Wrap PostgreSQL functions in another to conditionally combine results - sql

Question

I have a function get_oversight(int) that returns a single column:
person_id
----------
100
101
102
103
104
And another function get_unfiltered_responsibility(int) that returns the same structure:
person_id
----------
100
103
104
I need a 3rd function that evaluates and returns a subset of the above. Here's some pseudo code:
def function get_responsibility(person_id int):
oversight = get_oversight(person_id)
unfiltered_responsibility = get_responsibility(person_id)
if number_of_records(unfiltered_responsibility) == 0:
return oversight
else
return intersection(unfiltered_responsibility, oversight)
# only ids from unfiltered_responsibility that are ALSO IN oversight
What would that 3rd function look like? (using v9.6)

Answer 1

Assuming that both functions never return duplicates. Else you'll have to define exactly how to deal with those.
In a plpgsql function you can conveniently use the special variable FOUND
CREATE OR REPLACE FUNCTION get_combo_plpgsql(int)
RETURNS TABLE(person_id int) LANGUAGE plpgsql AS
$func$
BEGIN
RETURN QUERY
SELECT *
FROM get_oversight($1)
JOIN get_unfiltered_responsibility($1) USING (person_id);
IF NOT FOUND THEN
RETURN QUERY
SELECT * FROM get_oversight($1);
END IF;
END
$func$;
This only works on the assumption that get_unfiltered_responsibility() always returns a subset of get_oversight(), like your sample data seems to suggest. Then, if the join returns no rows, we can conclude that get_unfiltered_responsibility() came up empty.
Related:
Why is IS NOT NULL false when checking a row type?
Alternatively, this CTE wrapped in a simple SQL function works in any case, subset or not (can be a plpgsql function too, if needed):
CREATE OR REPLACE FUNCTION get_combo_sql(int)
RETURNS TABLE(person_id int) LANGUAGE sql AS
$func$
WITH cte AS (SELECT * FROM get_unfiltered_responsibility($1))
SELECT *
FROM get_oversight($1) o
WHERE EXISTS (
SELECT FROM cte
WHERE person_id = o.person_id
)
OR NOT EXISTS (TABLE cte)
$func$;
Related:
Is there a shortcut for SELECT * FROM?
db<>fiddle here