To: Karl Denninger <karl@denninger.net>
cc: pgsql-performance@postgresql.org
Subject: Re: Performance issue with 8.2.3 - "C" application 
In-reply-to: <46A6A549.1040606@denninger.net> 
References: <46A6A549.1040606@denninger.net>
Comments: In-reply-to Karl Denninger <karl@denninger.net>
	message dated "Tue, 24 Jul 2007 20:20:09 -0500"
Date: Tue, 24 Jul 2007 22:45:58 -0400
Message-ID: <29633.1185331558@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Karl Denninger <karl@denninger.net> writes:
> But.... here's the query that has a habit of taking the most time....

> select forum, * from post where toppost = 1 and (replied > (select 
> lastview from forumlog where login='theuser' and forum=post.forum and 
> number is null)) is not false AND (replied > (select lastview from 
> forumlog where login='theuser' and forum=post.forum and 
> number=post.number)) is not f
> alse order by pinned desc, replied desc offset 0 limit 20

Did that ever perform well for you?  It's the sub-selects that are
likely to hurt ... in particular,

>          ->  Index Scan using post_top on post  (cost=0.00..57266.37 
> rows=113 width=757)
>                Index Cond: (toppost = 1)
>                Filter: (((replied > (subplan)) IS NOT FALSE) AND 
> ((replied > (subplan)) IS NOT FALSE))

versus

>  Index Scan using post_top on post  (cost=0.00..632.03 rows=1013 width=11)
>    Index Cond: (toppost = 1)

The planner thinks that the two subplan filter conditions will eliminate
about 90% of the rows returned by the bare indexscan (IIRC this is
purely a rule of thumb, not based on any statistics) and that testing
them 1013 times will add over 50000 cost units to the basic indexscan.
That part I believe --- correlated subqueries are expensive.

			regards, tom lane