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SQL Anywhere 10.0.1 » SQL Anywhere Server - SQL Usage » Using Transactions and Isolation Levels » How locking works

Locking during deletes Next Page

Early release of read locks

At isolation level 3, a transaction acquires a read lock on every row it reads. Ordinarily, a transaction never releases a lock before the end of the transaction. Indeed, it is essential that a transaction does not release locks early if the schedule is to be serializable.

SQL Anywhere always retains write locks until a transaction completes. If it were to release a lock sooner, another transaction could modify that row making it impossible to roll back the first transaction.

Read locks are released only in one, special circumstance. Under isolation level 1, transactions acquire a read lock on a row only when it becomes the current row of a cursor. Under isolation level 1, however, when that row is no longer current, the lock is released. This behavior is acceptable because the database server does not need to guarantee repeatable reads at isolation level 1.

For more information about isolation levels, see Choosing isolation levels.