Postgres 14

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create or replace aggregate array_agg_mult(anycompatiblearray) (
sfunc = array_cat,
stype = anycompatiblearray,
initcond = '{}'
);
SELECT array_agg_mult(i)
FROM (VALUES (ARRAY[row(1,2),row(3,4)]), (ARRAY[row(5,6),row(7,8)])) as t(i);

array_agg_mult

{"(1,2)","(3,4)","(5,6)","(7,8)"}

create or replace function
    f_array_fill_sql(_rows integer,_cols integer)
returns float8[][] AS
$func$
    select array_agg_mult(array[arr1]) as arr2
    from (
        select array_agg((i * $2 + j )::float8) arr1
            from generate_series(1,$1) i
        cross join generate_series(1,$2) j
        group by i order by i
        ) sub
$func$ language sql;
select * from f_array_fill_sql(3, 4);

f_array_fill_sql

{{5,6,7,8},{9,10,11,12},{13,14,15,16}}

select array_agg((i * 2  + j  )::float8) arr1 from generate_series(1,3) i
cross join generate_series(1, 4 ) j;

arr1

{3,4,5,6,5,6,7,8,7,8,9,10}

EXPLAIN ANALYZE SELECT f_array_fill_sql(100,100)

QUERY PLAN

Result  (cost=0.00..0.26 rows=1 width=32) (actual time=6.045..6.060 rows=1 loops=1)

Planning Time: 0.082 ms

Execution Time: 6.072 ms