To: "Tyrrill, Ed" <tyrrill_ed@emc.com>
cc: "Scott Marlowe" <smarlowe@g2switchworks.com>,
	pgsql-performance@postgresql.org
Subject: Re: Slow queries on big table 
In-reply-to: <A23190A408F7094FAF446C1538222F7603EE4372@avaexch01.avamar.com> 
References: <A23190A408F7094FAF446C1538222F7603EE4245@avaexch01.avamar.com>
	<464E0036.3030804@g2switchworks.com>
	<1825.1179518769@sss.pgh.pa.us>
	<A23190A408F7094FAF446C1538222F7603EE4372@avaexch01.avamar.com>
Comments: In-reply-to "Tyrrill, Ed" <tyrrill_ed@emc.com>
	message dated "Fri, 18 May 2007 14:22:52 -0700"
Date: Fri, 18 May 2007 17:36:40 -0400
Message-ID: <3126.1179524200@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

"Tyrrill, Ed" <tyrrill_ed@emc.com> writes:
> Tom Lane <tgl@sss.pgh.pa.us> writes:
>> This combination of indexes:
>>
>>> Indexes:
>>> "backup_location_pkey" PRIMARY KEY, btree (record_id, backup_id)
>>> "backup_location_rid" btree (record_id)
>>
>> is really just silly.  You should have the pkey and then an index on
>> backup_id alone.

> Thanks for the help guys!  That was my problem.  I actually need the
> backup_location_rid index for a different query so I am going to keep
> it.

Well, you don't really *need* it; the two-column index on (record_id,
backup_id) will serve perfectly well for queries on its leading column
alone.  It'll be physically bigger and hence slightly slower to scan
than a single-column index; but unless the table is almost completely
read-only, the update overhead of maintaining all three indexes is
probably going to cost more than you can save with it.  Try that other
query with and without backup_location_rid and see how much you're
really saving.

>  Index Scan using backup_location_bid on backup_location
> (cost=0.00..9573.07 rows=415897 width=8) (actual time=0.106..3.486
> rows=2752 loops=1)
>    Index Cond: (backup_id = 1070)
>  Total runtime: 4.951 ms

That's more like it ;-)

			regards, tom lane