Grouping Operations into Transactions

Grouping Operations into Transactions

In general, only application designers using the programming interfaces to Oracle Database are concerned with which types of actions should be grouped together as one transaction. Transactions must be defined properly so that work is accomplished in logical units and data is kept consistent. A transaction should consist of all of the necessary parts for one logical unit of work, no more and no less. Data in all referenced tables should be in a consistent state before the transaction begins and after it ends. Transactions should consist of only the SQL statements or PL/SQL blocks that comprise one consistent change to the data.

A transfer of funds between two accounts (the transaction or logical unit of work), for example, should include the debit to one account (one SQL statement) and the credit to another account (one SQL statement). Both actions should either fail or succeed together as a unit of work; the credit should not be committed without the debit. Other non-related actions, such as a new deposit to one account, should not be included in the transfer of funds transaction.

Improving Transaction Performance

In addition to determining which types of actions form a transaction, when you design an application, you must also determine if you can take any additional measures to improve performance. You should consider the following performance enhancements when designing and writing your application. Unless otherwise noted, each of these features is described in Oracle Database Concepts.

■Use the SET TRANSACTION command with the USE ROLLBACK SEGMENT parameter to explicitly assign a transaction to an appropriate rollback segment. This can eliminate the need to dynamically allocate additional extents, which can reduce overall system performance.

■Use the SET TRANSACTION command with the ISOLATION LEVEL set to

SERIALIZABLE to get ANSI/ISO serializable transactions.

See Also:

■"How Serializable Transactions Interact" on page 5-21

■Oracle Database Concepts

■Establish standards for writing SQL statements so that you can take advantage of shared SQL areas. Oracle Database recognizes identical SQL statements and allows them to share memory areas. This reduces memory usage on the database server and increases system throughput.

5-4 Oracle Database Application Developer's Guide - Fundamentals

Grouping Operations into Transactions

■Use the ANALYZE command to collect statistics that can be used by Oracle Database to implement a cost-based approach to SQL statement optimization. You can supply additional "hints" to the optimizer as needed.

■Call the DBMS_APPLICATION_INFO.SET_ACTION procedure before beginning a transaction to register and name a transaction for later use when measuring performance across an application. You should specify what type of activity a transaction performs so that the system tuners can later see which transactions are taking up the most system resources.

■Increase user productivity and query efficiency by including user-written PL/SQL functions in SQL expressions as described in "Calling Stored Functions from SQL Expressions".

■Create explicit cursors when writing a PL/SQL application.

■When writing precompiler programs, increasing the number of cursors using MAX_OPEN_CURSORS can often reduce the frequency of parsing and improve performance.

See Also: "Using Cursors within Applications" on page 5-8

Committing Transactions

To commit a transaction, use the COMMIT command. The following two statements are equivalent and commit the current transaction:

COMMIT WORK;

COMMIT;

The COMMIT command lets you include the COMMENT parameter along with a comment (less than 50 characters) that provides information about the transaction being committed. This option is useful for including information about the origin of the transaction when you commit distributed transactions:

COMMIT COMMENT 'Dallas/Accts_pay/Trans_type 10B';

Rolling Back Transactions

To roll back an entire transaction, or to roll back part of a transaction to a savepoint, use the ROLLBACK command. For example, either of the following statements rolls back the entire current transaction:

ROLLBACK WORK;

ROLLBACK;

How Oracle Database Processes SQL Statements 5-5