From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Ben <bench(at)silentmedia(dot)com> |
Cc: | Bill Moran <wmoran(at)collaborativefusion(dot)com>, pgsql-performance(at)postgresql(dot)org |
Subject: | Re: quickly getting the top N rows |
Date: | 2007-10-04 18:52:10 |
Message-ID: | 3785.1191523930@sss.pgh.pa.us |
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Lists: | pgsql-performance |
Ben <bench(at)silentmedia(dot)com> writes:
> No, the tables are recently analyzed and there are a couple hundred
> thousand rows in there. But I think I just figured it out.... it's a
> 3-column index, and two columns of that index are the same for every row.
> When I drop those two columns from the ordering restriction, the index
> gets used and things speed up 5 orders of magnitude.
> Maybe the planner is smart enough to think that if a column in the order
> by clause is identical for most rows, then using an index won't help....
> but not smart enough to realize that if said column is at the *end* of the
> order by arguments, after columns which do sort quite well, then it should
> use an index after all.
You're being about as clear as mud here, except that you obviously lied
about what you were doing in your first message. If you have a planner
problem, show us the *exact* query, the *exact* table definition, and
unfaked EXPLAIN ANALYZE output.
regards, tom lane
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