From: Karl Denninger <karl@denninger.net>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance@postgresql.org
Subject: Re: Performance issue with 8.2.3 - "C" application
Date: Tue, 24 Jul 2007 21:55:45 -0500
Message-ID: <46A6BBB1.1020502@denninger.net> (raw)
In-Reply-To: <29633.1185331558@sss.pgh.pa.us>
References: <46A6A549.1040606@denninger.net>
<29633.1185331558@sss.pgh.pa.us>
Yeah, the problem doesn't appear to be there. As I said, if I look at
the PS of the system when its bogging, there aren't a whole bunch of
processes stuck doing these, so while this does take a second or two to
come back, that's not that bad.
Its GENERAL performance that just bites - the system is obviously out of
CPU, but what I can't get a handle on is WHY. It does not appear to be
accumulating large amounts of runtime in processes I can catch, but the
load average is quite high.
This is why I'm wondering if what I'm taking here is a hit on the
fork/exec inside the portmaster, in the setup internally in there, in
the IPC between my process via libPQ, etc - and how I can profile what's
going on.
Karl Denninger (karl@denninger.net)
http://www.denninger.net
Tom Lane wrote:
> 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
>
> ---------------------------(end of broadcast)---------------------------
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>
>
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>
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To: pgsql-performance@postgresql.org
Cc: karl@denninger.net, tgl@sss.pgh.pa.us
Subject: Re: Performance issue with 8.2.3 - "C" application
In-Reply-To: <46A6BBB1.1020502@denninger.net>
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