Re: The usual sequential scan, but with LIMIT !

    Yes, you're right as usual.
    I had not thought about playing with ORDER BY on a field which has only
one value in the result set.


> 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.