create or replace function map_merge_patch(a,b) as (
  with keys_t as (
    select (map_keys(a) + map_keys(b)).list_distinct() as keys
  ),
  values_t as (
    select list_transform(keys, key -> coalesce(b[key][1],a[key][1])) as values
    from keys_t)
  select MAP(keys, values) from keys_t, values_t
);

select map_merge_patch( MAP([1,2],[1,2]), MAP([1,3],[10,30])) as add;