Postgresql 9.4: How to find out the stack level of nested function

Question

Here are the two cases:

• Function fA fetches some rows and returns a table
• Function fB calls function fA and returns a table

Within fA, how could I know whether it's been called from the program or by fB?

Is there some @@nestlevel that can count how deep in the stack are the functions running?

Big thank you in advance,

• Craig Ringer: I'm not aware of any good way to do this. Access to the call stack via GET DISAGNOSTICS has been discussed and may be implemented for 9.6. Ask on pgsql-general and suggest it as a feature - or even better, implement it and send in a patch to plpgsql.

Answer 1

There's a different, simpler, solution for this. Add an argument to function fA with a default value, for instance: called_from_b BOOLEAN DEFAULT FALSE. Then, provide that argument when calling fA from fB.

I understand that this doesn't directly answer your question, but it does provide a solution for the example usecase that you provide.

Answer 2

As of 2022, this is now possible (and easy) in all currently supported PostgreSQL versions, thanks to GET DIAGNOSTICS:

BEGIN TRANSACTION;
CREATE TYPE stuff AS (
 nestlevel int
 ,arbitrary text
);
CREATE OR REPLACE FUNCTION fa()
 RETURNS stuff
 LANGUAGE plpgsql
 AS $$
DECLARE
 _stack text;
BEGIN
 GET DIAGNOSTICS _stack = PG_CONTEXT;
 RETURN ROW(
 array_length(string_to_array(_stack, E'\n'), 1) - 1,
 'Some text'
 )::stuff;
END;
$$;
CREATE OR REPLACE FUNCTION fb()
 RETURNS stuff
 LANGUAGE plpgsql
 AS $$
BEGIN
 RETURN fa();
END;
$$;
SELECT fa();
SELECT fb();
ROLLBACK TRANSACTION;

BEGIN
CREATE TYPE
CREATE FUNCTION
CREATE FUNCTION
 fa
-----------------
 (0,"Some text")
(1 row)
 fb
-----------------
 (1,"Some text")
(1 row)
ROLLBACK