Re: 512,600ms query becomes 7500ms... but why? Postgres 8.3 query planner quirk?

Karl Denninger <karl@denninger.net> wrote:
Kevin Grittner wrote:

>> I suspect that the above might do pretty well in 8.4.

> "Exists" can be quite slow.  So can "not exists"
>
> See if you can re-write it using a sub-select - just replace the
> "exists ...." with "(select ...) is not null"
>
> Surprisingly this often results in a MUCH better query plan under
> Postgresql.  Why the planner evaluates it "better" eludes me (it
> shouldn't) but the differences are often STRIKING - I've seen
> factor-of-10 differences in execution performance.

Have you seen such a difference under 8.4?  Can you provide a
self-contained example?

-Kevin