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George Orwell: Animal Farm
This chapter defines a transaction and describes how you can manage your work using transactions. It includes:
A transaction begins with the first executable SQL statement. A transaction ends when it is committed or rolled back, either explicitly (with a COMMIT or ROLLBACK statement) or implicitly (when a DDL statement is issued).
To illustrate the concept of a transaction, consider a banking database. When a bank customer transfers money from a savings account to a checking account, the transaction might consist of three separate operations: decrement the savings account, increment the checking account, and record the transaction in the transaction journal.
Oracle must allow for two situations. If all three SQL statements can be performed to maintain the accounts in proper balance, the effects of the transaction can be applied to the database. However, if something (such as insufficient funds, invalid account number, or a hardware failure) prevents one or two of the statements in the transaction from completing, the entire transaction must be rolled back so that the balance of all accounts is correct.
Figure 12 - 1 illustrates the banking transaction example.
Figure 12 - 1. A Banking Transaction
Executing successfully means that a single statement was parsed and found to be a valid SQL construction, and that the entire statement executed without error as an atomic unit (for example, all rows of a multi-row update are changed). However, until the transaction that contains the statement is committed, the transaction can be rolled back, and all of the changes of the statement can be undone. A statement, rather than a transaction, executes successfully.
Committing means that a user has said either explicitly or implicitly "make the changes in this transaction permanent". The changes made by the SQL statement(s) of your transaction only become permanent and visible to other users after your transaction has been committed. Only other users' transactions that started after yours will see the committed changes.
Errors that cause statement rollbacks are errors discovered during the execution stage of SQL statement processing (such as attempting to insert a duplicate value in a primary key or an invalid number into a numeric column), not the parsing stage (such as syntax errors in a SQL statement). Single SQL statements can also be rolled back to resolve deadlocks (competition for the same data); see "Deadlock Detection" ![[*]](jump.gif)
.
Therefore, a SQL statement that fails causes the loss only of any work it would have performed itself; it does not cause the loss of any work that preceded it in the current transaction. If the statement is a DDL statement, the implicit commit that immediately preceded it is not undone. This is a statement-level rollback.
Note: Users cannot directly refer to implicit savepoints in rollback statements.
When a transaction begins, Oracle assigns the transaction to an available rollback segment to record the rollback entries for the new transaction. See "Transactions and Rollback Segments" for more information about this topic.
A transaction ends when any of the following occurs:
Note: Applications should always explicitly commit or roll back transactions before program termination.
Before a transaction that has modified data is committed, the following will have occurred:
for a discussion of locks). for more information about the LGWR and DBWR.
Oracle allows you to roll back an entire uncommitted transaction. Alternatively, you can roll back the trailing portion of an uncommitted transaction to a marker called a savepoint; see the following section, "Savepoints", for a complete explanation of savepoints.
In rolling back an entire transaction, without referencing any savepoints, the following occurs:
for a discussion of locks).
for a discussion of locks).Using savepoints, you can arbitrarily mark your work at any point within a long transaction. This allows you the option of later rolling back work performed before the current point in the transaction (the end of the transaction) but after a declared savepoint within the transaction. For example, you can use savepoints throughout a long complex series of updates so that if you make an error, you do not need to resubmit every statement.
Savepoints are also useful in application programs in a similar way. If a procedure contains several functions, you can create a savepoint before each function begins. Then, if a function fails, it is easy to return the data to its state before the function began and then to re-execute the function with revised parameters or perform a recovery action.
After a rollback to a savepoint, Oracle releases the data locks obtained by rolled back statements. Other transactions that were waiting for the previously locked resources can proceed. Other transactions that want to update previously locked rows can do so.
During a discrete transaction, all changes made to any data are deferred until the transaction commits. Of course, other concurrent transactions are unable to see the uncommitted changes of a transaction whether the transaction is discreet or not.
Oracle generates redo information, but stores it in a separate location in memory. When the transaction issues a commit request, Oracle writes the redo information to the redo log file (along with other group commits), and applies the changes to the database block directly to the block. Oracle returns control to the application once the commit completes. This eliminates the need to generate undo information, since the block actually is not modified until the transaction is committed, and the redo information is stored in the redo log buffers.
For more information on discrete transactions, see Oracle7 Server Tuning.
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