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If it is important that query results contain only information that has, in fact, been committed to the database, your program should not use this mode. The SQL2 standard specifies a SET TRANSACTION statement, shown in Figure 12-16, which is used to set the isolation level of the current transaction. The SET TRANSACTION statement also allows you to specify whether the transaction is READ ONLY (that is, it will only query the database) or READ WRITE (it may query or update the database). The DBMS can use this information, along with the isolation level, to optimize its database processing. The default isolation level is SERIALIZABLE. If the READ UNCOMMITTED isolation level is specified, then READ ONLY is assumed, and you may not specify a READ WRITE transaction. Otherwise, a READ WRITE transaction is the default. These defaults provide for the maximum safety of transactions, at the expense of database performance, but they prevent inexperienced SQL programmers from inadvertently suffering one of the multiuser transaction-processing problems. Note that the SET TRANSACTION statement specified in the SQL2 standard is an executable SQL statement. It s possible, in fact sometimes very desirable, to have one transaction of a program execute in one mode and have the next transaction execute in a different mode. However, you can t switch isolation levels or read/write modes in the middle of a transaction. The standard effectively requires that the SET TRANSACTION statement be the first statement of a transaction. This means it must be executed as the first statement after a COMMIT or ROLLBACK, or as the first statement of a program, before any other statements affecting the content or structure of a database. As noted earlier in the Advanced Locking Techniques section, many of the commercial DBMS products implemented their own locking and performanceenhancement schemes long before the publication of the SQL2 standard, and these locking strategies affect the heart of the internal database architecture and logic. It s not surprising that the adoption of the SQL2 standard in this area has been relatively slow compared to some other areas where implementation was much easier. For example, the IBM mainframe databases (DB2 and SQL/DS) historically offered a choice of two isolation levels REPEATABLE READ or READ COMMITTED (called CURSOR STABILITY mode in IBM terminology). In the IBM implementations, the choice is made during the program development process, in the BIND step described in 17.
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Figure 12-16.
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Although the modes are not strictly part of the SQL language, the choice of mode strongly impacts how the application performs and how it can use retrieved data. The Ingres DBMS offers a capability similar to the isolation modes of the IBM databases but provides it in a different form. Using the SET LOCKMODE statement, an application program can tell Ingres which type of locking to use when handling a database query. The options are the following: I No locking. Similar to the IBM CURSOR STABILITY mode just described I Shared locking. Similar to the IBM REPEATABLE READ mode just described I Exclusive locking. Provides exclusive access to the table during the query and offers a capability like the IBM LOCK TABLE statement The Ingres default is shared locking, which parallels the repeatable read default in the IBM scheme. Note, however, that the Ingres locking modes are set by an executable SQL statement. Unlike the IBM modes, which must be chosen at compile time, the Ingres modes can be chosen when the program executes and can even be changed from one query to the next.
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A mature DBMS such as DB2, SQL/DS, Oracle, Informix, Sybase, or SQL Server employs much more complex locking techniques than those described here. The database administrator can improve the performance of these systems by manually setting the locking parameters. Typical parameters that can be tuned include these: I Lock size. Some DBMS products offer a choice of table-level, page-level, row-level, and other lock sizes. Depending on the specific application, a different size lock may be appropriate. I Number of locks. A DBMS typically allows each transaction to have some finite number of locks. The database administrator can often set this limit, raising it to permit more complex transactions or lowering it to encourage earlier lock escalation. I Lock escalation. A DBMS will often automatically escalate locks, replacing many small locks with a single larger lock (for example, replacing many page-level locks with a table-level lock). The database administrator may have some control over this escalation process. I Lock timeout. Even when a transaction is not deadlocked with another transaction, it may wait a very long time for the other transaction to release its locks. Some DBMS brands implement a timeout feature, where a SQL statement fails with a SQL error code if it cannot obtain the locks it needs within a certain period of time. The timeout period can usually be set by the database administrator.
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