Re: Poor Performance with Distinct Subqueries with EXISTS and EXCEPT - Mailing list pgsql-general

=?iso-8859-15?Q?Pierre-Fr=E9d=E9ric_Caillaud?= <lists@boutiquenumerique.com> writes:
>     I may be mistaken, but you may be doing the same thing twice : you're
> basically writing :

>     SELECT DISTINCT X WHERE Y EXCEPT SELECT DISTINCT X WHERE NOT Y
>     Is this not a way to get an empty result set ?

No, because some X values may appear in rows where Y, and also in rows
where NOT Y.

The DISTINCTs are wastes of time, though, because EXCEPT implies
elimination of duplicates.

            regards, tom lane