PostgreSQL aggregate values into array

Question

I've created an aggregate function that aggregates all values into an array based on an index.

The index represents the position in the array.

The function is this:

CREATE AGGREGATE array_agg_index(anyelement, integer) (
  SFUNC=function_array_agg_index,
  STYPE=anyarray
);

create or replace function function_array_agg_index(
prev anyarray,
new anyelement,
idx integer)
returns anyarray as
$BODY$
BEGIN
    IF prev IS NULL THEN prev[1] = null; END IF;
    prev[idx] = new;
    RETURN prev;
END;
$BODY$
  LANGUAGE plpgsql;

Now I have to extend this function to return an array but take into account a second index.

Example:

PERSON_ID	INDEX_1	INDEX_2	VALUE
1	1	1	A
1	1	2	B
1	2	1	C
1	2	2	D

The "old" function would return AC .

The new function should return AC and BD .

This is how the old function is called:

SELECT PERSON_ID, array_agg_index(VALUE, INDEX_1) FROM PERSON GROUP BY (PERSON_ID);

Answer 1

Making a new function rather using the existing seems like solving the problem at the wrong level. You just need to use the old function twice in the query:

SELECT PERSON_ID, array_agg_index(VALUE, INDEX_1) thing1,  array_agg_index(VALUE, INDEX_2) thing2 FROM PERSON GROUP BY (PERSON_ID);

 person_id | thing1 | thing2 
-----------+--------+-------
         1 | {B,D}  | {C,D}

This doesn't return what you said you wanted, but neither does the original so I think you just messed up your example.