Re: EXCEPT performace - Mailing list pgsql-general

"K. Ari Krupnikov" <ari@cogsci.ed.ac.uk> writes:
> I noticed an unexpected difference in performance between seemingly equivalent queries:
> SELECT id FROM short_table EXCEPT SELECT id FROM long_table;

> and

> SELECT id FROM short_table EXCEPT
>     (SELECT id FROM short_table JOIN long_table
>      ON short_table.id = long_table.id);

But they're not equivalent.  Consider what happens when NULLs are
present in both tables.

            regards, tom lane