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Re: Avoiding bad prepared-statement plans.

From: Mark Mielke <mark(at)mark(dot)mielke(dot)cc>
To: Yeb Havinga <yebhavinga(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, Jeroen Vermeulen <jtv(at)xs4all(dot)nl>, Alex Hunsaker <badalex(at)gmail(dot)com>, Greg Stark <gsstark(at)mit(dot)edu>, Bart Samwel <bart(at)samwel(dot)tk>, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Avoiding bad prepared-statement plans.
Date: 2010-02-26 20:29:11
Message-ID: 4B882F17.8050909@mark.mielke.cc (view raw or flat)
Thread:
Lists: pgsql-hackers
On 02/26/2010 03:11 PM, Yeb Havinga wrote:
> Tom Lane wrote:
>> Right, but if the parameter is unknown then its distribution is also
>> unknown.  In any case that's just nitpicking, because the solution is
>> to create a custom plan for the specific value supplied.  Or are you
>> suggesting that we should create a way for users to say "here is the
>> expected distribution of this parameter", and then try to fold that into
>> the planner estimates?
> Or instead of letting users give the distribution, gather it 
> automatically in some plan statistics catalog? I suspect in most 
> applications queries stay the same for months and maybe years, so 
> after some number of iterations it is possible to have decent call 
> statistics / parameter distributions. Maybe the the parameter value 
> distribution could even be annotated with actual cached plans.

The problem with the last - actual cached plans - is that it implies the 
other aspect I have been suggesting: In order to have a custom cached 
plan, the primary model must be to use custom plans. If PREPARE/EXECUTE 
uses generic plans normally, than the only cached plans available will 
be generic plans.

Cheers,
mark


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