Re: Strange workaround for slow query

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> It does seem like once the materialize step is done we could notice
> that the tuplestore is empty and, given that uses no outer variables
> or parameters and therefore will never be re-executed, we could skip
> the rest of the index scan.

Yeah, the same thing occurred to me while looking at this example.

Right now, nodeNestloop is not really aware of whether the inner scan
depends on any parameters from the outer scan, so it's a bit hard to
determine whether the join can be abandoned. However, I have 9.1
plans to change that --- I'd like to get rid of the current
pass-the-outer-tuple-to-ReScan hack entirely, in favor of having
nodeNestloop explicitly set PARAM_EXEC parameters for the inner scan.
Once that's in, it would be pretty easy to apply this optimization.
(I've added a note to my private TODO file about it.)

Another possible objection is that if the inner scan has any volatile
functions in its quals, it might yield a different result upon rescan
even without parameters. However, since we are already willing to stick
a Materialize node on it at the whim of the planner, I don't see how
such a short-circuit in the executor would make things any worse.

regards, tom lane