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Re: Avoiding bad prepared-statement plans.

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Bart Samwel <bart(at)samwel(dot)tk>
Cc: 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 12:41:12
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Lists: pgsql-hackers
On Thu, Feb 11, 2010 at 7:39 AM, Bart Samwel <bart(at)samwel(dot)tk> wrote:
> On Thu, Feb 11, 2010 at 13:25, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
> wrote:
>> 2010/2/11 Bart Samwel <bart(at)samwel(dot)tk>:
>> > Perhaps this could be based on a (configurable?) ratio of observed
>> > planning
>> > time and projected execution time. I mean, if planning it the first time
>> > took 30 ms and projected execution time is 1 ms, then by all means NEVER
>> > re-plan. But if planning the first time took 1 ms and resulted in a
>> > projected execution time of 50 ms, then it's relatively cheap to re-plan
>> > every time (cost increase per execution is 1/50 = 2%), and the potential
>> > gains are much greater (taking a chunk out of 50 ms adds up quickly).
>> It could be a good idea. I don't belive to sophisticate methods. There
>> can be a very simply solution. The could be a "limit" for price.  More
>> expensive queries can be replaned every time when the price will be
>> over limit.
> I guess the required complexity depends on how variable planning costs are.
> If planning is typically <= 2 ms, then a hard limit on estimated price is
> useful and can be set as low as (the equivalent of) 15 ms. However, if
> planning costs can be 50 ms, then the lowest reasonable "fixed" limit is
> quite a bit larger than that -- and that does not solve the problem reported
> earlier in this thread, where a query takes 30 ms using a generic plan and 1
> ms using a specialized plan.
> Anyhow, I have no clue how much time the planner takes. Can anybody provide
> any statistics in that regard?

It depends a great deal on the query, which is one of the things that
makes implementing this rather challenging.


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