Re: Performance issue with 8.2.3 - "C" application

От: Karl Denninger
Тема: Re: Performance issue with 8.2.3 - "C" application
Дата: ,
Msg-id: 46A6BBB1.1020502@denninger.net
(см: обсуждение, исходный текст)
Ответ на: Re: Performance issue with 8.2.3 - "C" application  (Tom Lane)
Ответы: Re: Performance issue with 8.2.3 - "C" application  ("Merlin Moncure")
Список: pgsql-performance

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Performance issue with 8.2.3 - "C" application  (Karl Denninger, )
 Re: Performance issue with 8.2.3 - "C" application  (Tom Lane, )
  Re: Performance issue with 8.2.3 - "C" application  (Karl Denninger, )
   Re: Performance issue with 8.2.3 - "C" application  ("Merlin Moncure", )
    Re: Performance issue with 8.2.3 - "C" application  (Karl Denninger, )
     Re: Performance issue with 8.2.3 - "C" application  (Tom Lane, )
      Re: Performance issue with 8.2.3 - "C" application  (Karl Denninger, )
       Re: Performance issue with 8.2.3 - "C" application  (Karl Denninger, )
 Re: Performance issue with 8.2.3 - "C" application  (Nis Jørgensen, )
 Re: Performance issue with 8.2.3 - "C" application  (Gregory Stark, )

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 ()
http://www.denninger.net



Tom Lane wrote:
Karl Denninger <> 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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