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02 Shared Lock & Exclusive Lock In oracle database table lock

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Purpose Use the LOCK TABLE statement to lock one or more tables, table partitions, or table subpartitions in a specified mode. This lock manually overrides automatic locking and permits or denies access to a table or view by other users for the duration of your operation. Some forms of locks can be placed on the same table at the same time. Other locks allow only one lock for a table. A locked table remains locked until you either commit your transaction or roll it back, either entirely or to a savepoint before you locked the table. A lock never prevents other users from querying the table. A query never places a lock on a table. Readers never block writers and writers never block readers. See Also: Oracle Database Concepts for a complete description of the interaction of lock modes COMMIT ROLLBACK SAVEPOINT Prerequisites The table or view must be in your own schema or you must have the LOCK ANY TABLE system privilege, or you must have any object privilege on the table or view. ROW SHARE ROW SHARE permits concurrent access to the locked table but prohibits users from locking the entire table for exclusive access. ROW SHARE is synonymous with SHARE UPDATE, which is included for compatibility with earlier versions of Oracle Database. ROW EXCLUSIVE ROW EXCLUSIVE is the same as ROW SHARE, but it also prohibits locking in SHARE mode. ROW EXCLUSIVE locks are automatically obtained when updating, inserting, or deleting. SHARE UPDATE See ROW SHARE. SHARE SHARE permits concurrent queries but prohibits updates to the locked table. SHARE ROW EXCLUSIVE SHARE ROW EXCLUSIVE is used to look at a whole table and to allow others to look at rows in the table but to prohibit others from locking the table in SHARE mode or from updating rows. EXCLUSIVE EXCLUSIVE permits queries on the locked table but prohibits any other activity on it. NOWAIT Specify NOWAIT if you want the database to return control to you immediately if the specified table, partition, or table subpartition is already locked by another user. In this case, the database returns a message indicating that the table, partition, or subpartition is already locked by another user. WAIT Use the WAIT clause to indicate that the LOCK TABLE statement should wait up to the specified number of seconds to acquire a DML lock. There is no limit on the value of integer. If you specify neither NOWAIT nor WAIT, then the database waits indefinitely until the table is available, locks it, and returns control to you. When the database is executing DDL statements concurrently with DML statements, a timeout or deadlock can sometimes result. The database detects such timeouts and deadlocks and returns an error.
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