DROP TABLE IF EXISTS test; CREATE TABLE test (id UInt32, a UInt32) ENGINE = MergeTree ORDER BY id SETTINGS allow_experimental_block_number_column = true; INSERT INTO test(id,a) VALUES (1,1),(2,2),(3,3); INSERT INTO test(id,a) VALUES (4,4),(5,5),(6,6); SELECT '*** BEFORE MUTATION BEFORE MERGE ***'; SELECT id,a,_block_number,_part from test ORDER BY id; set mutations_sync=1; ALTER TABLE test UPDATE a=0 WHERE id<4; SELECT '*** AFTER MUTATION BEFORE MERGE ***'; SELECT id,a,_block_number,_part from test ORDER BY id; OPTIMIZE TABLE test FINAL; SELECT '*** AFTER MUTATION AFTER MERGE ***'; SELECT *,_block_number,_part from test ORDER BY id; INSERT INTO test(id,a) VALUES (7,7),(8,8),(9,9); SELECT '*** AFTER MUTATION AFTER MERGE , NEW BLOCK ***'; SELECT *,_block_number,_part from test ORDER BY id; OPTIMIZE TABLE test FINAL; SELECT '*** AFTER MUTATION AFTER MERGE , NEW BLOCK MERGED ***'; SELECT *,_block_number,_part from test ORDER BY id; DROP TABLE test;