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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 13:04:46
Message-ID: 603c8f071002110504yb07519do9dd258c5a0ee6143@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, Feb 11, 2010 at 7:48 AM, Bart Samwel <bart(at)samwel(dot)tk> wrote:
> On Thu, Feb 11, 2010 at 13:41, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>
>> On Thu, Feb 11, 2010 at 7:39 AM, Bart Samwel <bart(at)samwel(dot)tk> wrote:
>> > 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.
>
> But I guess you can probably expect it to be on the same order for the same
> query in generic form and with filled-in parameters?

I think so.... but I wouldn't bet the farm on it without testing.

> 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, so the other assumption is that the planning-to-execution
ratio is more or less constant.  Whether that's sufficiently true to
make the proposed system useful and reliable is not clear to me.

...Robert

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