To: Christian Brink <cbrink@r-stream.com>
cc: pgsql-performance@postgresql.org
Subject: Re: PostgreSQL upgraded to 8.2 but forcing index scan on query
	produces faster
In-reply-to: <4BA7C057.9090909@r-stream.com> 
References: <4BA7C057.9090909@r-stream.com>
Comments: In-reply-to Christian Brink <cbrink@r-stream.com>
	message dated "Mon, 22 Mar 2010 15:09:11 -0400"
Date: Mon, 22 Mar 2010 15:21:35 -0400
Message-ID: <12444.1269285695@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Christian Brink <cbrink@r-stream.com> writes:
>           ->  Nested Loop  (cost=0.01..2416.59 rows=22477 width=4) 
> (actual time=0.595..2.150 rows=225 loops=1)
>                 ->  Index Scan using sysstrings_pkey on sysstrings  
> (cost=0.00..8.27 rows=1 width=182) (actual time=0.110..0.112 rows=1 loops=1)
>                       Index Cond: (id = 'net/Console/Employee/Day End 
> Time'::text)
>                 ->  Index Scan using sales_tranzdate_index on sales s  
> (cost=0.01..1846.40 rows=22477 width=12) (actual time=0.454..1.687 
> rows=225 loops=1)
>                       Index Cond: ((s.tranzdate >= ('2010-02-15'::date + 
> (sysstrings.data)::time without time zone)) AND (s.tranzdate < 
> ('2010-02-16'::date + (sysstrings.data)::time without time zone)))
>                       Filter: ((NOT void) AND (NOT suspended))

The fundamental reason why you're getting a bad plan choice is the
factor-of-100 estimation error here.  I'm not sure you can do a whole
lot about that without rethinking the query --- in particular I would
suggest trying to get rid of the non-constant range bounds.  You're
apparently already plugging in an external variable for the date,
so maybe you could handle the time of day similarly instead of joining
to sysstrings for it.

			regards, tom lane