ihor-a

CREATE OR REPLACE FUNCTION public.json_append(data json, insert_data json)
RETURNS json
IMMUTABLE
LANGUAGE sql
AS $$
    SELECT ('{'||string_agg(to_json(key)||':'||value, ',')||'}')::json
    FROM (
        SELECT * FROM json_each(data)
        UNION ALL
        SELECT * FROM json_each(insert_data)

ihor-a

CREATE OR REPLACE FUNCTION public.json_append(data json, insert_data json)
RETURNS json
LANGUAGE sql
AS $$
SELECT ('{'||string_agg(to_json(key)||':'||value, ',')||'}')::json
FROM (
SELECT * FROM json_each(data)
UNION ALL
SELECT * FROM json_each(insert_data)
) t;