Re: [GENERAL] Query runs fast or slow
On Sun, Apr 16, 2006 at 04:32:25PM -0400, Tom Lane wrote:
> [EMAIL PROTECTED] writes:
> > EXPLAIN ANALYZE doesn't show the slow timing
> > because it requires values, not $n placeholders,
>
> To analyze the plan used for a parameterized query, try
>
> PREPARE foo(...) AS SELECT ... $n ...
>
> EXPLAIN ANALYZE EXECUTE foo(...)
>
> But I already know what you're going to find: the planner's estimates
> for the range query are not going to be very good when it has no idea
> what the range bounds are. This is a situation where it may be best
> to absorb the hit of re-planning each time instead of using a generic
> parameterized plan.
OK, here is the new explain analyze. I eliminated cache effects by
dumping the tables and picking random values with an editor.
felix=> PREPARE foo(TEXT, INT, INT) AS SELECT s.data, g.key, g.val, g.sid FROM
key k, val_int v, sid s, glue_int g WHERE (k.data = $1 AND k.id = g.key) AND
(v.data >= $2 AND v.data <= $3) AND v.id = g.val AND g.sid = s.id;
PREPARE
felix=> explain analyze execute foo('mthNQFrmVs3Q4bVruCxIAGy', 1973028023,
1973028223);
QUERY PLAN
Nested Loop (cost=1380.11..404223.36 rows=499 width=60) (actual
time=5785.012..77823.688 rows=1 loops=1)
-> Hash Join (cost=1380.11..402713.38 rows=499 width=16) (actual
time=5766.308..77804.969 rows=1 loops=1)
Hash Cond: ("outer".val = "inner".id)
-> Nested Loop (cost=0.00..400829.78 rows=99701 width=16) (actual
time=115.154..77401.159 rows=10 loops=1)
-> Index Scan using key_data_key on "key" k (cost=0.00..5.82
rows=1 width=4) (actual time=0.125..0.132 rows=1 loops=1)
Index Cond: (data = $1)
-> Index Scan using glue_int_key_idx on glue_int g
(cost=0.00..399577.70 rows=99701 width=16) (actual time=115.011..76570.366
rows=10 loops=1)
Index Cond: ("outer".id = g."key")
-> Hash (cost=1378.86..1378.86 rows=500 width=4) (actual
time=11.580..11.580 rows=0 loops=1)
-> Index Scan using val_int_data_key on val_int v
(cost=0.00..1378.86 rows=500 width=4) (actual time=11.556..11.561 rows=1
loops=1)
Index Cond: ((data >= $2) AND (data <= $3))
-> Index Scan using sid_pkey on sid s (cost=0.00..3.01 rows=1 width=52)
(actual time=18.682..18.687 rows=1 loops=1)
Index Cond: ("outer".sid = s.id)
Total runtime: 77823.897 ms
(14 rows)
A repeat shows it faster, from 77 seconds to 3.
felix=> explain analyze execute foo('mthNQFrmVs3Q4bVruCxIAGy', 1973028023,
1973028223);
QUERY PLAN
-
Nested Loop (cost=1380.11..404223.36 rows=499 width=60) (actual
time=205.137..2931.899 rows=1 loops=1)
-> Hash Join (cost=1380.11..402713.38 rows=499 width=16) (actual
time=205.056..2931.803 rows=1 loops=1)
Hash Cond: ("outer".val = "inner".id)
-> Nested Loop (cost=0.00..400829.78 rows=99701 width=16) (actual
time=0.148..2564.255 rows=10 loops=1)
-> Index Scan using key_data_key on "key" k (cost=0.00..5.82
rows=1 width=4) (actual time=0.031..0.039 rows=1 loops=1)
Index Cond: (data = $1)
-> Index Scan using glue_int_key_idx on glue_int g
(cost=0.00..399577.70 rows=99701 width=16) (actual time=0.105..1808.068
rows=10 loops=1)
Index Cond: ("outer".id = g."key")
-> Hash (cost=1378.86..1378.86 rows=500 width=4) (actual
time=0.090..0.090 rows=0 loops=1)
-> Index Scan using val_int_data_key on val_int v
(cost=0.00..1378.86 rows=500 width=4) (actual time=0.074..0.080 rows=1 loops=1)
Index Cond: ((data >= $2) AND (data <= $3))
-> Index Scan using sid_pkey on sid s (cost=0.00..3.01 rows=1 width=52)
(actual time=0.061..0.066 rows=1 loops=1)
Index Cond: ("outer".sid = s.id)
Total runtime: 2932.013 ms
(14 rows)
And running it as a simple query shows it much faster, 72 ms.
felix=> EXPLAIN ANALYZE SELECT s.data, g.key, g.val, g.sid FROM key k, val_int
v, sid s, glue_int g WHERE (k.data = 'mthNQFrmVs3Q4bVruCxIAGy' AND k.id =
g.key) AND (v.data >= 1973028023 AND v.data <= 1973028223) AND v.id = g.val AND
g.sid = s.id;
QUERY
PLAN
--
Re: [GENERAL] Query runs fast or slow
On Sun, Apr 16, 2006 at 04:32:25PM -0400, Tom Lane wrote: > To analyze the plan used for a parameterized query, try > > PREPARE foo(...) AS SELECT ... $n ... > > EXPLAIN ANALYZE EXECUTE foo(...) > > But I already know what you're going to find: the planner's estimates > for the range query are not going to be very good when it has no idea > what the range bounds are. This is a situation where it may be best > to absorb the hit of re-planning each time instead of using a generic > parameterized plan. I will try this Monday, but isn't 75 seconds an awful long time? It almost seems like even the worst plan could find records faster than that, and if it were actually scanning everything sequentially, there would be a fair amount of variation, say 25 seconds, 50 seconds, 100 seconds. The most I have seen is a range of, say, 75-77. That just seems way too slow. -- ... _._. ._ ._. . _._. ._. ___ .__ ._. . .__. ._ .. ._. Felix Finch: scarecrow repairman & rocket surgeon / [EMAIL PROTECTED] GPG = E987 4493 C860 246C 3B1E 6477 7838 76E9 182E 8151 ITAR license #4933 I've found a solution to Fermat's Last Theorem but I see I've run out of room o ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [GENERAL] Query runs fast or slow
[EMAIL PROTECTED] writes: > EXPLAIN ANALYZE doesn't show the slow timing > because it requires values, not $n placeholders, To analyze the plan used for a parameterized query, try PREPARE foo(...) AS SELECT ... $n ... EXPLAIN ANALYZE EXECUTE foo(...) But I already know what you're going to find: the planner's estimates for the range query are not going to be very good when it has no idea what the range bounds are. This is a situation where it may be best to absorb the hit of re-planning each time instead of using a generic parameterized plan. regards, tom lane ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
[GENERAL] Query runs fast or slow
I have a benchmark test which runs a query very slowly under certain
circumstances. I used Ethereal to capture the packet traffic, and
also enabled debug5 logging out of curiousity. While the slow query
is in progress, there is no log or packet activity, but the cpu is
busy. These packets are below; look for SLOW PACKETS HERE to skip all
this explanatory drudge.
This WHERE clause is fast under all conditions:
... AND (val.data = $2) AND ...
This WHERE clause is fast as a simple query, but is excruciatingly
slow as prepare / execute / fetch:
... AND (val.data > $2 AND val.data < $3) AND ...
My test program is in Perl and uses DBI/DBD::Pg. Postgresql version
is 8.0.3 on a dual core dual opteron with 2G of RAM. DBI is version
1.48. DBD::Pg is version 1.42. The OS is rPath Linux 2.6.15.
The test runs each SQL statement three times, first as a simple query
to preload caches, then as prepare / execute / fetch, and lastly as a
simple query again.
$sth = $dbh->prepare(sql_with_placeholders);
$dbh->selectall_arrayref(sql_with_values_substituted);
$sth->execute(@values);
$sth->fetchall_arrayref();
$dbh->selectall_arrayref(sql_with_values_substituted);
I captured packet traffic and tailed the log while these were running.
Everything is fine except one query, which took 75 seconds to run,
when the others took 3 milliseconds. During this 75 seconds, the
postmaster log showed no activity, but top showed the postmaster quite
busy.
75 seconds! That's an eternity. I can't imagine any circumstances
where it makes sense. EXPLAIN ANALYZE doesn't show the slow timing
because it requires values, not $n placeholders, and it is the prepare
/ execute operation which is so slow. I will be glad to send the log,
the packet capture file, the test program itself, and anything else
which helps. Here are the EXPLAIN statements in case it helps.
EXPLAIN for the equality WHERE clause:
felix=> explain analyze SELECT sid.data, glue.key, glue.val, glue.sid FROM
key, val, sid, glue WHERE (key.data =
'x6ATArB_k1cgLp1mD5x2nzVVf2DQw4Lw1-Ow5NCzzs5Pupg6K' AND key.id = glue.key) AND
(val.data = 357354306) AND val.id = glue.val AND glue.sid = sid.id;
QUERY
PLAN
Nested Loop (cost=5.82..1119.29 rows=1 width=60) (actual
time=2.271..36.184 rows=1 loops=1)
-> Hash Join (cost=5.82..1116.27 rows=1 width=16) (actual
time=2.079..35.976 rows=1 loops=1)
Hash Cond: ("outer"."key" = "inner".id)
-> Nested Loop (cost=0.00..1105.43 rows=1001 width=16) (actual
time=0.315..31.820 rows=1000 loops=1)
-> Index Scan using val_data_key on val (cost=0.00..6.01
rows=1 width=4) (actual time=0.119..0.123 rows=1 loops=1)
Index Cond: (data = 357354306)
-> Index Scan using glue_val_idx on glue
(cost=0.00..702.58 rows=31747 width=16) (actual time=0.181..24.438 rows=1000
loops=1)
Index Cond: ("outer".id = glue.val)
-> Hash (cost=5.82..5.82 rows=1 width=4) (actual
time=0.292..0.292 rows=0 loops=1)
-> Index Scan using key_data_key on "key" (cost=0.00..5.82
rows=1 width=4) (actual time=0.266..0.271 rows=1 loops=1)
Index Cond: (data =
'x6ATArB_k1cgLp1mD5x2nzVVf2DQw4Lw1-Ow5NCzzs5Pupg6K'::text)
-> Index Scan using sid_pkey on sid (cost=0.00..3.01 rows=1 width=52)
(actual time=0.179..0.183 rows=1 loops=1)
Index Cond: ("outer".sid = sid.id)
Total runtime: 37.880 ms
(14 rows)
EXPLAIN for the range WHERE clause:
felix=> explain analyze SELECT sid.data, glue.key, glue.val, glue.sid FROM
key, val, sid, glue WHERE (key.data = 'kOSkZ5iN6sz-KqGo51aTwqZnvCKQRUH2SZ8k'
AND key.id = glue.key) AND (val.data > 183722006 AND val.data < 183722206) AND
val.id = glue.val AND glue.sid = sid.id;
QUERY
PLAN
---
Nested Loop (cost=5.82..1119.30 rows=1 width=60) (actual
time=15.016..15.525 rows=1 loops=1)
-> Hash Join (cost=5.82..1116.27 rows=1 width=16) (actual
time=14.879..15.374 rows=1 loops=1)
Hash Cond: ("outer"."key" = "inner".id)
-> Nested Loop (cost=0.00..1105.43 rows=1001 width=16) (actual
time=0.211..11.666 rows=1000 loops=1)
-> Index Scan using val_data_key on val (cost=0.00..6.01
rows=1 width=4) (actual time=0.071..0.076 rows=1 loops=1)
Index Cond:
