From: | "Florian G(dot) Pflug" <fgp(at)phlo(dot)org> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Rod Taylor <pg(at)rbt(dot)ca>, Simon Riggs <simon(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org, Martin Lesser <ml-pgsql(at)bettercom(dot)de> |
Subject: | Re: "Constraint exclusion" is not general enough |
Date: | 2006-08-08 08:17:18 |
Message-ID: | 44D8488E.10206@phlo.org |
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Lists: | pgsql-hackers |
Tom Lane wrote:
> "Florian G. Pflug" <fgp(at)phlo(dot)org> writes:
>> Tom Lane wrote:
>>> But you don't have any cost numbers until after you've done the plan.
>
>> Couldn't this work similar to geqo_effort? The planner could
>> try planning the query using only cheap algorithmns, and if
>> the cost exceeds a certain value, it'd restart, and use
>> more sophisticated methods.
>
> AFAICS this would be a net loss on average. Most of the time, the
> constraint exclusion code doesn't win, and so throwing away all your
> planning work to try it is going to be a loser most of the time.
On the other hand, if the "consider-replanning" threshold is high enough,
than that additional time really doesn't matter - If a query runs for minutes,
or even hours, a few wasted cycles during planning don't hurt.
greetings, Florian Pflug
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