On Wed, Jul 28, 2010 at 6:55 AM, Dimitri Fontaine
<dfontaine@hi-media.com> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> But here you want to have different paths for
>> the same relation that generate *different output*, and the planner
>> doesn't understand that concept.
>
> Sorry? I though what Equivalence Class provides is the "proving" that
> using this qualification or another will *not* affect the output.
In a query like...
SELECT d1.ID, d2.IDFROM DocPrimary d1 JOIN DocPrimary d2 ON d2.BasedOn=d1.IDWHERE (d1.ID=234409763) or
(d2.ID=234409763)
...you're going to scan d1, scan d2, and then join the results. The
scan of d1 is going to produce different results depending on whether
you evaluate or not d1.ID=234409763, and the scan of d2 is going to
produce different results depending on whether or not you evaluate
d2.BasedOn=234409763.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company
