To: Craig Ringer <craig@postnewspapers.com.au>
cc: Einars <einars@gmail.com>, pgsql-performance@postgresql.org
Subject: Re: Chaotically weird execution plan 
In-reply-to: <48D9943B.9060801@postnewspapers.com.au> 
References: <4f8bd5c90809231753xbcdacd2xfba4d38c2eb34faf@mail.gmail.com>
	<48D9943B.9060801@postnewspapers.com.au>
Comments: In-reply-to Craig Ringer <craig@postnewspapers.com.au>
	message dated "Wed, 24 Sep 2008 09:13:31 +0800"
Date: Tue, 23 Sep 2008 23:01:10 -0400
Message-ID: <17643.1222225270@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Craig Ringer <craig@postnewspapers.com.au> writes:
> I'd already written: "If you need the test for status = 1, consider a
> partial index" when I noticed your schema definition:

>> "comments_created_by" btree (created_by) WHERE status = 1

> I find it hard to guess why it's having to recheck the WHERE clause
> given the use of a partial index that should cover that nicely.

No, that's operating as designed.  A bitmap scan's RECHECK condition
is only applied when the bitmap has become lossy due to memory
pressure.  In that case we have to look at each row on each of the pages
fingered by the index as containing possible matches ... and we'd better
check the partial-index qual too, since maybe not all the rows on those
pages will satisfy it.  In a plain indexscan there is no lossiness
involved and so the partial-index qual need never be rechecked.

			regards, tom lane