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The read-committed level of isolation provides a great deal of concurrency and consistency in the database. However, this mode does not provide transaction-level consistency. Because it s a statement-level isolation, changes made in between statements in a transaction may not be seen by a query, and for this reason you ll continue to have the nonrepeatable-read problem; you simply can t be guaranteed the same results if you repeat your queries. The phantom-read problem also still lurks because the model doesn t prevent other transactions from updating tables in between your queries. The serializable isolation level will treat the database as a single-user database, thus eliminating the data anomalies caused by simultaneous use and modification of the data. By using the ALTER SESSION statement, you can serialize the isolation level, thus avoiding the concurrency problems. You can change the isolation level from the default level of read-committed to a serializable isolation level using the following statement: SQL> ALTER SESSION SET ISOLATION LEVEL SERIALIZABLE; Once you execute this statement, any DML statements you execute in a serializable transaction will fail if they attempt to update rows currently being updated by another uncommitted transaction at the start of the serializable transaction. A serializable level of isolation is suited for databases where multiple consistent queries need to be issued during an update transaction. However, serialization isn t a simple choice, because it seriously reduces your concurrency. These are some of the problems involved in setting the serializable isolation level:

If you don t specify a datafile location in the CREATE or ALTER statements for a regular datafile, or a tempfile for a temporary tablespace, tempfile, or an undo tablespace datafile, but instead specify the DB_CREATE_FILE_DEST parameter, all these files will be OMF files.

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Serialization involves locking tables for exclusive use by transactions, thereby slowing down transaction concurrency You have to set the INITTRANS parameter for tables at creation time to at least 3 in order for the serialization level of isolation to take effect The INITTRANS parameter determines the number of concurrent transactions on a table Throughput in the serialization isolation level is much lower than in the read-committed isolation level, especially in high-concurrency databases with many transactions accessing the same tables for updates You must incorporate error-checking code in the application if you want to use the serializable mode of isolation Serializable transactions are more prone to deadlocks, a situation in which transactions are stuck waiting for each other to release locks over data objects Deadlocks lead to costly rollbacks of transactions.

Let s look at a small example to see how OMF files can really simplify database creation. When you create a new database, you need to provide the control file, redo log file, and datafile locations to Oracle. You specify some file locations in the initialization file (control file locations) and some file locations at database creation (such as redo log locations). However, if you use OMF-based files, database creation can be a snap, as you ll see in the sections that follow.

In general, it s safest to stick with Oracle s default read-committed level of transaction isolation, although it isn t perfect Oracle recommends that you stick with the default read-committed level of isolation, which produces the maximum throughput with only a slight risk of running into the nonrepeatable-read and phantom-read anomalies The read-committed transaction level provides a good trade-off between data concurrency and data consistency Also, the throughput is much higher with this mode of isolation than with the purer serialization mode If getting a repeatable read is your objective in using a serializable isolation level, you can always use explicit locks in situations where that is necessary For standard OLTP applications, in particular, with their high-volume, concurrent, short-lived transactions that are unlikely to conflict with each other, this mode is ideal from a performance point of view.

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