Re: Changed SRF in targetlist handling

On Mon, May 23, 2016 at 01:28:11PM -0500, Merlin Moncure wrote:
> On Mon, May 23, 2016 at 12:10 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > 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
> >> list.
> >
> > 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
> > node types.
> >
> >> 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.
>
> +1 on removing LCM.

As a green field project, that would make total sense. As a thing
decades in, it's not clear to me that that would break less stuff or
break it worse than simply disallowing SRFs in the target list, which
has been rejected on bugward-compatibility grounds. I suspect it
would be even worse because disallowing SRFs in target lists would at
least be obvious and localized when it broke code.

Cheers,
David.
--
David Fetter <david(at)fetter(dot)org> http://fetter.org/
Phone: +1 415 235 3778 AIM: dfetter666 Yahoo!: dfetter
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