Re: Assert single row returning SQL-standard functions - Mailing list pgsql-hackers

On Fri, Aug 29, 2025, at 10:30, Pavel Stehule wrote:
> pá 29. 8. 2025 v 10:16 odesílatel Joel Jacobson <joel@compiler.org> napsal:
>> Can we think of some SQL-standard function way to also prevent against 0 rows?
>>
>
> I am afraid there is not nothing. NULL is the correct result in SQL.
> SQL allow to check ROW_COUNT by using GET DIAGNOSTICS commands and
> raising an error when something is unexpected
>
> I can imagine allowing the NOT NULL flag for functions, and then the
> result can be checked on NOT NULL value.

I like the idea of a NOT NULL flag for functions.
What syntax could we image for that?

Regarding DML functions, could we make the RETURN () trick work somehow?

Here is a failed attempt:

CREATE OR REPLACE FUNCTION test_update(_a int)
RETURNS bool
RETURN (
    WITH update_cte AS (
        UPDATE footab SET id = _a WHERE footab.id = _a RETURNING footab.id
    )
    SELECT id FROM update_cte
);

ERROR:  WITH clause containing a data-modifying statement must be at the top level
LINE 4:     WITH update_cte AS (
                 ^

I'm not sure if this is a standard requirement, or if it's just a PostgreSQL-specific limitation?

SELECT oldtbl.empno FROM OLD TABLE (DELETE FROM emp WHERE deptno = 2) AS oldtbl;
SELECT newtbl.empno FROM NEW TABLE (UPDATE emp SET salary = 0 WHERE deptno = 2) AS newtbl;

/Joel