On Fri, 2008-05-02 at 16:17 +0100, Heikki Linnakangas wrote:
> Simon Riggs wrote:
> > * We've said here http://www.postgresql.org/docs/faqs.TODO.html that we
> > "Don't want hints". If that's what we really think, then this patch must
> > surely be rejected because its a hint... That isn't my view. I *now*
> > think we do need hints of various kinds.
>
> cursors_tuple_fraction or OPTIMIZE FOR xxx ROWS isn't the kind of hints
> we've said "no" to in the past. We don't want hints that work-around
> planner deficiencies, for example where we get the row count of a node
> completely wrong. This is different. This is about telling how the
> application is going to use the result set. It's relevant even assuming
> that the planner got the estimates spot on.
Yes, thats what I see.
> Which plan is the best
> depends on whether the application can start processing the data as it
> comes in, or whether it's loading it all in memory first, for example.
Agreed, which is why people want to tell us that also, when they know
the answer in the context of their application.
--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com