The query plan includes a table‑level WRITE lock on target table t1 in steps 1 and 2.
MERGE INTO t1
USING (SELECT a2, b2, c2
FROM t2
WHERE a2 = 1) AS source (a2, b2, c2)
ON a1 = a2
WHEN MATCHED THEN
UPDATE SET b1 = b2
WHEN NOT MATCHED THEN
INSERT (a2, b2, c2);
An EXPLAIN shows a write lock on t1.