Re: Implement targetlist SRFs using ROWS FROM() (was Changed SRF in targetlist handling)

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Andres Freund <andres(at)anarazel(dot)de>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Implement targetlist SRFs using ROWS FROM() (was Changed SRF in targetlist handling)
Date: 2017-01-19 00:08:04
Message-ID: 17806.1484784484@sss.pgh.pa.us
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Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> So, one of the big reasons I use CASE is to avoid evaluating
> expressions in cases where they might throw an ERROR. Like, you know:
> CASE WHEN d != 0 THEN n / d ELSE NULL END
> I guess it's not the end of the world if that only works for
> non-set-returning functions, but it's something to think about.

Well, refusing CASE-containing-SRF at all isn't going to make your
life any better in that regard :-(

It's possibly worth noting that this is also true for aggregates and
window functions: wrapping those in a CASE doesn't stop them from being
evaluated, either. People seem to be generally used to that, although
I think I've seen one or two complaints about it from folks who seemed
unclear on the concept of aggregates.

In the end I think this is mostly about backwards compatibility:
are we sufficiently worried about that that we'd rather throw an
error than have a silent change of behavior? TBH I'm not sure.
We've certainly got two other silent changes of behavior in this
same patch. The argument for treating this one differently,
I think, is that it's changing from a less surprising behavior
to a more surprising one whereas the other changes are the reverse,
or at worst neutral.

regards, tom lane

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