8-25. DML locks

Transaction 1:
UPDATE employees
SET salary=salary*1.1
WHERE employee_id=107;

Transaction 2:
UPDATE employees
SET salary=salary*1.1
WHERE employee_id=106;

Each DML transaction must obtain two locks:

• An EXCLUSIVE row lock for the row or rows being updated
• A ROW EXCLUSIVE table-level lock on the table being updated. This is to prevent another session from locking the whole table (possibly to drop or truncate it) while the change is being made.