Re: Performance of ORDER BY

Glenn Sullivan <glenn(dot)sullivan(at)varianinc(dot)com> 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