"mikelin" <mikelin.ca@xxxxxxxxx> writes: > which sounds like caching, so I created an index on that function, > expecting stellar performance, but the performance turned out to be > pretty bad: > words=# explain analyse select * from word order by > word_difficulty(word) limit 100; > I wouldn't have expected that Index Scan to be so slow. The index scan actually is not slow; the problem is that the planner neglects to suppress the computation of the sort-key columns in the output rows, even though they're not referenced anywhere. Normally that doesn't matter a whole lot, but if it's a really expensive function then it does matter. We just noticed this problem a few weeks ago: http://archives.postgresql.org/pgsql-performance/2006-11/msg00054.php I'm hoping to fix this for 8.3, but suspect that the fix will be too invasive to consider back-patching to older releases. regards, tom lane
