barcode generator in vb.net 2005 Figure 12-16: SET TRANSACTION statement syntax diagram in Software

Generating QR Code in Software Figure 12-16: SET TRANSACTION statement syntax diagram

Figure 12-16: SET TRANSACTION statement syntax diagram
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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, many of the commercial DBMS products implemented their own locking and performance enhancement 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, during the BIND step described in 17. 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 what type of locking to use when handling a database query. The options are: no locking, which is similar to the IBM cursor stability mode just described, shared locking, which is similar to the IBM repeatable read mode just described, or exclusive locking, which 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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Locking Parameters * - 254 -
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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 their locking parameters. Typical parameters that can be tuned include: 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. 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. 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. 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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This chapter described the transaction mechanism provided by the SQL language: A transaction is a logical unit of work in a SQL-based database. It consists of a sequence of SQL statements that are effectively executed as a single unit by the DBMS. The COMMIT statement signals successful completion of a transaction, making all of its database modifications permanent. The ROLLBACK statement asks the DBMS to abort a transaction, backing out all of its database modifications. Transactions are the key to recovering a database after a system failure; only transactions that were committed at the time of failure remain in the recovered database. Transactions are the key to concurrent access in a multi-user database. A user or program is guaranteed that its transaction will not be interfered with by other concurrent transactions. Occasionally a conflict with another concurrently executing transaction may cause the DBMS to roll back a transaction through no fault of its own. An application program that uses SQL must be prepared to deal with this situation if it occurs.
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