|Programmer's Guide to the Oracle Precompilers, 1.8||
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, 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 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.
The coming sections show you how to define and control transactions. Specifically, you learn how to
You end a transaction in one of the following ways:
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.
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.
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.
Only work started by the failed SQL statement is lost; work done before that statement in the current transaction is kept. Thus, if a data definition statement fails, the automatic commit that precedes it is not undone.
Note: 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 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.
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
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 per 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.
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;
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.
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 below, 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
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.
The FOR UPDATE OF clause is optional. For instance, instead of
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" .
ORA-01002: fetch out of sequence
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 the Oracle7 Server Application Developer's Guide.
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 table lock 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.
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;
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.
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 Oracle7 Server Concepts.
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.
EXEC SQL INSERT INTO EMP ... EXEC SQL EXECUTE BEGIN UPDATE emp ... ... EXCEPTION WHEN DUP_VAL_ON_INDEX THEN ROLLBACK; END; END-EXEC; ...
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