Release 8.0
A54661_01
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| Programmer's Guide to the Oracle Pro*C/C++ Precompiler Release 8.0 A54661_01 |
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This chapter explains how to do transaction processing. You learn the basic techniques that safeguard the consistency of your database, including how to control whether changes to Oracle data are made permanent or undone. The following topics are discussed:
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 invoked when you run an application program or a tool such as SQL*Forms, and connect to Oracle.
Oracle allows user sessions to work "simultaneously" and share computer resources. To do this, Oracle must control concurrence, the accessing of the same data by many users. Without adequate concurrency controls, there might be a loss of data integrity. That is, changes to data or structures might be made in the wrong order.
Oracle uses locks (sometimes called enqueues) 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, once 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.
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 coming sections show you how to define and control transactions. Specifically, you learn how to
For details about the SQL statements discussed in this chapter, see Oracle8 Server SQL Reference.
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:
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 interrupt. 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.
If you do not subdivide your program with the COMMIT or ROLLBACK statement, Oracle treats the whole program as a single transaction (unless the program contains data definition statements, which issue automatic COMMITS).
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. Specifically, the COMMIT statement
The COMMIT statement has no effect on the values of host variables or on the flow of control in your program.
When MODE=ORACLE, explicit cursors that are not referenced in a CURRENT OF clause remain open across COMMITs. This can boost performance. For an example, see "Fetching Across COMMITs" on page 10-12.
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.
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 re-execute the function.
To undo part of a transaction, you use savepoints with the ROLLBACK statement and its TO SAVEPOINT clause. In the following 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 the third element of sqlerrd 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 (;;)
{
printf("Customer number? ");
gets(temp);
cust_number = atoi(temp);
printf("Customer name? ");
gets(cust_name);
EXEC SQL INSERT INTO mail_list (custno, cname, stat)
VALUES (:cust_number, :cust_name, 'ACTIVE');
...
}
for (;;)
{
printf("Customer number? ");
gets(temp);
cust_number = atoi(temp);
printf("New status? ");
gets(new_status);
EXEC SQL UPDATE mail_list
SET stat = :new_status
WHERE custno = :cust_number;
}
/* mark savepoint */
EXEC SQL SAVEPOINT start_delete;
EXEC SQL DELETE FROM mail_list
WHERE stat = 'INACTIVE';
if (sqlca.sqlerrd[2] < 25) /* check number of rows deleted */
printf("Number of rows deleted is %d\n", sqlca.sqlerrd[2]);
else
{
printf("Undoing deletion of %d rows\n", sqlca.sqlerrd[2]);
EXEC SQL WHENEVER SQLERROR GOTO sql_error;
EXEC SQL ROLLBACK TO SAVEPOINT start_delete;
}
EXEC SQL WHENEVER SQLERROR CONTINUE;
EXEC SQL COMMIT WORK RELEASE;
exit(0);
sql_error:
EXEC SQL WHENEVER SQLERROR CONTINUE;
EXEC SQL ROLLBACK WORK RELEASE;
printf("Processing error\n");
exit(1);
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.
If you give two savepoints the same name, the earlier savepoint is erased. A COMMIT or ROLLBACK statement erases all savepoints.
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 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.
If you start a transaction that you cannot finish (a SQL statement might not execute successfully, for example), ROLLBACK lets you return to the starting point, so that the database is not left in an inconsistent state. Specifically, the ROLLBACK statement
The ROLLBACK statement has no effect on the values of host variables or on the flow of control in your program.
When MODE=ORACLE, explicit cursors not referenced in a CURRENT OF clause remain open across ROLLBACKs.
Specifically, the ROLLBACK TO SAVEPOINT statement
Note that you cannot specify the RELEASE option in a ROLLBACK TO SAVEPOINT statement.
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 disconnects from 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, as shown in the following example:
EXEC SQL WHENEVER SQLERROR GOTO sql_error;
for (;;)
{
printf("Employee number? ");
gets(temp);
emp_number = atoi(temp);
printf("Employee name? ");
gets(emp_name);
EXEC SQL INSERT INTO emp (empno, ename)
VALUES (:emp_number, :emp_name);
...
}
...
sql_error:
EXEC SQL WHENEVER SQLERROR CONTINUE;
EXEC SQL ROLLBACK WORK RELEASE;
printf("Processing error\n");
exit(1);
Oracle automatically rolls back transactions if your program terminates abnormally. Refer to "Using the RELEASE Option" on page 10-9.
Before executing any SQL statement, Oracle marks an implicit savepoint (not available to you). Then, if the statement fails, Oracle automatically rolls it back and returns the applicable error code to sqlcode in the SQLCA. For example, if an INSERT statement causes an error by trying to insert a duplicate value in a unique index, the statement is rolled back.
Oracle can also roll back single SQL statements to break deadlocks. Oracle signals an error to one of the participating transactions and rolls back the current statement in that transaction.
Only work started by the failed SQL statement is lost; work done before that statement in the current transaction is saved. Thus, if a data definition statement fails, the automatic commit that precedes it is not undone.
Before executing a SQL statement, Oracle must parse it, that is, examine it to make sure it follows syntax rules and refers to valid database objects. Errors detected while executing a SQL statement cause a rollback, but errors detected while parsing the statement do not.
Oracle automatically rolls back changes 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.
You use the SET TRANSACTION statement to begin 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. 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, COMMIT, and ROLLBACK statements are allowed in a read-only transaction. For example, including an INSERT, DELETE, or SELECT FOR UPDATE OF statement causes an error.
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.
A COMMIT, ROLLBACK, or data definition statement ends a read-only transaction. (Recall that data definition statements issue an implicit COMMIT.)
In the following 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 */
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.
When you DECLARE a cursor that is referenced in the CURRENT OF clause of an UPDATE or DELETE statement, you use the FOR UPDATE OF clause to acquire exclusive row locks. SELECT FOR UPDATE OF identifies the rows that will be updated or deleted, then locks each row in the active set. This is useful, for example, when you want to base an update on the existing values in a row. You must make sure the row is not changed by another user before your update.
The FOR UPDATE OF clause is optional. For example, instead of coding
EXEC SQL DECLARE emp_cursor CURSOR FOR
SELECT ename, job, sal FROM emp WHERE deptno = 20
FOR UPDATE OF sal;
you can drop the FOR UPDATE OF clause and simply code
EXEC SQL DECLARE emp_cursor CURSOR FOR
SELECT ename, job, sal FROM emp WHERE deptno = 20;
The CURRENT OF clause signals the precompiler to add a FOR UPDATE clause if necessary. You use the CURRENT OF clause to refer to the latest row FETCHed from a cursor. For an example, see "Using the CURRENT OF Clause" on page 5-17.
If you use the FOR UPDATE OF clause, you cannot reference multiple tables.
An explicit FOR UPDATE OF or an implicit FOR UPDATE acquires exclusive row locks. All rows are locked at the OPEN, not as they are FETCHed. Row locks are released when you COMMIT or ROLLBACK (except when you ROLLBACK to a savepoint). Therefore, you cannot FETCH from a FOR UPDATE cursor after a COMMIT.
You use the LOCK TABLE statement to lock one or more tables in a specified lock mode. For example, the statement below locks the EMP table in row share mode. Row share locks allow concurrent access to a table; they prevent other users from locking the entire table for exclusive use.
EXEC SQL LOCK TABLE EMP IN ROW SHARE MODE NOWAIT;
The lock mode determines what other locks can be placed on the table. For example, many users can acquire row share locks on a table at the same time, but only one user at a time can acquire an exclusive lock. While one user has an exclusive lock on a table, no other users can INSERT, UPDATE, or DELETE rows in that table.
For more information about lock modes, see Oracle8 Server Concepts.
The optional keyword NOWAIT tells Oracle not to wait for a table if it has been locked by another user. Control is immediately returned to your program, so it can do other work before trying again to acquire the lock. (You can check sqlcode in the SQLCA to see if the LOCK TABLE failed.) If you omit NOWAIT, Oracle waits until the table is available; the wait has no set limit.
A table lock never keeps other users from querying a table, and a query never acquires a table lock. So, a query never blocks another query or an update, and an update never blocks a query. Only if two different transactions try to update the same row will one transaction wait for the other to complete.
Table locks are released when your transaction issues a COMMIT or ROLLBACK.
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. An example follows:
...
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 ...
for (;;)
{
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;
...
}
Note, however, that the FETCHed rows are not locked. So, you might get inconsistent results if another user modifies a row after you read it but before you update or delete it.
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 instead you issue a ROLLBACK, all the changes are undone. However, if a network or machine 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 Oracle8 Server Concepts.
The following guidelines will help you avoid some common problems.
When designing your application, group logically related actions together in one transaction. A well-designed transaction includes all the steps necessary to accomplish a given task-no more and no less.
Data in the tables you reference must be left in a consistent state. So, the SQL statements in a transaction should change the data in a consistent way. For example, a transfer of funds between two bank accounts should include a debit to one account and a credit to another. Both updates should either succeed or fail together. An unrelated update, such as a new deposit to one account, should not be included in the transaction.
If your application programs include SQL locking statements, make sure the Oracle users requesting locks have the privileges needed to obtain the locks. Your DBA can lock any table. Other users can lock tables they own or tables for which they have a privilege, such as ALTER, SELECT, INSERT, UPDATE, or DELETE.
If a PL/SQL block is part of a transaction, COMMITs and ROLLBACKs inside the block affect the whole transaction. In the following example, the ROLLBACK undoes changes made by the UPDATE and the INSERT:
EXEC SQL INSERT INTO EMP ...
EXEC SQL EXECUTE
BEGIN
UPDATE emp ...
...
EXCEPTION
WHEN DUP_VAL_ON_INDEX THEN
ROLLBACK;
...
END;
END-EXEC;
...
