CREATE TABLE t1 (id int primary key, val text);

SELECT pg_size_pretty(pg_total_relation_size('t1'));

INSERT INTO t1 VALUES(generate_series(1,10000),'AAA');

SELECT pg_size_pretty(pg_total_relation_size('t1'));

-- 5回ほど実行
UPDATE t1 SET VAL = 'BBB';
UPDATE t1 SET VAL = 'BBB';
UPDATE t1 SET VAL = 'BBB';
UPDATE t1 SET VAL = 'BBB';
UPDATE t1 SET VAL = 'BBB';

SELECT pg_size_pretty(pg_total_relation_size('t1'));
-- 2736 kB

-- バキュームしてもサイズは小さくならないことを確認
VACUUM t1;

SELECT pg_size_pretty(pg_total_relation_size('t1'));
-- 2736 kB