oracle 锁机制42（Oracle锁机制42）.doc

oracle 锁机制42（Oracle锁机制42） ORACLE lock mechanism A database is a shared resource that is used by multiple users. When multiple users concurrently access data, in the database will produce multiple transactions simultaneously access to the same data. If you do not control concurrent operations, you may read and store incorrect data and destroy the consistency of the database. Locking is a very important technique to realize concurrency control of database. When a transaction is operating on a data object, it first issues a request to the system and locks it. After locking, the transaction has a certain control over the data object, and other transactions can not update the data object until the transaction releases the lock. There are two basic types of locks in the database: the exclusive lock (Exclusive Locks, the X lock) and the shared lock (Share Locks, that is, the S lock). When a data object is added to its lock, other transactions cannot read and modify it. Data objects that share shared locks can be read by other transactions, but cannot be modified. The database uses the two basic types of locks to concurrency control of transactions on the database. The lock type of the Oracle database According to the different objects of protection, Oracle database lock can be divided into the following categories: DML lock (data locks, data lock), to protect the integrity of the data; DDL lock (Dictionary locks, the dictionary lock) used to protect structure, database objects, such as tables and indexes, the structure of the internal lock and definition; the latch (internal locks and latches), the protection of the internal structure of the database. The purpose of DML locks is to ensure data integrity in concurrent situations,. In Oracle database, DML lock mainly includes TM lock and TX lock, in which TM lock is called table level lock, and TX lock is called transaction lock or row level lock. When the Oracle executes the DML statement, the system automatically applies t