On Mon, Jun 6, 2016 at 2:53 PM, Tom Lane <t...@sss.pgh.pa.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
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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