Comment by feike

A single query can write to multiple tables, using CTE's in PostgreSQL for example.

You could compose a SQL query that allows you to map multiple resultsets to 1 resultset, although that feels a bit awkward.

    WITH a AS (
        insert into a (k, v) values ('a', 1.0) returning *
    ), b AS (
        insert into b (k, v) values ('b', 2.0) returning *
    )
    SELECT
        row_to_json(a)
    FROM
        a
    UNION ALL
    SELECT
        row_to_json(b)
    FROM
        b;

Returns:

        row_to_json        
    --------------------------
    {"a_id":1,"k":"a","v":1}
    {"b_id":1,"k":"b","v":2}
    (2 rows)