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Re: Avoiding bad prepared-statement plans.

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Bart Samwel <bart(at)samwel(dot)tk>, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, Jeroen Vermeulen <jtv(at)xs4all(dot)nl>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Avoiding bad prepared-statement plans.
Date: 2010-02-11 16:17:40
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Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Thu, Feb 11, 2010 at 7:48 AM, Bart Samwel <bart(at)samwel(dot)tk> wrote:
>> Because that's the
>> underlying assumption of the "ratio" criterion -- that re-planning with
>> filled-in parameters takes about as much time as the initial planning run
>> took.

> We only want to replan when replanning is relatively cheap compared to
> execution,

Well, no, consider the situation where planning takes 50 ms, the generic
plan costs 100ms to execute, but a parameter-specific plan would take 1ms
to execute.  Planning is very expensive compared to execution but it's
still a win to do it.

The problem that we face is that we don't have any very good way to tell
whether a fresh planning attempt is likely to yield a plan significantly
better than the generic plan.  I can think of some heuristics --- for
example if the query contains LIKE with a parameterized pattern or a
partitioned table --- but that doesn't seem like a particularly nice
road to travel.

A possible scheme is to try it and keep track of whether we ever
actually do get a better plan.  If, after N attempts, none of the custom
plans were ever more than X% cheaper than the generic one, then give up
and stop attempting to produce custom plans.  Tuning the variables might
be challenging though.

			regards, tom lane

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