Re: Avoiding bad prepared-statement plans.

From: Jeroen Vermeulen <jtv(at)xs4all(dot)nl>
To: Yeb Havinga <yebhavinga(at)gmail(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Avoiding bad prepared-statement plans.
Date: 2010-02-09 13:46:55
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Yeb Havinga wrote:

>> I've been discussing this with Josh, Heikki, and Peter E. over the
>> past few weeks.
> Is this searchable in the archives? I'm interested in ideas discussed.

No, sorry. These were face-to-face discussions at and FOSDEM.

>> If a prepared statement takes parameters, and the generic plan has a
>> high projected cost, re-plan each EXECUTE individually with all its
>> parameter values bound. It may or may not help, but unless the
>> planner is vastly over-pessimistic, re-planning isn't going to
>> dominate execution time for these cases anyway.

> This sounds like a really nice to have feature. Maybe it'd also be
> possible to skip replanning between executes if the current bound values
> are 'indexwise-equivalent' to the values used at previous planning, i.e.
> nothing in the statistics indicates that execution cost would be (much)
> different. Are there more ways to cut down on planning time? Obviously
> some plannedstatement/plannerinfo structures could be kept, but maybe
> it'd also be possible to plan only that part of the join tree where the
> params are used in a scan/join qual.

I think we should be careful not to over-think this. Planning isn't
*that* costly, so apply Amdahl's Law liberally. I'm proposing some easy
things we could do without adding much overhead or maintenance burden;
I've been assuming that getting intimate with the planner would risk
those advantages.


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