Message-ID: <433C1C2D.7090908@agliodbs.com>
Date: Thu, 29 Sep 2005 09:54:05 -0700
From: Josh Berkus <josh@agliodbs.com>
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To: "Jeffrey W. Baker" <jwbaker@acm.org>
Cc: Ron Peacetree <rjpeace@earthlink.net>,
	pgsql-hackers@postgresql.org, pgsql-performance@postgresql.org
Subject: Re: [PERFORM] A Better External Sort?
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Jeff, Ron,

First off, Jeff, please take it easy.  We're discussing 8.2 features at 
this point and there's no reason to get stressed out at Ron.  You can 
get plenty stressed out when 8.2 is near feature freeze.  ;-)


Regarding use cases for better sorts:

The biggest single area where I see PostgreSQL external sort sucking is 
  on index creation on large tables.   For example, for free version of 
TPCH, it takes only 1.5 hours to load a 60GB Lineitem table on OSDL's 
hardware, but over 3 hours to create each index on that table.  This 
means that over all our load into TPCH takes 4 times as long to create 
the indexes as it did to bulk load the data.

Anyone restoring a large database from pg_dump is in the same situation. 
  Even worse, if you have to create a new index on a large table on a 
production database in use, because the I/O from the index creation 
swamps everything.

Following an index creation, we see that 95% of the time required is the 
external sort, which averages 2mb/s.  This is with seperate drives for 
the WAL, the pg_tmp, the table and the index.  I've confirmed that 
increasing work_mem beyond a small minimum (around 128mb) had no benefit 
on the overall index creation speed.


--Josh Berkus