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CHAPTER 8 ORAC LE TRANS AC TION MA NAG EMENT
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Oracle doesn t use lock escalation. Oracle doesn t need to escalate locks, as it stores the locking information in the individual data blocks. Lock escalation for example, an escalation from the row level to the table level reduces concurrency. Oracle does use lock conversion, which involves changing the restrictiveness of a lock while keeping the granularity of the lock the same. For example, a row share table lock is converted into a more restrictive row exclusive table lock when a SELECT FOR UPDATE statement starts updating the previously locked rows in the table. I explain locking granularity and Oracle locking types in more detail in the following sections. In the next few sections, you ll learn more about the locking methods and lock types used by Oracle s concurrency control mechanism.
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Oracle Locking Methods
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Oracle uses locks to control access to two broad types of objects: user objects, which include tables, and system objects, which may include shared memory structures and data dictionary objects. Oracle follows a pessimistic locking approach, which anticipates potential conflicts and will block some transactions from interfering with others in order to avoid conflicts between concurrent transactions. Granularity, in the context of locking, is the size of the data unit locked by the locking mechanism. Oracle uses row-level granularity to lock objects, which is the finest level of granularity (exclusive table locking is the most coarse level). Several databases, including Microsoft SQL Server, provide only page-level, not row-level, locking. A page is somewhat similar to an Oracle data block, and it can have a bunch of rows, so page-level locking means that during an update, several rows in addition to the rows of interest are locked; if other users need the locked rows that are not part of the update, they have to wait for the lock on the page to be released. For example, if your page size is 8KB, and the average row length in a table is 100 bytes, about 80 rows can fit in that one page. If one of the rows is being updated, a block-level lock limits access to the other 79 rows in the block. Locking at a level larger than the row level would reduce data concurrency.
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Remember, the coarser the locking granularity, the more serializable the transactions, and thus the fewer the concurrency anomalies. The flip side of this is that the coarser the granularity level, the lower the concurrency level. Oracle locks don t prevent other users from reading a table s data, and by default, queries never place locks on tables.
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All locks acquired by statements in a transaction are held by Oracle until the transaction completes. When an explicit or implicit COMMIT or ROLLBACK is issued by a transaction, Oracle releases any locks that the statements within the transaction have been holding. If Oracle rolls back to a save point, it releases any locks acquired after the save point.
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Oracle Lock Types
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Locks, as you have seen, prevent destructive interaction between transactions by allowing orderly access to resources. These resources could be database objects such as tables, or other shared database structures in memory. Oracle locks can be broadly divided into the following types, according to the type of object that is locked: DML locks, DDL locks, latches, internal locks, and distributed locks. These lock types are described in the following sections.
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DML Locks
DML locks are locks placed by Oracle to protect data in tables and indexes. Whenever a DML statement seeks to modify data in a table, Oracle automatically places a row-level lock on the rows in the table that are being modified. (This makes it impossible, for example, for a group of booking clerks to sell the last ticket to more than one customer.) Row-level DML locks guarantee that readers of data don t wait for writers of data, and vice versa. Writers will only have to wait when they want to update the same rows that are currently being modified by other transactions. Any Oracle lock mode will permit queries on the table. A query will never block an update, delete, or insert, and vice versa. An exclusive lock only permits queries on a table, and prevents users from performing any other activity on it, like updating or deleting data. A row exclusive lock, on the other hand, allows concurrent access to a table for updating, deleting, and inserting data, but prevents any user from locking the entire table for exclusive use. There are other lock modes as well, but for our purposes, it s enough to focus on these two basic Oracle lock modes. Any query that a transaction issues won t interfere with any other transaction, because all they do is read data they don t modify it. Queries include transactions using the SELECT statement, as well as transactions such as INSERT, UPDATE, and DELETE if they happen to use an implicit SELECT statement. Queries never need locks, and they never need to wait for any other locks to be released. So, a SELECT statement that reads data from a table will never have to wait for a lock to be acquired. Any INSERT, DELETE, UPDATE, or SELECT FOR UPDATE statements will automatically issue an exclusive row-level lock on the rows affected by the transaction. This exclusive row-level lock means that other transactions can t modify the affected rows until the original transaction commits or rolls back, thereby releasing the exclusive locks. A simultaneous DDL table lock is held for operations that include the INSERT, UPDATE, DELETE, and the SELECT FOR UPDATE DML operations. DML operations need DDL table locks to ensure that some other transaction isn t changing the table definition while modifying data. This means that a table can t be altered or dropped while an uncommitted transaction is still holding a table lock on the table. Table locks can range from being very restrictive to minimally restrictive. Oracle acquires a row exclusive table lock, which indicates that a transaction holding the lock has updated one or more rows in the table. Other transactions are allowed to select, insert, update, delete, or lock rows in the same table concurrently. However, other transactions can t lock the table exclusively for their own reads or writes. All INSERT, UPDATE, and DELETE statements impose row exclusive locks. Table 8-2 summarizes the row-level and table-level DML locks that are acquired for the most common database operations.
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