Re: More tablescanning fun

On Fri, 25 Apr 2003 09:38:01 -0500, "Jim C. Nasby" <jim(at)nasby(dot)net>
wrote:
>In this case, the interpolation can't be at fault, because correlation
>is 1 (unless the interpolation is backwards, but that doesn't appear to
>be the case).

But your index has 3 columns which causes the index correlation to be
assumed as 1/3. So the interpolation uses 1/9 (correlation squared)
and you get a cost estimation that almost equals the upper bound.

If you want to play around with other interpolation methods, you might
want to get this patch: http://www.pivot.at/pg/16-correlation-732.diff

A short description of the GUC parameters introduced by this patch can
be found here:
http://archives.postgresql.org/pgsql-performance/2002-11/msg00256.php

As a short term workaround for an unmodified Postgres installation,
you can create an index on email_contrib(project_id).

Servus
Manfred