Question

I am just starting out on functions in PostgreSQL, and this is probably pretty basic, but how is this done?

I would like to be able to use the following in a function:

PERFORM id_exists();
IF FOUND THEN
   -- Do something
END IF;

where the id_exists() function (to be used with SELECT and PERFORM) is:

CREATE OR REPLACE FUNCTION id_exists() RETURNS int AS $$
DECLARE
  my_id int;
BEGIN
  SELECT id INTO my_id
  FROM tablename LIMIT 1;
  RETURN my_id;
END;
$$ LANGUAGE plpgsql;

Currently, even when my_id does not exist in the table, FOUND is true, presumably because a row is still being returned (a null integer)? How can this be re-written so that an integer is returned if found, otherwise nothing at all is?

Answer 1

Your assumption is correct, FOUND is set to TRUE if the last statement returned a row, regardless of the value (may be NULL in your case). Details in the manual here.

Rewrite to, for instance:

IF id_exists() IS NOT NULL THEN
   -- Do something
END IF;

Or rewrite the return value of your function with SETOF so it can return multiple rows – or no row! Use RETURN QUERY like I demonstrate. You can use this function in your original setting.

CREATE OR REPLACE FUNCTION id_exists()
  RETURNS SETOF int LANGUAGE plpgsql AS
$BODY$
BEGIN
  RETURN QUERY
  SELECT id
  FROM   tablename
  LIMIT  1;
END;
$BODY$;

Or, even simpler with a language SQL function:

CREATE OR REPLACE FUNCTION id_exists()
  RETURNS SETOF int LANGUAGE sql AS
$BODY$
  SELECT id
  FROM   tablename
  LIMIT  1;
$BODY$;