=?iso-8859-15?Q?Pierre-Fr=E9d=E9ric_Caillaud?= <[EMAIL PROTECTED]> 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
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