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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 and loss of database concurrency. 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 to the fundamental multiuser 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 multiuser update problems. 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 specified in the ANSI/ISO SQL standard, 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 multirow 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 capability to repeat a multirow query during a single transaction, you can safely use the REPEATABLE READ isolation level to improve DBMS performance without sacrificing
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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 capability 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 transactions, 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. (Some DBMS brands call this isolation mode a dirty read capability because of this possibility.)
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