Tom,
Yes, the distribution must be what's doing it. I guess I knew that subconciously, but was looking for something like hints to force the planner to do what I wanted. Instead it looks like I'll have to do a bit of tweaking with my indexes. Probably a partial index on the 'not_displayed_id' column. It'll be very small and shouldn't cause much overhead. I was trying to keep my index count down, and have had a dual-column index on (replaced_by_id, not_displayed_id) to this point.
Thanks once again for your help!
________________________________________
From: Tom Lane [tgl@xxxxxxxxxxxxx]
Sent: Tuesday, December 18, 2007 10:36 AM
To: Weber, Geoffrey M.
Cc: pgsql-general@xxxxxxxxxxxxxx
Subject: Re: Problem with index not being chosen inside PL/PgSQL function...
"Weber, Geoffrey M." <Geoffrey.Weber@xxxxxxxxxxxxx> writes:
> Hmm - good question! However, it is - both the id and
> not_displayed_id are INTEGERs.
Well, in that case it must be a statistics issue --- does the indexed
column have a badly skewed distribution?
You could investigate how many rows the planner thinks will be fetched
via
PREPARE foo(int) AS
SELECT ah.* FROM alert ah where ( (ah.replaced_by_id = '0') AND (not_displayed_id = $1 ) ) ORDER BY replaced_by_id, not_displayed_id;
EXPLAIN EXECUTE foo(42);
which will set up exactly the same planning situation as occurs in the
plpgsql function: no knowledge of the exact value being compared to.
regards, tom lane
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