| From: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> |
|---|---|
| To: | Merlin Moncure <mmoncure(at)gmail(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: EXISTS clauses not being optimized in the face of 'one time pass' optimizable expressions |
| Date: | 2016-07-01 16:45:06 |
| Message-ID: | 20160701164505.GA304870@alvherre.pgsql |
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| Lists: | pgsql-hackers |
Merlin Moncure wrote:
> It's pretty easy to craft a query where you're on the winning side,
> but what's the worst case of doing two pass...is constant folding a
> non trivial fraction of planning time?
One thing that has been suggested is to re-examine the plan after
planning is done, and if execution time is estimated to be large (FSVO),
then run a second planning pass with more expensive optimizations
enabled to try and find better plans. The guiding principle would be to
continue to very quickly find good enough plans for
frequent/small/simple queries, but spend more planning effort on more
complex ones where execution is likely to take much longer than planning
time.
So doing constant-folding twice would be enabled for the second planning
pass.
--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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