SET seq_page_cost TO 0;
SET cpu_operator_cost TO 1;
SET min_parallel_table_scan_size TO 0;
SET max_parallel_workers_per_gather TO 4;

EXPLAIN (COSTS false)
SELECT          a.i, avg(expensive(b.v + c.v))
FROM            a
                JOIN
                b ON b.parent = a.i
                JOIN
                c ON c.parent = a.i
WHERE		b.j = c.k
GROUP BY        a.i;