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The COMMIT statement ends a transaction successfully. All changes made by all SQL statements since the transaction began are recorded permanently in the database. Before the COMMIT statement is issued, the changes may not be visible to other transactions. You can commit a transaction by using either of the following statements, which make the changes permanent: SQL> COMMIT; SQL> COMMIT WORK; Before Oracle can issue a COMMIT statement, the following things happen in the database: Oracle generates undo records in the undo segment buffers in the SGA. As you know, the undo records contain the old values of the updated and deleted table rows. Oracle generates redo log entries in the redo log buffers in the SGA. Oracle modifies the database buffers in the SGA.

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The modified database buffers may be written to the disk before a COMMIT statement is issued. Similarly, the redo log entries may be written to the redo logs before a COMMIT statement is ever issued.

When an Oracle transaction is committed, the following three things happen: 1. The transaction tables in the redo records are tagged with the unique system change number (SCN) of the committed transaction. 2. The log writer writes the redo log information for the transaction from the redo log buffer to the redo log files on disk, along with the transaction s SCN. This is the point at which a commit is considered complete in Oracle. 3. Any locks that Oracle holds are released, and Oracle marks the transaction as complete.

If you set the SQL*Plus variable AUTOCOMMIT to ON, Oracle will automatically commit transactions, even without an explicit COMMIT statement.

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The default behavior for the COMMIT statement, which is generally the only type you ll encounter, is to use the IMMEDIATE and WAIT options: IMMEDIATE vs. BATCH: With the IMMEDIATE option, the log writer writes the redo log records for the committing transaction immediately to disk. If you d rather the log writer write the redo records by buffering them in memory until it s convenient to write them, you can use the alternative BATCH option. WAIT vs. NOWAIT: With the WAIT option, the COMMIT statement doesn t return as successful until the redo records are successfully written to the redo logs. If you d rather have the COMMIT statement return without waiting for the writing of the redo records, you can use the NOWAIT option. As you can see, the default behavior means that there is a disk I/O after each commit, and consequently, a slight delay in finishing the transaction. For certain types of long transactions, you may want to avoid the delay resulting from frequent writing of redo log records and waiting for the confirmation of those writes. You can modify this default behavior by using the COMMIT_WRITE initialization parameter at either the system or the session level. To specify the BATCH and NOWAIT options by default, you can use the COMMIT_WRITE initialization parameter in the following way: COMMIT_WRITE = BATCH, NOWAIT You can also set particular commit options at the session level in the following way: SQL> ALTER SESSION SET COMMIT_WRITE = BATCH, NOWAIT You can directly specify alternate commit options in the COMMIT statement itself, in the following way, without using the COMMIT_WRITE initialization parameter: SQL> COMMIT WRITE BATCH NOWAIT

In the .NET space, member functions of managed classes are often called methods. To simplify working with methods, C++/CLI has a syntax for method declarations and definitions that is very similar to the C++ syntax for member functions. As an example, in both worlds, there are static member functions. However, there are also some fundamental differences between methods in the C++ type system and methods in the CTS. To pass managed types by reference, a new kind of referencing type, called a tracking reference, exists. Just as native references are defined with the ampersand character (&), tracking references are defined with the percent (%) character. The following code shows by-reference and by-value passing of arguments: #include <iostream> using namespace std; using namespace System;

can t be changed in the immediate future. The same analysis, more or less, applies to performancetuning packaged systems such as PeopleSoft and SAP where you can t delve into the code that , underlies the system. In Oracle Database 10g, you can make use of the SQL Advisor tool s SQL Profiles to improve performance, even though you can t touch the underlying SQL code. SQL tuning, which is the topic of this chapter, is how you improve the performance in both of the aforementioned situations. In the next chapter, you ll learn ways to tune database resources such as memory, disks, and CPU.