really expensive?
> 
> I think you'd have to instead spill the query results into a tuplestore
> (like we do for FOR HOLD queries at end of xact), and then do the
> computations based on that.


I should think that spilling anything to a tuplestore would only be needed
if the query contains an ORDER BY expression.  If you query

FETCH FIRST 50 PERCENT * FROM foo;

you should just return every other row, discarding the rest, right?  It's
only when an explicit ordering is given that the need to store the results
arises.  Even with

FETCH FIRST 50 PERCENT name FROM foo ORDER BY name;

you can return one row for every two rows that you get back from the
sort node, reducing the maximum number you need to store at any time to
no more than 25% of all rows.

Or am I missing something?

mark

Re: FETCH FIRST clause PERCENT option

[Thomas Munro]

On Fri, Aug 31, 2018 at 11:42 PM Surafel Temesgen  wrote:
> On Tue, Aug 28, 2018 at 7:33 PM Erik Rijkers  wrote:
>>
>> ;
>>
>> TRAP: FailedAssertion("!(slot != ((void *)0))", File: "execTuples.c",
>> Line: 42
>
> The attache patch include a fix for the crash .can you check it again?

FYI this fails[1] in src/test/modules/test_ddl_deparse's create_table
test because of the keyword:

  CREATE TABLE stud_emp (
  percent int4
  ) INHERITS (emp, student);
! ERROR:  syntax error at or near "percent"

[1] https://travis-ci.org/postgresql-cfbot/postgresql/builds/430777123

--
Thomas Munro
http://www.enterprisedb.com

Re: FETCH FIRST clause PERCENT option

[Surafel Temesgen]

On Tue, Aug 28, 2018 at 7:33 PM Erik Rijkers  wrote:

> ;
>
> TRAP: FailedAssertion("!(slot != ((void *)0))", File: "execTuples.c",
> Line: 42

The attache patch include a fix for the crash .can you check it again?
Regards
Surafel
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 4db8142afa..8491b7831a 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -44,7 +44,7 @@ SELECT [ ALL | DISTINCT [ ON ( expressionexpression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
 [ LIMIT { count | ALL } ]
 [ OFFSET start [ ROW | ROWS ] ]
-[ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
+[ FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY ]
 [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 where from_item can be one of:
@@ -1397,7 +1397,7 @@ OFFSET start
 which PostgreSQL also supports.  It is:
 
 OFFSET start { ROW | ROWS }
-FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
+FETCH { FIRST | NEXT } [ count ] [ PERCENT ] { ROW | ROWS } ONLY
 
 In this syntax, the start
 or count value is required by
@@ -1407,7 +1407,8 @@ FETCH { FIRST | NEXT } [ count ] {
 ambiguity.
 If count is
 omitted in a FETCH clause, it defaults to 1.
-ROW
+with PERCENT count specifies the maximum number of rows to return 
+in percentage.ROW
 and ROWS as well as FIRST
 and NEXT are noise words that don't influence
 the effects of these clauses.
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index bb28cf7d1d..e842e17542 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -43,6 +43,7 @@ ExecLimit(PlanState *pstate)
 	LimitState *node = castNode(LimitState, pstate);
 	ScanDirection direction;
 	TupleTableSlot *slot;
+	TupleDesc   tupleDescriptor;
 	PlanState  *outerPlan;
 
 	CHECK_FOR_INTERRUPTS();
@@ -52,6 +53,8 @@ ExecLimit(PlanState *pstate)
 	 */
 	direction = node->ps.state->es_direction;
 	outerPlan = outerPlanState(node);
+	slot = node->ps.ps_ResultTupleSlot;
+	tupleDescriptor = node->ps.ps_ResultTupleSlot->tts_tupleDescriptor;
 
 	/*
 	 * The main logic is a simple state machine.
@@ -60,6 +63,23 @@ ExecLimit(PlanState *pstate)
 	{
 		case LIMIT_INITIAL:
 
+			if (node->limitOption == PERCENTAGE)
+			{
+
+			/*
+			 * Find all rows the plan will return.
+			 */
+for (;;)
+{
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		break;
+	}
+	tuplestore_puttupleslot(node->totalTuple, slot);
+}
+			}
+
 			/*
 			 * First call for this node, so compute limit/offset. (We can't do
 			 * this any earlier, because parameters from upper nodes will not
@@ -87,24 +107,46 @@ ExecLimit(PlanState *pstate)
 return NULL;
 			}
 
-			/*
-			 * Fetch rows from subplan until we reach position > offset.
-			 */
-			for (;;)
+			if (node->limitOption == PERCENTAGE)
 			{
-slot = ExecProcNode(outerPlan);
-if (TupIsNull(slot))
+for (;;)
 {
-	/*
-	 * The subplan returns too few tuples for us to produce
-	 * any output at all.
-	 */
-	node->lstate = LIMIT_EMPTY;
-	return NULL;
+	slot = MakeSingleTupleTableSlot(tupleDescriptor);
+	if (!tuplestore_gettupleslot(node->totalTuple, true, true, slot))
+	{
+		node->lstate = LIMIT_EMPTY;
+		return NULL;
+	}
+	else
+	{
+		node->subSlot = slot;
+		if (++node->position > node->offset)
+		break;
+	}
+}
+			}
+			else if (node->limitOption == EXACT_NUMBER)
+			{
+
+/*
+ * Fetch rows from subplan until we reach position > offset.
+ */
+for (;;)
+{
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		/*
+		 * The subplan returns too few tuples for us to produce
+		 * any output at all.
+		 */
+		node->lstate = LIMIT_EMPTY;
+		return NULL;
+	}
+	node->subSlot = slot;
+	if (++node->position > node->offset)
+		break;
 }
-node->subSlot = slot;
-if (++node->position > node->offset)
-	break;
 			}
 
 			/*
@@ -144,18 +186,34 @@ ExecLimit(PlanState *pstate)
 
 	return NULL;
 }
+if (node->limitOption == PERCENTAGE)
+{
+	if (tuplestore_gettupleslot(node->totalTuple, true, false, slot))
+	{
+		node->subSlot = slot;
+		node->position++;
+	}
+	else
+	{
+		node->lstate = LIMIT_SUBPLANEOF;
+		return NULL;
+	}
+}
+else if (node->limitOption == EXACT_NUMBER)
+{
 
-/*
- * Get next tuple from subplan, if any.
- */
-slot = ExecProcNode(outerPlan);
-if (TupIsNull(slot))
-{
-	node->lstate = LIMIT_SUBPLANEOF;
-	return NULL;
+	 /*
+	 * Get next tuple from subplan, if any.
+	 */
+	slot = ExecProcNode(outerPlan);
+	if (TupIsNull(slot))
+	{
+		node->lstate = LIMIT_SUBPLANEOF;
+		return

Re: FETCH FIRST clause PERCENT option

[Erik Rijkers]

On 2018-08-28 14:14, Surafel Temesgen wrote:
On Tue, Aug 21, 2018 at 3:50 PM Andres Freund  
wrote:




Imagine volatile functions being used. That can be problematic because
of repeated side-effects, but also will lead to plainly wrong
results. Consider what happens with your approach where somebody does
something like WHERE value < random().


Thanks for explaining . The attach patch use tuplestore instead



[fetch-wth-percent-v2.patch]


I had a quick look at this.  Apply, compile, make check are ok.

In straightforward usage seems to work ok.


But I also ran across this crash:

create table if not exists t as
   select i from generate_series(1, 100) as f(i);

select * from t
  offset 60 rows
  fetch next 3 percent rows only
  FOR UPDATE
;

TRAP: FailedAssertion("!(slot != ((void *)0))", File: "execTuples.c", 
Line: 428)





Erik Rijkers