Re: [HACKERS] Query fails when SRFs are part of FROM clause (Commit id: 69f4b9c85f)

Andres Freund <andres@anarazel.de> writes:
> On 2017-01-27 17:58:04 +0530, Rushabh Lathia wrote:
>> SELECT  *
>> FROM pg_constraint pc,
>> CAST(CASE WHEN pc.contype IN ('f','u','p') THEN generate_series(1,
>> array_upper(pc.conkey, 1)) ELSE NULL END AS int) AS position;
>> 
>> Above query is failing with "set-valued function called in context that
>> cannot accept a set".

> I think that's correct. Functions in FROM are essentially a shorthand
> for ROWS FROM(). And ROWS FROM doesn't allow arbitrary expressions.

No, but it allows whatever looks syntactically like a function, including
casts.  IIRC, we made func_expr work that way ages ago to deflect
complaints that it wasn't very clear why some things-that-look-like-
functions were allowed in CREATE INDEX and others not.

> If, I didn't check, that worked previously, I think that was more
> accident than intent.

Yeah, probably.  But are we prepared to break working queries?
As I understood it, the agreement on this whole tlist-SRF change
was that we would not change any behavior that wasn't ill-defined.

We could probably fix this with the modification that was discussed
previously, to allow FunctionScan nodes to project a scalar tlist
from the outputs of their SRFs.
        regards, tom lane