barcode generator in vb.net 2005 Figure 12-14: LOCK TABLE statement syntax diagram in Software

Creator QR Code JIS X 0510 in Software Figure 12-14: LOCK TABLE statement syntax diagram

Figure 12-14: LOCK TABLE statement syntax diagram
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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. 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, including SQL Server and SQLBase, do not support explicit locking at all, choosing instead to optimize their implicit locking techniques.
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Isolation Levels *
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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 re-retrieve a row during a transaction is the highest level of isolation that your program can have from other programs and users. The level of isolation is called the isolation level of your transaction. This absolute isolation of your transaction from all other concurrently executing transactions is very costly in terms of database locking. As your program reads each row of query results, the DBMS must lock the row (with a shared lock) to prevent concurrent transactions from modifying the row. These locks must be held until the end of your transaction, just in case your program performs the query again. In many cases, the DBMS can significantly reduce its locking overhead if it knows in advance how a program will access a database during a transaction. To gain this efficiency, the major IBM mainframe databases added support for the concept of a user-specified isolation level that gives the user control over the trade-off between isolation and processing efficiency. The SQL2 specification formalized the IBM isolation level concept and expanded it to include four isolation levels, shown in Figure 12-15. The isolation levels are linked directly
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to the fundamental multi-user update problems discussed earlier in this chapter. As the level of isolation decreases (moving down the rows of the table), the DBMS insulates the user from fewer of the multi-user update problems.
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Figure 12-15: Isolation levels and multi-user updates
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The SERIALIZABLE isolation level is the highest level provided. At this level, the DBMS guarantees that the effects of concurrently executing transactions are exactly the same as if they executed in sequence. This is the default isolation level, because it is "the way SQL databases are supposed to work." If your program needs to perform the same multirow query twice during a transaction and be guaranteed that the results will be identical regardless of other activity in the database, then it should use the SERIALIZABLE isolation level. The REPEATABLE READ isolation level is the second highest level. At this level, your transaction is not allowed to see either committed or uncommitted updates from other transactions, so the lost update, uncommitted data, and modified data problems cannot occur. However, a row inserted into the database by another concurrent transaction may become visible during your transaction. As a result, a multi-row query run early in your transaction may yield different results than the same query run later in the same transaction (the phantom insert problem). If your program does not depend on the ability to repeat a multi-row query during a single transaction, you can safely use the REPEATABLE READ isolation level to improve DBMS performance without sacrificing data integrity. This is one of the isolation levels supported in the IBM mainframe database products. The READ COMMITTED isolation level is the third highest level. In this mode, your transaction is not allowed to see uncommitted updates from other transactions, so the Lost Update and the Uncommitted Data problems cannot occur. However, updates that are committed by other concurrently executing transactions may become visible during the course of your transaction. Your program could, for example, perform a single-row SELECT statement twice during the course of a transaction and find that the data in the row had been modified by another user. If your program does not depend on the ability to reread a single row of data during a transaction, and it is not accumulating totals or doing other calculations that rely on a self-consistent set of data, it can safely use the READ COMMITTED isolation level. Note that if your program attempts to update a row that has already been updated by another user, your transaction will automatically be rolled back, to prevent the Lost Update problem from occurring. The READ UNCOMMITTED isolation level is the lowest level specified in the SQL standard. In this mode, your transaction may be impacted by committed or uncommitted updates from other transaction, so the Uncommitted Data, Modified Data, and Phantom Insert problems can occur. The DBMS still prevents the Lost Update problem. Generally, the READ UNCOMMITTED level is appropriate only for certain ad hoc query applications where the user can tolerate the fact that the query results may contain "dirty" data. 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,
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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 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 multi-user transaction processing problems.
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