From: Tom Lane <tgl@sss.pgh.pa.us>
To: Karl Denninger <karl@denninger.net>
Cc: pgsql-performance@postgresql.org
Subject: Re: Performance issue with 8.2.3 - "C" application
Date: Tue, 24 Jul 2007 22:45:58 -0400
Message-ID: <29633.1185331558@sss.pgh.pa.us> (raw)
In-Reply-To: <46A6A549.1040606@denninger.net>
References: <46A6A549.1040606@denninger.net>
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
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, karl@denninger.net
Subject: Re: Performance issue with 8.2.3 - "C" application
In-Reply-To: <29633.1185331558@sss.pgh.pa.us>
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