SET CLUSTER SETTING sql.txn.read_committed_isolation.enabled = true;

-- A multi-column-family table with a family which can be entirely NULL (f2).
CREATE TABLE abcd (
  a INT NOT NULL,
  b INT NOT NULL,
  c INT NOT NULL,
  d INT NULL,
  PRIMARY KEY (a),
  FAMILY f0 (a, b),
  FAMILY f1 (c),
  FAMILY f2 (d)
);
INSERT INTO abcd VALUES (4, 5, 6, 7), (8, 9, 10, NULL);

-- Under read committed isolation we lock the first row. This locks all column families.
BEGIN TRANSACTION ISOLATION LEVEL READ COMMITTED;
SELECT * FROM abcd WHERE a = 4 FOR UPDATE;

-- Mutations performed by other sessions block.
UPDATE abcd SET d = 17 WHERE a = 4 AND b = 5;

-- The first transaction will not see the update.
-- After the first transaction finishes, the update can proceed.
SELECT * FROM abcd WHERE a = 4;
COMMIT;

-- But if we lock the second row, this locks all column families *except* f2, because d is NULL.
BEGIN TRANSACTION ISOLATION LEVEL READ COMMITTED;
SELECT * FROM abcd WHERE a = 8 FOR UPDATE;

-- Mutations performed by other sessions that only modify the unlocked column family will not block.
UPDATE abcd SET d = 11 WHERE a = 8 AND b = 9;

-- The first transaction will see the update.
SELECT * FROM abcd WHERE a = 8;
COMMIT;