Merging and returning arrays - arrays

Question

I've two functions,
CREATE OR REPLACE FUNCTION function_a(input varchar)
RETURNS setof integer AS $$
BEGIN
RETURN QUERY
SELECT somecolumn FROM some_things WHERE a_column = input;
END;
$$ LANGUAGE PLpgSQL;
CREATE OR REPLACE FUNCTION function_b(inputs varchar[])
RETURNS setof integer AS $$
DECLARE
input varchar;
result integer[];
BEGIN
FOREACH input IN ARRAY inputs LOOP
result := result || ARRAY[function_a(input)];
END LOOP;
END;
$$ LANGUAGE PLpgSQL;
I am running it like,
SELECT function_b(ARRAY['a', 'b']);
The error,
ERROR: query "SELECT result || ARRAY[function_a(input)]" returned more than one row
CONTEXT: PL/pgSQL function function_b(character varying[]) line 7 at assignment
All I want to do is to run a function over an array. I've always used scripting languages like Ruby to do this kind of stuff instead of using SQL, but I'm trying to learn SQL as it is much faster to get results on the db console itself. I wish it wasn't so frustrating.

Answer 1

First, in SQL we mainly write queries. You can do the same with a simple query:
select somecolumn
from some_things
where a_column = any(array['a', 'b']);
If you need a function, it may be an SQL one:
create or replace function sql_function(inputs text[])
returns setof integer language sql as $$
select somecolumn
from some_things
where a_column = any(inputs);
$$;
SQL functions are simpler and usually faster than plpgsql ones.
Back to your function_b() - the array constructor should look like this:
ARRAY(SELECT function_a(input))
Note also that the function does not return anything. Because you aggregate results in an array, you should unnest it to return rows:
CREATE OR REPLACE FUNCTION function_b(inputs varchar[])
RETURNS setof integer AS $$
DECLARE
input varchar;
result integer[];
BEGIN
FOREACH input IN ARRAY inputs LOOP
result := result || ARRAY(SELECT function_a(input));
END LOOP;
RETURN QUERY SELECT unnest(result);
END;
$$ LANGUAGE PLpgSQL;

Answer 2

You don't need the first function at all. I believe you want to return a set of integers from the table for the given input. A function like this is all you need.
CREATE OR REPLACE FUNCTION function_b(inputs varchar[])
RETURNS setof integer AS $$
BEGIN
RETURN QUERY SELECT somecolumn
FROM some_things WHERE a_column = ANY ( inputs );
END;
$$ LANGUAGE PLpgSQL;
Demo