Thanks, Tom. Looks like that was the issue.
I changed the function to use groupid = 57925 instead of groupid = $1
(I can do the same change in the JDBC prepare statement), and the
performance is much better.
It is still more than twice that of the simple query: 401.111 ms vs.
155.544 ms, which, however, is more acceptable than 3000ms.
Will upgrade to 8.1 at some point, but would like to get reasonable
performance with 7.4 until then. I did increase the statistics target
to 1000.
Claire
On Jan 28, 2008, at 12:51 PM, Tom Lane wrote:
> Claire McLister <mclister@zeesource.net> writes:
>> When I do an EXPLAIN ANALYZE on one query that returns 3261 rows, it
>> executes in a reasonable 159ms:
>> ...
>> If I issue the same query over JDBC or use a PSQL stored procedure,
>> it
>> takes over 3000 ms, which, of course is unacceptable!
>
> I suspect that the problem is with "groupid = $1" instead of
> "groupid = 57925". The planner is probably avoiding an indexscan
> in the parameterized case because it's guessing the actual value will
> match so many rows as to make a seqscan faster. Is the distribution
> of groupid highly skewed? You might get better results if you
> increase
> the statistics target for that column.
>
> Switching to something newer than 7.4.x might help too. 8.1 and up
> support "bitmap" indexscans which work much better for large numbers
> of hits, and correspondingly the planner will use one in cases where
> it wouldn't use a plain indexscan.
>
> regards, tom lane
>
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