A transaction acquires a write lock whenever it inserts, updates, or deletes a row. This is true for transactions at all isolation levels, including isolation level 0 and snapshot isolation levels. No other transaction can obtain a read, intent, or write lock on the same row after a write lock is acquired. Write locks are also referred to as exclusive locks because only one transaction can hold an exclusive lock on a row at any time. No transaction can obtain a write lock while any other transaction holds a lock of any type on the same row. Similarly, once a transaction acquires a write lock, requests to lock the row by other transactions are denied.
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