RE: (When) can a single SQL statement return multiple result sets?

-----Original Message-----
From: Jan Behrens <jbe-mlist(at)magnetkern(dot)de>
Sent: Wednesday, April 10, 2024 11:23 PM
To: pgsql-general(at)lists(dot)postgresql(dot)org
Subject: (When) can a single SQL statement return multiple result sets?

Hello,

While writing a PostgreSQL client library for Lua supporting Pipelining (using PQsendQueryParams), I have been wondering if there are any single SQL commands that return multiple result sets. It is indeed possible to create such a case by using the RULE system:

db=> CREATE VIEW magic AS SELECT;
CREATE VIEW
db=> CREATE RULE r1 AS ON DELETE TO magic
db-> DO INSTEAD SELECT 42 AS "answer";
CREATE RULE
db=> CREATE RULE r2 AS ON DELETE TO magic
db-> DO ALSO SELECT 'Hello' AS "col1", 'World!' AS "col2";
CREATE RULE
db=> DELETE FROM magic; -- single SQL statement!
answer
--------
42
(1 row)

col1 | col2
-------+--------
Hello | World!
(1 row)

DELETE 0

Here, "DELETE FROM magic" returns multiple result sets, even though it is only a single SQL statement.

(Note that this isn't possible with rules ON SELECT because it is only allowed to create a single SELECT rule on a view.)

The case outlined above seems to be a somewhat special case. I haven't found any other way to return multiple results (other than sending several semicolon-separated statements, which is not supported by PQsendQueryParams). So is there any (other) case where I reasonably should expect several result sets returned by PQgetResult (before PQgetResult returns NULL)? Wouldn't it make sense to disallow such behavior altogether? And if not, why can't I write a stored procedure or function that returns multiple result sets?

These questions are relevant to me because it may have an effect on the API design if a statement can return several result sets.

Kind regards,
Jan Behrens
-----Original Message-----

Hi, you can declare a function which returns multiple CURSORS...

RETURNS SETOF REFCURSOR

Then, in your function, you have to write something like this
DECLARE
rc_1 refcursor;
rc_2 refcursor;
rc_3 refcursor;
...
OPEN rc_1 FOR SELECT ...
OPEN rc_2 FOR SELECT ...
OPEN rc_3 FOR SELECT ...
RETURN NEXT rc_1;
RETURN NEXT rc_2;
RETURN NEXT rc_3;

Regards,