Re: The usual sequential scan, but with LIMIT ! - Mailing list pgsql-performance

=?iso-8859-15?Q?Pierre-Fr=E9d=E9ric_Caillaud?= <lists@boutiquenumerique.com> writes:
>     suppose I SELECT WHERE topic_id=2 ORDER BY topic_id ASC,id DESC.
>     Postgres does a seq scan, but it could think a bit more and start at
> "first index node which has topic_id>2" (simple to find in a btree) then
> go backwards in the index.

If you write it as
    SELECT WHERE topic_id=2 ORDER BY topic_id DESC,id DESC.
then an index on (topic_id, id) will work fine.  The mixed ASC/DESC
ordering is not compatible with the index.

            regards, tom lane