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Re: Memory usage during sorting

From: Peter Geoghegan <peter(at)2ndquadrant(dot)com>
To: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Memory usage during sorting
Date: 2012-01-22 00:51:58
Message-ID: (view raw or whole thread)
Lists: pgsql-hackers
On 16 January 2012 00:59, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
> I think it would be better to pre-deduct the tape overhead amount we
> will need if we decide to switch to tape sort from the availMem before
> we even start reading (and then add it back if we do indeed make that
> switch).  That way we wouldn't over-run the memory in the first place.
>  However, that would cause apparent regressions in which sorts that
> previously fit into maintenance_work_mem no longer do.  Boosting
> maintenance_work_mem to a level that was actually being used
> previously would fix those regressions, but pointing out that the
> previous behavior was not optimal doesn't change the fact that people
> are used to it and perhaps tuned to it.  So the attached patch seems
> more backwards-friendly.

Hmm. Are people really setting maintenance_work_mem such that it is
exactly large enough to quicksort when building an index in one case
but not another? Is the difference large enough to warrant avoiding

Peter Geoghegan
PostgreSQL Development, 24x7 Support, Training and Services

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