To: Daniele Varrazzo <piro@develer.com>
cc: pgsql-performance@postgresql.org
Subject: Re: An "obvious" index not being used 
In-reply-to: <4859B13F.3070103@develer.com> 
References: <4859B13F.3070103@develer.com>
Comments: In-reply-to Daniele Varrazzo <piro@develer.com>
	message dated "Thu, 19 Jun 2008 02:07:11 +0100"
Date: Wed, 18 Jun 2008 21:43:34 -0400
Message-ID: <10923.1213839814@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Daniele Varrazzo <piro@develer.com> writes:
> There is an index in the field "foo.account_id" but is not used. The resulting 
> query plan is:

>   Aggregate  (cost=300940.70..300940.71 rows=1 width=0) (actual
> time=13412.088..13412.089 rows=1 loops=1)
>     ->  Hash IN Join  (cost=11.97..299858.32 rows=432953 width=0) (actual
> time=0.678..13307.074 rows=92790 loops=1)
>           Hash Cond: (foo.account_id = accounts.id)
>           ->  Seq Scan on foo  (cost=0.00..275591.14 rows=5313514 width=4)
> (actual time=0.014..7163.538 rows=5313514 loops=1)

Well, if the estimate of 432953 rows selected were correct, it'd be
right not to use the index.  Fetching one row in ten is not a chore
for an indexscan.  (I'm not sure it'd prefer an indexscan even with an
accurate 92K-row estimate, but at least you'd be in the realm where
tweaking random_page_cost would make a difference.)

I'm not sure why that estimate is so bad, given that you said you
increased the stats target on the table.  Is there anything particularly
skewed about the distribution of the account IDs?

			regards, tom lane