Horton

Re: Various performance questions

Neil Conway <neilc@samurai.com>

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From: Neil Conway <neilc@samurai.com>
To: Tarhon-Onu Victor <mituc@iasi.rdsnet.ro>
Cc: Dror Matalon <dror@zapatec.com>, PostgreSQL Performance <pgsql-performance@postgresql.org>
Date: 2003-10-27T15:34:53Z
Lists: pgsql-performance 
On Mon, 2003-10-27 at 10:15, Tarhon-Onu Victor wrote:
> select count(*) from items where channel < 
> 5000; will never use any of the current indexes because none matches 
> your WHERE clause (channel appears now only in multicolumn indexes).

No -- a multi-column index can be used to answer queries on a prefix of
the index's column list. So an index on (channel, xyz) can be used to
answer queries on (just) "channel".

-Neil