Re: Strangely Variable Query Performance

Okay -- I started leaving indexes on one by one.

The explain broke when the detail_summary_receipt_encounter_idx index was
left on (receipt, encounter_id).

Just dropping that index had no effect, but there's a LOT of indexes that
refer to receipt. So on a hunch I tried dropping all indexes that refer
to receipt date and that worked -- so it's the indexes that contain
receipt date that are teh problem.

For more fun, I tried leaving the index that's just receipt date alone
(index detail_summary_receipt_id_idx) and THAT produced the correct query;
it's all these multi-column queries that are fouling things up, it would
seem!

.... So does this mean I should experiment with dropping those indexes?
I'm not sure if that will result in 'bad things' as there are other
complicated actions like sorts that may go real slow if I drop those
indexes. BUT I think it'll be easy to convince the customer to drop their
absurdly complicated sorts if I can come back with serious results like
what we've worked out here.

And thanks again -- have a good dinner! :)

Steve

On Thu, 12 Apr 2007, Tom Lane wrote:

> Steve <cheetah(at)tanabi(dot)org> writes:
>> Either way, it runs perfectly fast. So it looks like the indexes are
>> confusing this query like you suspected. Any advise?
>
> Wow --- sometimes grasping at straws pays off. I was testing here with
> just a subset of the indexes to save build time, but I bet that one of
> the "irrelevant" ones is affecting this somehow. Time to re-test.
>
> If you have some time to kill, it might be interesting to vary that
> begin/rollback test script to leave one or two other indexes in place,
> and see if you can identify exactly which other index(es) get it
> confused.
>
> I'm about to go out to dinner with the wife, but will have a closer
> look when I get back, or tomorrow morning. We'll figure this out.
>
> regards, tom lane
>
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