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Re: Performance issue with 8.2.3 - "C" application

From: Karl Denninger <karl(at)denninger(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Performance issue with 8.2.3 - "C" application
Date: 2007-07-25 02:55:45
Message-ID: 46A6BBB1.1020502@denninger.net (view raw or flat)
Thread:
Lists: pgsql-performance
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(at)denninger(dot)net)
http://www.denninger.net




Tom Lane wrote:
> Karl Denninger <karl(at)denninger(dot)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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>   

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Next:From: Merlin MoncureDate: 2007-07-25 03:15:19
Subject: Re: Performance issue with 8.2.3 - "C" application
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