Re: Different execution plans for semantically equivalent queries

Mikkel Lauritsen <renard@tala.dk> writes:
> I would like to do a query which retrieves the newest record for each
> type, and the persistence framework that I'm using does something
> which is structurally like

> SELECT * FROM table t1 WHERE 0 = (SELECT COUNT(*) FROM table t2 WHERE
>     t2.type = t1.type AND t2.timestamp > t1.timestamp)

I suspect that *any* database is going to have trouble optimizing that.
You'd be well advised to lobby the persistence framework's authors to
produce less brain-dead SQL.  The NOT EXISTS formulation seems to
express what's wanted much less indirectly.

            regards, tom lane