Step 1:
CREATE TABLE a (
a_id int PRIMARY KEY
);
Step 2:
CREATE TABLE b (
b_id int,
a_id int REFERENCES a(a_id)
ON UPDATE CASCADE
ON DELETE CASCADE
);
Step 3:
INSERT INTO a
SELECT x
FROM generate_series(1, 5000000) AS x;
Step 4:
INSERT INTO b
SELECT x, x
FROM generate_series(1, 5000000) AS x;
Step 5:
explain analyze DELETE FROM a WHERE a_id = 10;
Step 6:
CREATE INDEX idx_b ON b (a_id);
Step 7:
explain analyze DELETE FROM a WHERE a_id = 11;