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From: Simon Riggs <simon@2ndQuadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Jonah H. Harris <jonah.harris@gmail.com>
Cc: Alex Hunsaker <badalex@gmail.com>
Cc: Kenneth Marshall <ktm@rice.edu>
Cc: Xiao Meng <mx.cogito@gmail.com>
Cc: Zdenek Kotala <Zdenek.Kotala@sun.com>
Cc: pgsql-patches@postgresql.org
Subject: Re: hash index improving v3
Date: Tue, 23 Sep 2008 14:34:39 +0100
Message-ID: <1222176879.4445.361.camel@ebony.2ndQuadrant> (raw)
In-Reply-To: <26331.1222175594@sss.pgh.pa.us>
References: <34d269d40809052249h59750f83v4b19a50d0555580@mail.gmail.com>
	<34d269d40809091823t204e252fl246443be71aed9ff@mail.gmail.com>
	<20080910130401.GG6714@it.is.rice.edu>
	<34d269d40809102049y2599a381kf409c0b5f18b6170@mail.gmail.com>
	<34d269d40809102117g470c4d25l55be7898fc43fc40@mail.gmail.com>
	<20080911152437.GM6714@it.is.rice.edu>
	<34d269d40809111951s7bac44cfk5dc9244693dd777c@mail.gmail.com>
	<20080912142921.GB26773@it.is.rice.edu>
	<7246.1221445012@sss.pgh.pa.us>
	<34d269d40809222025k2b2252dft89db471649c8eef8@mail.gmail.com>
	<36e682920809222030k49293e85jc57429296c5d2d76@mail.gmail.com>
	<17323.1222142759@sss.pgh.pa.us>
	<1222144262.4445.265.camel@ebony.2ndQuadrant>
	<18049.1222145316@sss.pgh.pa.us>
	<1222146936.4445.290.camel@ebony.2ndQuadrant>
	<25193.1222172194@sss.pgh.pa.us>
	<1222175115.4445.344.camel@ebony.2ndQuadrant>
	<26331.1222175594@sss.pgh.pa.us>


On Tue, 2008-09-23 at 09:13 -0400, Tom Lane wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
> > Thinks: Why not just sort all of the time and skip the debate entirely?
> 
> The sort is demonstrably a loser for smaller indexes.  Admittedly,
> if the index is small then the sort can't cost all that much, but if
> the (correct) threshold is some large fraction of shared_buffers then
> it could still take awhile on installations with lots-o-buffers.

The other realisation is that for large indexes, giving them more
maintenance_work_mem probably will make them build faster 'cos we'll be
sorting. So "give big indexes more memory" is still true *enough* to be
broadly consistent, explainable and understandable. I do explain things
in more detail on some courses, but pithy rules help busy people.

-- 
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support




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