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;