Re: Performance issue with 8.2.3 - "C" application

Karl Denninger ()
http://www.denninger.net

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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