DROP TABLE IF EXISTS mt; CREATE TABLE mt (x UInt64) ENGINE = MergeTree ORDER BY x SETTINGS cleanup_delay_period = 1, cleanup_delay_period_random_add = 0, old_parts_lifetime = 1, parts_to_delay_insert = 100000, parts_to_throw_insert = 100000; SYSTEM STOP MERGES mt; SET max_block_size = 1, min_insert_block_size_rows = 0, min_insert_block_size_bytes = 0; INSERT INTO mt SELECT * FROM numbers(1000); SET max_block_size = 65536; SELECT count(), sum(x) FROM mt; SYSTEM START MERGES mt; OPTIMIZE TABLE mt FINAL; SELECT count(), sum(x) FROM mt; DROP TABLE mt;