Re: Extremely irregular query performance

Simon Riggs wrote:
> On Wed, 2006-01-11 at 22:23 -0500, Tom Lane wrote:
> > =?iso-8859-1?Q?Jean-Philippe_C=F4t=E9?= <jean-philippe.cote@crt.umontreal.ca> writes:
> > > Thanks a lot for this info, I was indeed exceeding the genetic
> > > optimizer's threshold.  Now that it is turned off, I get
> > > a very stable response time of 435ms (more or less 5ms) for
> > > the same query. It is about three times slower than the best
> > > I got with the genetic optimizer on, but the overall average
> > > is much lower.
> >
> > Hmm.  It would be interesting to use EXPLAIN ANALYZE to confirm that the
> > plan found this way is the same as the best plan found by GEQO, and
> > the extra couple hundred msec is the price you pay for the exhaustive
> > plan search.  If GEQO is managing to find a plan better than the regular
> > planner then we need to look into why ...
>
> It seems worth noting in the EXPLAIN whether GEQO has been used to find
> the plan, possibly along with other factors influencing the plan such as
> enable_* settings.

I thought the best solution would be to replace "QUERY PLAN" with "GEQO
QUERY PLAN" when GEQO was in use.  However, looking at the code, I see
no way to do that cleanly.

Instead, I added documentation to EXPLAIN to highlight the fact the
execution plan will change when GEQO is in use.

(I also removed a documentation mention of the pre-7.3 EXPLAIN output
behavior.)

--
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
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Index: doc/src/sgml/ref/explain.sgml
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RCS file: /cvsroot/pgsql/doc/src/sgml/ref/explain.sgml,v
retrieving revision 1.35
diff -c -c -r1.35 explain.sgml
*** doc/src/sgml/ref/explain.sgml    4 Jan 2005 00:39:53 -0000    1.35
--- doc/src/sgml/ref/explain.sgml    20 Jan 2006 16:18:53 -0000
***************
*** 151,161 ****
    </para>

    <para>
!    Prior to <productname>PostgreSQL</productname> 7.3, the plan was
!    emitted in the form of a <literal>NOTICE</literal> message.  Now it
!    appears as a query result (formatted like a table with a single
!    text column).
    </para>
   </refsect1>

   <refsect1>
--- 151,162 ----
    </para>

    <para>
!    Genetic query optimization (<acronym>GEQO</acronym>) randomly
!    tests execution plans.  Therefore, when the number of tables
!    exceeds <varname>geqo</> and genetic query optimization is in use,
!    the execution plan will change each time the statement is executed.
    </para>
+
   </refsect1>

   <refsect1>