Re: Endgame for all those SELECT FOR UPDATE changes: fix plan node order

Tom Lane escribió:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:

> > This seems like it could potentially introduce a performance
> > regression, but the current behavior is so bizarre that it seems like
> > we should still change it.
>
> Yeah, it could definitely run slower than the existing code --- in
> particular the combination of all three (FOR UPDATE ORDER BY LIMIT)
> would tend to become a seqscan-and-sort rather than possibly just
> reading one end of an index. However, I quote the old aphorism that
> it can be made indefinitely fast if it doesn't have to give the right
> answer. The reason the current behavior is fast is it's giving the
> wrong answer :-(

So this probably merits a warning in the release notes for people to
check that their queries continue to run with the performance they
expect.

--
Alvaro Herrera http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support