To: "Benjamin Krajmalnik" <kraj@servoyant.com>
cc: pgsql-performance@postgresql.org
Subject: Re: Quesion on the use of indexes 
In-reply-to: <F4E6A2751A2823418A21D4A160B689887B0362@fletch.stackdump.local> 
References: <F4E6A2751A2823418A21D4A160B689887B0362@fletch.stackdump.local>
Comments: In-reply-to "Benjamin Krajmalnik" <kraj@servoyant.com>
	message dated "Mon, 16 Aug 2010 21:22:57 -0600"
Date: Mon, 16 Aug 2010 23:33:29 -0400
Message-ID: <29298.1282016009@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

"Benjamin Krajmalnik" <kraj@servoyant.com> writes:
> A little background - I have various multi-column indexes whenever I
> have queries which restrict the output based on the values of the 2
> fields (for example, a client code and the date of a transaction).

> Is there a performance gain using this approach as opposed to using 2
> separate indexes, one on the first column and one on the second column?

Maybe, maybe not ... it's going to depend on a bunch of factors, one of
which is what your update load is like compared to the queries that read
the indexes.  There's a bit of coverage of this in the fine manual: see
http://www.postgresql.org/docs/8.4/static/indexes-multicolumn.html
and the next few pages.

The short of it is that there's no substitute for doing your own
experiments for your own application ...

			regards, tom lane