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@xxxxxxxxxxxxx> 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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