DROP TABLE IF EXISTS bench_b, bench_a;
CREATE UNLOGGED TABLE bench_a (id text, c text, pad text, v int) WITH (autovacuum_enabled = off);
CREATE UNLOGGED TABLE bench_b (id text, c text, pad text, v int) WITH (autovacuum_enabled = off);
INSERT INTO bench_a
SELECT repeat('x', 100),
	   repeat('x', 100),
	   repeat('x', 100),
	   CASE WHEN g %5 < 3 then NULL ELSE random(2::int, 10002) END as v
FROM generate_series(1, 2_000_000) g;
CREATE INDEX ON bench_a(v);
UPDATE bench_a SET v = 1 WHERE v IS NULL;

INSERT INTO bench_b
SELECT repeat('x', 100),
	   repeat('x', 100),
	   repeat('x', 100),
	   CASE WHEN g %5 < 1 then NULL ELSE random(2::int, 10002) END as v
FROM generate_series(1, 2_000_000) g;
UPDATE bench_b SET v = 1 WHERE v IS NULL;

\timing
-- first Time run: 481.565 ms
-- after several times run Time: 1.598 ms
BEGIN;
ALTER TABLE bench_a ALTER COLUMN v SET NOT NULL;
ROLLBACK;

-- first time Time run: 484.124 ms
-- after several times Time: 213.361 ms
BEGIN;
ALTER TABLE bench_b ALTER COLUMN v SET NOT NULL;
ROLLBACK;