Re: Planner matching constants across tables in a join

Richard Huxton <dev@archonet.com> writes:

> I know this has been covered on one of the lists in the past, but I'm damned
> if I can find the keywords to locate it.
>
> If I join two tables with a comparison to a constant on one, why can't the
> planner see that the comparison applies to both tables:

It sure does. Postgres does an impressive job of tracing equality clauses
around for just this purpose.

> SELECT a.id FROM a JOIN b ON a.id=b.id WHERE a.id=1;
>
> runs much slower than
>
> SELECT a.id FROM a JOIN b ON a.id=b.id WHERE a.id=1 AND b.id=1;

Really? They produce virtually the same plan for me.

Why do you think it'll run slower?
What query are you actually finding slow?

--
greg