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Re: increasing collapse_limits?

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: increasing collapse_limits?
Date: 2011-04-30 19:38:53
Message-ID: C1156ACD-C74A-4F04-B011-AD374221B14D@gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Apr 30, 2011, at 7:38 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
>> Actually we had to solve a issue with slow SELECT. The problem was in
>> low value of JOIN_COLLAPSE_LIMITS. Can we increase a default of this
>> value. I checked some complex query, and planner needed about 200ms
>> for JOIN_COLLAPSE_LIMIT = 16. So some around 12 can be well.
> 
> I'd like to see a rather larger survey of cases before changing that.
> Also, amount of memory consumed is at least as large a concern here
> as runtime.

I seem to remember that I was the last one to suggest raising these limits and someone demonstrated rather convincingly that for certain classes of queries that would cause really big problems.

...Robert

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