Re: Set Returning Functions and array_agg()

[Stephen Scheck <singularsyntax@xxxxxxxxx>]

I'm guessing the reason is something like this: even though the "things" returned by these two statements are the same logical entity (from a mathematics/set theory standpoint):

pg_dev=# select * from unnest(array[1,2,3]);

 unnest 

--------

      1

      2

      3

(3 rows)

pg_dev=# select unnest(array[1,2,3]);

 unnest 

--------

      1

      2

      3

(3 rows)

The processing code-path for an aggregate function gets fed row-by-row and is not just handed a complete set to work on. That would explain why set-returning functions are allowed in the columns-clause (no general prohibition on that) but not passable to aggregate functions.

But then, shouldn't it be possible to write something like array_agg that takes a set as input and returns an array, that is not an aggregate function, and is callable from the columns-clause?

On Wed, Apr 24, 2013 at 2:26 PM, Stephen Scheck <singularsyntax@xxxxxxxxx> wrote:

Possibly due to my lack of thorough SQL understanding. Perhaps there's a better way of doing what I'm ultimately trying to accomplish, but still the question remains - why does this work:

pg_dev=# select unnest(array[1,2,3]);

 unnest 

--------

      1

      2

      3

(3 rows)

But not this:

pg_dev=# select array_agg(unnest(array[1,2,3]));

ERROR:  set-valued function called in context that cannot accept a set

The solution to the problem is actually of less interest right now then in understanding what's going on in the two statements above. It seems a bit inconsistent to me. If an aggregate function cannot handle rows generated in the columns-part of the statement, then why is a single-column row(s) result acceptable in the first statement?

On Wed, Apr 24, 2013 at 1:29 PM, hubert depesz lubaczewski <depesz@xxxxxxxxxx> wrote:

On Wed, Apr 24, 2013 at 12:48:44PM -0700, Stephen Scheck wrote:
> I have a UDF (written in C) that returns SETOF RECORD of an anonymous
> record type
> (defined via OUT parameters). I'm trying to use array_agg() to transform
> its output to
> an array:
> pg_dev=# SELECT array_agg((my_setof_record_returning_func()).col1);
> ERROR:  set-valued function called in context that cannot accept a set

Is there any reason why you're not using normal syntax:
select array_agg(col1) from my_setof_record_returning_func();
?

Best regards,

depesz