|From:||Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>|
|To:||Andres Freund <andres(at)anarazel(dot)de>|
|Subject:||Re: Changed SRF in targetlist handling|
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Andres Freund <andres(at)anarazel(dot)de> writes:
> discussing executor performance with a number of people at pgcon,
> several hackers - me included - complained about the additional
> complexity, both code and runtime, required to handle SRFs in the target
Yeah, this has been an annoyance for a long time.
> One idea I circulated was to fix that by interjecting a special executor
> node to process SRF containing targetlists (reusing Result possibly?).
> That'd allow to remove the isDone argument from ExecEval*/ExecProject*
> and get rid of ps_TupFromTlist which is fairly ugly.
Would that not lead to, in effect, duplicating all of execQual.c? The new
executor node would still have to be prepared to process all expression
> Robert suggested - IIRC mentioning previous on-list discussion - to
> instead rewrite targetlist SRFs into lateral joins. My gut feeling is
> that that'd be a larger undertaking, with significant semantics changes.
Yes, this was discussed on-list awhile back (I see David found a reference
already). I think it's feasible, although we'd first have to agree
whether we want to remain bug-compatible with the old
least-common-multiple-of-the-periods behavior. I would vote for not,
but it's certainly a debatable thing.
> If we accept bigger semantical changes, I'm inclined to instead just get
> rid of targetlist SRFs in total; they're really weird and not needed
This seems a bridge too far to me. It's just way too common to do
"select generate_series(1,n)". We could tell people they have to
rewrite to "select * from generate_series(1,n)", but it would be far
more polite to do that for them.
> One issue with removing targetlist SRFs is that they're currently
> considerably faster than SRFs in FROM:
I suspect that depends greatly on your test case. But in any case
we could put more effort into optimizing nodeFunctionscan.
regards, tom lane
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