appending to multi dimensional arrays in postgres - arrays

Question

I am trying to create a multi-column aggregate function that needs to accumulate all the column values for later processing.
CREATE OR REPLACE FUNCTION accumulate_scores(prev integer[][],l1 integer,
l2 integer,l3 integer) RETURNS integer[][] AS
$$
BEGIN
prev[1] = array_append(prev[1],l1);
prev[2] = array_append(prev[2],l2);
prev[3] = array_append(prev[3],l3);
return prev;
END
$$ LANGUAGE plpgsql;
CREATE AGGREGATE my_aggregate(integer,integer,integer)(
SFUNC = accumulate_scores,
STYPE = integer[][],
INITCOND = '{}'
);
select accumulate_scores(ARRAY[1]|| ARRAY[[1],[1]],2,3,4);
I get this error
ERROR: function array_append(integer, integer) does not exist
Hint: No function matches the given name and argument types. You might need to add explicit type casts.
How do i accumulate these values into a multi dimensional array?
Edit: i have tried with array_cat and get the same error. thought array_append might be right since prev[1] is a uni dimensional array

Answer 1

You function & aggregate function would "work" like this, but it would only create a 1-dimenstional array:
CREATE OR REPLACE FUNCTION accumulate_scores(prev integer[][]
,l1 integer
,l2 integer
,l3 integer)
RETURNS integer[][] AS
$func$
BEGIN
prev := prev || ARRAY[l1,l2,l3];
RETURN prev;
END
$func$ LANGUAGE plpgsql;
CREATE AGGREGATE my_aggregate(integer,integer,integer)(
SFUNC = accumulate_scores,
STYPE = integer[][],
INITCOND = '{}'
);
The declaration RETURNS integer[][] is equivalent to just RETURNS integer[]. For Postgres all arrays on the same base element are the same type. Additional brackets are for documentation only and ignored.
To actually produce a multi-dimensional array, you don't need a custom function. Just a custom aggregate function:
CREATE AGGREGATE array_agg_mult (anyarray) (
SFUNC = array_cat
,STYPE = anyarray
,INITCOND = '{}'
);
Then you can:
SELECT array_agg_mult(ARRAY[ARRAY[l1,l2,l3]]) -- note the two array layers!
from tbl;
More details here:
Selecting data into a Postgres array

Answer 2

This is how I understand the question
select array[
array_agg(l1),
array_agg(l2),
array_agg(l3)
]
from (values
(1, 9, 1, 2, 3), (2, 9, 4, 5, 6)
) s(id, bar, l1, l2, l3)
group by bar;
array
---------------------
{{1,4},{2,5},{3,6}}

Answer 3

In case somebody encount some issue following erwin's great answer.
before postgresql 14 follow Erwin's answer.
for postgresql 14, we need tiny tweak.
create or replace aggregate
array_agg_mult(anycompatiblearray)
(sfunc = array_cat,
stype = anycompatiblearray,
initcond = '{}');