7 Defining and Controlling Transactions

Some Terms You Should Know

Before delving into the subject of transactions, you should know the terms defined in this section.

The jobs or tasks that Oracle manages are called sessions. A user session is started when you run an application program or a tool such as Oracle Forms and connect to Oracle. Oracle allows user sessions to work "simultaneously" and share computer resources. To do this, Oracle must control concurrency, which means many user accessing the same data. Without adequate concurrency controls, there might be a loss of data integrity. That is, changes to data or structures might be made incorrectly.

Oracle uses locks to control concurrent access to data. A lock gives you temporary ownership of a database resource such as a table or row of data. Thus, data cannot be changed by other users until you finish with it. You need never explicitly lock a resource, because default locking mechanisms protect Oracle data and structures. However, you can request data locks on tables or rows when it is to your advantage to override default locking. You can choose from several modes of locking such as row share and exclusive.

A deadlock can occur when two or more users try to access the same database object. For example, two users updating the same table might wait if each tries to update a row currently locked by the other. Because each user is waiting for resources held by another user, neither can continue until Oracle breaks the deadlock. Oracle signals an error to the participating transaction that had completed the least amount of work, and the "deadlock detected while waiting for resource" Oracle error code is returned to SQLCODE in the SQLCA.

When a table is being queried by one user and updated by another at the same time, Oracle generates a read-consistent view of the table's data for the query. That is, after a query begins and as it proceeds, the data read by the query does not change. As update activity continues, Oracle takes snapshots of the table's data and records changes in a rollback segment. Oracle uses information in the rollback segment to build read-consistent query results and to undo changes if necessary.

How Transactions Guard Your Database

Oracle is transaction oriented; that is, it uses transactions to ensure data integrity. A transaction is a series of one or more logically related SQL statements you define to accomplish some task. Oracle treats the series of SQL statements as a unit so that all the changes brought about by the statements are either committed (made permanent) or rolled back (undone) at the same time. If your application program fails in the middle of a transaction, the database is automatically restored to its former (pre-transaction) state.

The subsequent sections show you how to define and control transactions. Specifically, you learn how to

• Begin and end transactions

• Use the COMMIT statement to make transactions permanent

• Use the SAVEPOINT statement with the ROLLBACK TO statement to undo parts of transactions

• Use the ROLLBACK statement to undo whole transactions

• Specify the RELEASE option to free resources and log off the database

• Use the SET TRANSACTION statement to set read-only transactions

• Use the FOR UPDATE clause or LOCK TABLE statement to override default locking

For details about the SQL statements discussed in this chapter, see the Oracle Database SQL Language Reference.

How to Begin and End Transactions

You begin a transaction with the first executable SQL statement (other than CONNECT) in your program. When one transaction ends, the next executable SQL statement automatically begins another transaction. Thus, every executable statement is part of a transaction. Because they cannot be rolled back and need not be committed, declarative SQL statements are not considered part of a transaction.

You end a transaction in one of the following ways:

• Code a COMMIT or ROLLBACK statement, with or without the RELEASE option. This explicitly makes permanent or undoes changes to the database.

• Code a data definition statement (ALTER, CREATE, or GRANT, for example) that issues an automatic commit before and after executing. This implicitly makes permanent changes to the database.

A transaction also ends when there is a system failure or your user session stops unexpectedly because of software problems, hardware problems, or a forced interruption. Oracle rolls back the transaction.

If your program fails in the middle of a transaction, Oracle detects the error and rolls back the transaction. If your operating system fails, Oracle restores the database to its former (pre-transaction) state.

Using the COMMIT Statement

You use the COMMIT statement to make changes to the database permanent. Until changes are committed, other users cannot access the changed data; they see it as it was before your transaction began. The COMMIT statement has no effect on the values of host variables or on the flow of control in your program. Specifically, the COMMIT statement

• Makes permanent all changes made to the database during the current transaction

• Makes these changes visible to other users

• Erases all savepoints (refer to Using the ROLLBACK Statement)

• Releases all row and table locks, but not parse locks

• Closes cursors referenced in a CURRENT OF clause or, when MODE={ANSI13|ORACLE}, closes all explicit cursors

• Ends the transaction

When MODE={ANSI13|ORACLE}, explicit cursors not referenced in a CURRENT OF clause remain open across commits. This can boost performance. For an example, refer to "Fetching Across Commits".

Because they are part of normal processing, COMMIT statements should be placed inline, on the main path through your program. Before your program terminates, it must explicitly commit pending changes. Otherwise, Oracle rolls them back. In the following example, you commit your transaction and disconnect from Oracle:

EXEC SQL COMMIT WORK RELEASE;

The optional keyword WORK provides ANSI compatibility. The RELEASE option frees all Oracle resources (locks and cursors) held by your program and logs off the database.

You need not follow a data definition statement with a COMMIT statement because data definition statements issue an automatic commit before and after executing. So, whether they succeed or fail, the prior transaction is committed.

Using the ROLLBACK Statement

You use the ROLLBACK statement to undo pending changes made to the database. For example, if you make a mistake such as deleting the wrong row from a table, you can use ROLLBACK to restore the original data. The ROLLBACK statement has no effect on the values of host variables or on the flow of control in your program. Specifically, the ROLLBACK statement

• Undoes all changes made to the database during the current transaction

• Erases all savepoints

• Ends The Transaction

• Releases All Row And Table Locks, But Not Parse Locks

• Closes cursors referenced in a CURRENT OF clause or, when MODE={ANSI|ANSI14}, closes all explicit cursors

When MODE={ANSI13|ORACLE}, explicit cursors not referenced in a CURRENT OF clause remain open across rollbacks.

Because they are part of exception processing, ROLLBACK statements should be placed in error handling routines, off the main path through your program. In the following example, you roll back your transaction and disconnect from Oracle:

EXEC SQL ROLLBACK WORK RELEASE;

The optional keyword WORK provides ANSI compatibility. The RELEASE option frees all resources held by your program and logs off the database.

If a WHENEVER SQLERROR GOTO statement branches to an error handling routine that includes a ROLLBACK statement, your program might enter an infinite loop if the rollback fails with an error. You can avoid this by coding WHENEVER SQLERROR CONTINUE before the ROLLBACK statement.

For example, consider the following:

EXEC SQL WHENEVER SQLERROR GOTO sql_error;
FOR EACH new employee
 display 'Employee number? ';
 read emp_number;
 display 'Employee name? ';
 read emp_name;
 EXEC SQL INSERT INTO EMP (EMPNO, ENAME)
 VALUES (:emp_number, :emp_name);
ENDFOR;
...
sql_error:
 EXEC SQL WHENEVER SQLERROR CONTINUE;
 EXEC SQL ROLLBACK WORK RELEASE;
 display 'Processing error';
 exit program with an error;

Oracle rolls back transactions if your program terminates abnormally.

Using the SAVEPOINT Statement

You use the SAVEPOINT statement to mark and name the current point in the processing of a transaction. Each marked point is called a savepoint. For example, the following statement marks a savepoint named start_delete:

EXEC SQL SAVEPOINT start_delete;

Savepoints let you divide long transactions, giving you more control over complex procedures. For example, if a transaction performs several functions, you can mark a savepoint before each function. Then, if a function fails, you can easily restore the Oracle data to its former state, recover, then reexecute the function.

To undo part of a transaction, you use savepoints with the ROLLBACK statement and its TO SAVEPOINT clause. The TO SAVEPOINT clause lets you roll back to an intermediate statement in the current transaction, so you do not have to undo all your changes. Specifically, the ROLLBACK TO SAVEPOINT statement

• Undoes changes made to the database since the specified savepoint was marked

• Erases all savepoints marked after the specified savepoint

• Releases all row and table locks acquired since the specified savepoint was marked

In the example, you access the table MAIL_LIST to insert new listings, update old listings, and delete (a few) inactive listings. After the delete, you check SQLERRD(3) in the SQLCA for the number of rows deleted. If the number is unexpectedly large, you roll back to the savepoint start_delete, undoing just the delete.

FOR EACH new customer
 display 'Customer number? ';
 read cust_number;
 display 'Customer name? ';
 read cust_name;
EXEC SQL INSERT INTO MAIL_LIST (CUSTNO, CNAME, STAT)
 VALUES (:cust_number, :cust_name, 'ACTIVE');
ENDFOR;
FOR EACH revised status
 display 'Customer number? ';
 read cust_number;
 display 'New status? ';
 read new_status;
 EXEC SQL UPDATE MAIL_LIST
 SET STAT = :new_status WHERE CUSTNO = :cust_number;
ENDFOR;
-- mark savepoint
EXEC SQL SAVEPOINT start_delete;
EXEC SQL DELETE FROM MAIL_LIST WHERE STAT = 'INACTIVE';
IF sqlca.sqlerrd(3) < 25 THEN -- check number of rows deleted
 display 'Number of rows deleted is ', sqlca.sqlerrd(3);
ELSE
 display 'Undoing deletion of ', sqlca.sqlerrd(3), ' rows';
 EXEC SQL WHENEVER SQLERROR GOTO sql_error;
 EXEC SQL ROLLBACK TO SAVEPOINT start_delete;
ENDIF;
EXEC SQL WHENEVER SQLERROR CONTINUE;
EXEC SQL COMMIT WORK RELEASE;
exit program;
sql_error:
 EXEC SQL WHENEVER SQLERROR CONTINUE;
 EXEC SQL ROLLBACK WORK RELEASE;
 display 'Processing error';
 exit program with an error;

Note that you cannot specify the RELEASE option in a ROLLBACK TO SAVEPOINT statement.

Rolling back to a savepoint erases any savepoints marked after that savepoint. The savepoint to which you roll back, however, is not erased. For example, if you mark five savepoints, then roll back to the third, only the fourth and fifth are erased. A COMMIT or ROLLBACK statement erases all savepoints.

By default, the number of active savepoints in each user session is limited to 5. An active savepoint is one that you marked since the last commit or rollback. Your Database Administrator (DBA) can raise the limit by increasing the value of the Oracle initialization parameter SAVEPOINTS. If you give two savepoints the same name, the earlier savepoint is erased.

Using the RELEASE Option

Oracle rolls back changes automatically if your program terminates abnormally. Abnormal termination occurs when your program does not explicitly commit or roll back work and disconnect from Oracle using the RELEASE option.

Normal termination occurs when your program runs its course, closes open cursors, explicitly commits or rolls back work, disconnects from Oracle, and returns control to the user. Your program will exit gracefully if the last SQL statement it executes is either

EXEC SQL COMMIT RELEASE;

or

EXEC SQL ROLLBACK RELEASE;

Otherwise, locks and cursors acquired by your user session are held after program termination until Oracle recognizes that the user session is no longer active. This might cause other users in a multiuser environment to wait longer than necessary for the locked resources.

Using the SET TRANSACTION Statement

You use the SET TRANSACTION statement to begin a read-only or read/write transaction, or to assign your current transaction to a specified rollback segment. A COMMIT, ROLLBACK, or data definition statement ends a read-only transaction.

Because they allow "repeatable reads," read-only transactions are useful for running multiple queries against one or more tables while other users update the same tables. During a read-only transaction, all queries refer to the same snapshot of the database, providing a multitable, multiquery, read-consistent view. Other users can continue to query or update data as usual. An example of the SET TRANSACTION statement follows:

EXEC SQL SET TRANSACTION READ ONLY;

The SET TRANSACTION statement must be the first SQL statement in a read-only transaction and can appear only once in a transaction. The READ ONLY parameter is required. Its use does not affect other transactions. Only the SELECT (without FOR UPDATE), LOCK TABLE, SET ROLE, ALTER SESSION, ALTER SYSTEM, COMMIT, and ROLLBACK statements are allowed in a read-only transaction.

In the example, as a store manager, you check sales activity for the day, the past week, and the past month by using a read-only transaction to generate a summary report. The report is unaffected by other users updating the database during the transaction.

EXEC SQL SET TRANSACTION READ ONLY; 
 EXEC SQL SELECT SUM(SALEAMT) INTO :daily FROM SALES 
 WHERE SALEDATE = SYSDATE; 
 EXEC SQL SELECT SUM(SALEAMT) INTO :weekly FROM SALES 
 WHERE SALEDATE > SYSDATE - 7; 
 EXEC SQL SELECT SUM(SALEAMT) INTO :monthly FROM SALES 
 WHERE SALEDATE > SYSDATE - 30; 
 EXEC SQL COMMIT WORK; 
-- simply ends the transaction since there are no changes 
-- to make permanent 
-- format and print report

Overriding Default Locking

By default, Oracle implicitly (automatically) locks many data structures for you. However, you can request specific data locks on rows or tables when it is to your advantage to override default locking. Explicit locking lets you share or deny access to a table for the duration of a transaction or ensure multitable and multiquery read consistency.

With the SELECT FOR UPDATE OF statement, you can explicitly lock specific rows of a table to make sure they do not change before an update or delete is executed. However, Oracle automatically obtains row-level locks at update or delete time. So, use the FOR UPDATE OF clause only if you want to lock the rows before the update or delete.

You can explicitly lock entire tables using the LOCK TABLE statement.

EXEC SQL DECLARE emp_cursor CURSOR FOR 
 SELECT ENAME, JOB, SAL FROM EMP WHERE DEPTNO = 20 
 FOR UPDATE OF SAL;

EXEC SQL DECLARE emp_cursor CURSOR FOR 
 SELECT ENAME, JOB, SAL FROM EMP WHERE DEPTNO = 20;

ORA-01002: fetch out of sequence

EXEC SQL LOCK TABLE EMP IN ROW SHARE MODE NOWAIT;

Fetching Across Commits

If you want to intermix commits and fetches, do not use the CURRENT OF clause. Instead, select the rowid of each row, then use that value to identify the current row during the update or delete. Consider the following example:

EXEC SQL DECLARE emp_cursor CURSOR FOR
 SELECT ENAME, SAL, ROWID FROM EMP WHERE JOB = 'CLERK';
...
EXEC SQL OPEN emp_cursor;
EXEC SQL WHENEVER NOT FOUND GOTO ...
LOOP
 EXEC SQL FETCH emp_cursor INTO :emp_name, :salary, :row_id;
 ... 
 EXEC SQL UPDATE EMP SET SAL = :new_salary
 WHERE ROWID = :row_id;
 EXEC SQL COMMIT;
ENDLOOP;

Handling Distributed Transactions

A distributed database is a single logical database comprising multiple physical databases at different nodes. A distributed statement is any SQL statement that accesses a remote node using a database link. A distributed transaction includes at least one distributed statement that updates data at multiple nodes of a distributed database. If the update affects only one node, the transaction is non-distributed.

When you issue a commit, changes to each database affected by the distributed transaction are made permanent. If you issue a rollback instead, all the changes are undone. However, if a network or computer fails during the commit or rollback, the state of the distributed transaction might be unknown or in doubt. In such cases, if you have FORCE TRANSACTION system privileges, you can manually commit or roll back the transaction at your local database by using the FORCE clause. The transaction must be identified by a quoted literal containing the transaction ID, which can be found in the data dictionary view DBA_2PC_PENDING. Some examples follow:

EXEC SQL COMMIT FORCE '22.31.83';
...
EXEC SQL ROLLBACK FORCE '25.33.86';

FORCE commits or rolls back only the specified transaction and does not affect your current transaction. Note that you cannot manually roll back in-doubt transactions to a savepoint.

The COMMENT clause in the COMMIT statement lets you specify a comment to be associated with a distributed transaction. If ever the transaction is in doubt, Oracle stores the text specified by COMMENT in the data dictionary view DBA_2PC_PENDING along with the transaction ID. The text must be a quoted literal <= 50 characters in length. An example follows:

EXEC SQL COMMIT COMMENT 'In-doubt trans; notify Order Entry';

For more information about distributed transactions, see Oracle Database Concepts.

Guidelines

The following guidelines will help you avoid some common problems.

EXEC SQL INSERT INTO EMP ...
EXEC SQL EXECUTE
 BEGIN UPDATE emp 
...
 ...
 EXCEPTION
 WHEN DUP_VAL_ON_INDEX THEN
 ROLLBACK;
 END;
END-EXEC;
...