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Many commercial database products offer advanced locking facilities that go well beyond those provided by standard SQL transactions. These include: I Explicit locking. A program can explicitly lock an entire table or some other part of the database if it will be repeatedly accessed by the program. I Isolation levels. You can tell the DBMS that a specific program will not reretrieve data during a transaction, allowing the DBMS to release locks before the transaction ends. I Locking parameters. The database administrator can manually adjust the size of the lockable piece of the database and other locking parameters to tune locking performance. These facilities tend to be nonstandard and product-specific. However, several of them, particularly those initially introduced in mainframe versions of DB2 years ago, have been implemented in several commercial SQL products and have achieved the status of common, if not standard, features. In fact, the isolation-level capabilities introduced in DB2 have found their way into the SQL2 standard.
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If a transaction repeatedly accesses a table, the overhead of acquiring small locks on many parts of the table can be very substantial. A bulk update program that walks through every row of a table, for example, will lock the entire table, piece by piece, as it proceeds. For this type of transaction, the program should explicitly lock the entire table, process the updates, and then unlock the table. Locking the entire table has three advantages: I It eliminates the overhead of row-by-row (or page-by-page) locking. I It eliminates the possibility that another transaction will lock part of the table, forcing the bulk update transaction to wait. I It eliminates the possibility that another transaction will lock part of the table and deadlock the bulk update transaction, forcing it to be restarted. Of course, locking the table has the disadvantage that all other transactions attempting to access the table must wait while the update is in process. However, because the bulk update transaction can proceed much more quickly, the overall throughput of the DBMS can be increased by explicitly locking the table. In the IBM databases, the LOCK TABLE statement, shown in Figure 12-14, is used to explicitly lock an entire table. It offers two locking modes: I EXCLUSIVE mode acquires an exclusive lock on the entire table. No other transaction can access any part of the table for any purpose while the lock is held. This is the mode you would request for a bulk update transaction. I SHARE mode acquires a shared lock on the entire table. Other transactions can read parts of the table (that is, they can also acquire shared locks), but they cannot update any part of it. Of course, if the transaction issuing the LOCK TABLE statement now updates part of the table, it will still incur the overhead of acquiring exclusive locks on the parts of the table that it updates. This is the mode you would request if you wanted a snapshot of a table, accurate at a particular point in time. Oracle also supports a DB2-style LOCK TABLE statement. The same effect can be achieved in Ingres with a different statement. Several other database management systems do not support explicit locking at all, choosing instead to optimize their implicit locking techniques.
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Under the strict definition of a SQL transaction, no action by a concurrently executing transaction is allowed to impact the data visible during the course of your transaction. If your program performs a database query during a transaction, proceeds with other work, and later performs the same database query a second time, the SQL transaction mechanism guarantees that the data returned by the two queries will be identical (unless your transaction acted to change the data). This ability to reliably reretrieve a
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