Re: Performance of ORDER BY

[Glenn Sullivan <glenn.sullivan@xxxxxxxxxxxxx>]

Thanks to Luke and Tom for the input.  I guess this was good timing given that it looks like
8.2 was just released today.   I will upgade to that before doing anything else.

Glenn

Tom Lane wrote:

Glenn Sullivan <glenn.sullivan@xxxxxxxxxxxxx> writes:
  

I am wanting some ideas about improving the performance of ORDER BY in
our use.  I have a DB on the order of 500,000 rows and 50 columns.
The results are always sorted with ORDER BY.  Sometimes, the users end up
with a search that matches most of the rows.  In that case, I have a
LIMIT 5000 to keep the returned results under control.  However, the
sorting seems to take 10-60 sec.  If I do the same search without the
ORDER BY, it takes about a second.  
    

Does the ORDER BY match an index?  If so, is it using the index?
(See EXPLAIN.)

  

I am currently on version 8.0.1 on Windows XP using a Dell Optiplex 280
with 1Gb of ram.  I have set sort_mem=100000 set.
    

In 8.0 that might be counterproductively high --- we have seen cases
where more sort_mem = slower with the older sorting code.  I concur
with Luke's advice that you should update to 8.2 (not 8.1) to get the
improved sorting code.

			regards, tom lane