Re: Changed SRF in targetlist handling

On Mon, Jun 6, 2016 at 2:53 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Now, if we decide to try to rewrite tlist SRFs as LATERAL, it would likely
> behoove us to do that rewrite before expanding * not after, so that we can
> eliminate the multiple evaluation of foo() that happens currently. (That
> makes it a parser problem not a planner problem.) And maybe we should
> rewrite non-SRF composite-returning functions this way too, because people
> have definitely complained about the extra evaluations in that context.
> But my point here is that lockstep evaluation does have practical use
> when the SRFs are iterating over matching collections of generated rows.
> And that seems like a pretty common use-case.

Yeah, OK. I'm not terribly opposed to going that way. I think the
current behavior sucks badly enough - both because the semantics are
bizarre and because it complicates the whole executor for a niche
feature - that it's worth taking a backward compatibility hit to
change it. I guess I'd prefer #2 to #2.5, #2.5 to #3, and #3 to #1.
I really don't like #1 much - I think I'd almost rather do nothing.

--
Robert Haas
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