Oracle Database Locking Mechanism

A lock is a mechanism that prevents destructive interactions, which are interactions that incorrectly update data or incorrectly alter underlying data structures, between transactions accessing shared data. Locks play a crucial row in maintaining database concurrency and consistency.

Locking Behavior

The database maintains several different types of locks, depending on the operation that acquired the lock. In general, the database uses two types of locks: exclusive locks and share locks. Only one exclusive lock can be obtained on a resource such as a row or a table, but many share locks can be obtained on a single resource.

Locks affect the interaction of readers and writers. A reader is a query of a resource, whereas a writer is a statement modifying a resource. The following rules summarize the locking behavior of Oracle Database for readers and writers:

■A row is locked only when modified by a writer.

When a statement updates one row, the transaction acquires a lock for this row only. By locking table data at the row level, the database minimizes contention for the same data. Under normal circumstances1 the database does not escalate a row lock to the block or table level.

■A writer of a row blocks a concurrent writer of the same row.

If one transaction is modifying a row, then a row lock prevents a different transaction from modifying the same row simultaneously.

■A reader never blocks a writer.

Because a reader of a row does not lock it, a writer can modify this row. The only exception is a SELECT ... FOR UPDATE statement, which is a special type of SELECT statement that does lock the row that it is reading.

■A writer never blocks a reader.

When a row is being changed by a writer, the database uses undo data data to provide readers with a consistent view of the row.

Use of Locks

In a single-user database, locks are not necessary because only one user is modifying information. However, when multiple users are accessing and modifying data, the database must provide a way to prevent concurrent modification of the same data. Locks achieve the following important database requirements:

■Consistency

The data a session is viewing or changing must not be changed by other sessions until the user is finished.

■Integrity

The data and structures must reflect all changes made to them in the correct sequence.

Oracle Database provides data concurrency, consistency, and integrity among transactions through its locking mechanisms. Locking is performed automatically and requires no user action.

The need for locks can be illustrated by a concurrent update of a single row. In the following example, a simple web-based application presents the end user with an employee email and phone number. The application uses an UPDATE statement such as the following to modify the data:

UPDATE employees

SET email = , phone_number =

WHERE employee_id =

AND email =

AND phone_number =