Ashish wrote:
> I am thinking about starting with the following TODO item:
>
> --> Have EXPLAIN ANALYZE issue NOTICE messages when the estimated
> and actual row counts differ by a specified percentage.
>
> I picked this because it is somewhat related to query processing
> which is what I am most interested in. It also <seems> like a
> good start up project for a newbie like me. Before I start
> looking into what this would involve and start a conversation
> on designing a solution - I wanted to know what you guys think
> about this particular TODO, and it suitability to a newbie.
> Looking forward to your comments...
I even have a sample patch you can use as a start, attached.
--
Bruce Momjian <br...@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
Index: doc/src/sgml/ref/explain.sgml
===================================================================
RCS file: /cvsroot/pgsql/doc/src/sgml/ref/explain.sgml,v
retrieving revision 1.38
diff -c -c -r1.38 explain.sgml
*** doc/src/sgml/ref/explain.sgml 18 Sep 2006 19:54:01 -0000 1.38
--- doc/src/sgml/ref/explain.sgml 22 Dec 2006 17:09:05 -0000
***************
*** 64,72 ****
<para>
The <literal>ANALYZE</literal> option causes the statement to be actually executed, not only
planned. The total elapsed time expended within each plan node (in
! milliseconds) and total number of rows it actually returned are added to
! the display. This is useful for seeing whether the planner's estimates
! are close to reality.
</para>
<important>
--- 64,72 ----
<para>
The <literal>ANALYZE</literal> option causes the statement to be actually executed, not only
planned. The total elapsed time expended within each plan node (in
! milliseconds) and total number of rows it actually returned and variance are added to
! the display. A sign of the variance indicates whether the estimate was too high or too low.
! This is useful for seeing how close the planner's estimates are to reality.
</para>
<important>
***************
*** 222,229 ****
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
! HashAggregate (cost=39.53..39.53 rows=1 width=8) (actual time=0.661..0.672 rows=7 loops=1)
! -> Index Scan using test_pkey on test (cost=0.00..32.97 rows=1311 width=8) (actual time=0.050..0.395 rows=99 loops=1)
Index Cond: ((id > $1) AND (id < $2))
Total runtime: 0.851 ms
(4 rows)
--- 222,229 ----
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
! HashAggregate (cost=39.53..39.53 rows=1 width=8) (actual time=0.661..0.672 rows=7 var=-6.00 loops=1)
! -> Index Scan using test_pkey on test (cost=0.00..32.97 rows=1311 width=8) (actual time=0.050..0.395 rows=99 var=+12.24 loops=1)
Index Cond: ((id > $1) AND (id < $2))
Total runtime: 0.851 ms
(4 rows)
Index: src/backend/commands/explain.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/commands/explain.c,v
retrieving revision 1.152
diff -c -c -r1.152 explain.c
*** src/backend/commands/explain.c 4 Oct 2006 00:29:51 -0000 1.152
--- src/backend/commands/explain.c 22 Dec 2006 17:09:09 -0000
***************
*** 57,62 ****
--- 57,63 ----
static void show_sort_keys(Plan *sortplan, int nkeys, AttrNumber *keycols,
const char *qlabel,
StringInfo str, int indent, ExplainState *es);
+ static double ExplainVariance(double estimate, double actual);
/*
* ExplainQuery -
***************
*** 704,713 ****
{
double nloops = planstate->instrument->nloops;
! appendStringInfo(str, " (actual time=%.3f..%.3f rows=%.0f loops=%.0f)",
1000.0 * planstate->instrument->startup / nloops,
1000.0 * planstate->instrument->total / nloops,
planstate->instrument->ntuples / nloops,
planstate->instrument->nloops);
}
else if (es->printAnalyze)
--- 705,716 ----
{
double nloops = planstate->instrument->nloops;
! appendStringInfo(str, " (actual time=%.3f..%.3f rows=%.0f var=%+.2f loops=%.0f)",
1000.0 * planstate->instrument->startup / nloops,
1000.0 * planstate->instrument->total / nloops,
planstate->instrument->ntuples / nloops,
+ ExplainVariance(plan->plan_rows,
+ planstate->instrument->ntuples / nloops),
planstate->instrument->nloops);
}
else if (es->printAnalyze)
***************
*** 1205,1207 ****
--- 1208,1225 ----
appendStringInfo(str, "\n");
}
+
+
+ static double ExplainVariance(double estimate, double actual)
+ {
+ if (estimate == actual)
+ return 0;
+ else if (actual == 0)
+ return estimate;
+ else if (estimate == 0)
+ return -actual;
+ else if (estimate > actual)
+ return (estimate / actual) - 1;
+ else
+ return -(actual / estimate - 1);
+ }
--
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