To: Simon Riggs <simon@2ndquadrant.com>
Cc: pgsql-hackers@postgresql.org, pgsql-performance@postgresql.org
Subject: Re: Releasing memory during External sorting? 
In-reply-to: <1127468233.4145.178.camel@localhost.localdomain> 
References: <1127468233.4145.178.camel@localhost.localdomain>
Comments: In-reply-to Simon Riggs <simon@2ndquadrant.com>
	message dated "Fri, 23 Sep 2005 10:37:12 +0100"
Date: Fri, 23 Sep 2005 10:09:39 -0400
Message-ID: <3465.1127484579@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Simon Riggs <simon@2ndquadrant.com> writes:
> If not, I would propose that when we move from qsort to tapesort mode we
> free the larger work_mem setting (if one exists) and allocate only a
> lower, though still optimal setting for the tapesort. That way the
> memory can be freed for use by other users or the OS while the tapesort
> proceeds (which is usually quite a while...).

On most platforms it's quite unlikely that any memory would actually get
released back to the OS before transaction end, because the memory
blocks belonging to the tuplesort context will be intermixed with blocks
belonging to other contexts.  So I think this is pretty pointless.
(If you can't afford to have the sort using all of sort_mem, you've set
sort_mem too large, anyway.)

			regards, tom lane