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The ROLLBACK statement undoes, or rolls back, the changes made by SQL statements within a transaction, so long as you didn t already commit the transaction. Once you issue the ROLLBACK statement, none of the changes made to the tables by SQL statements since the transaction began are recorded to the database permanently. You can roll back an entire transaction by rolling back all
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CHAPTER 6 ORACLE TRANSACTION MANAGEMENT
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changes made by all the SQL statements within that transaction by simply using the ROLLBACK command as follows: SQL> ROLLBACK; You can also partially roll back the effects of a transaction by using save points in the transaction. Using a save point, you can roll back to the last SAVEPOINT command in the transaction, as follows: SQL> ROLLBACK TO SAVEPOINT POINT A; The SAVEPOINT statement acts like a bookmark for the uncommitted statements in the transaction. In the second of the preceding examples, the rollback is only up to point A in the transaction. Everything before point A is still committed. Oracle uses the undo records in the undo tablespace to roll back the transactions after a ROLLBACK command. It also releases any locks that are held, and it marks the transaction as complete. If the rollback is to a save point, the transaction is deemed incomplete, and you can continue the transaction. If a SQL statement errors out during its execution, all the changes made by it to that point are automatically rolled back. This is known as a statement-level rollback. A deadlock is a condition that occurs when SQL statements from two sessions contend for the same piece of data. In that situation, Oracle automatically rolls back one of the SQL statements to resolve deadlocks.
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Transaction Properties
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Transactions in RDBMSs must possess four important properties, symbolized by the ACID acronym, which stands for atomicity, consistency, isolation, and durability of transactions. Transaction management, in general, means supporting database transactions so the ACID properties are maintained. Let s look at the transaction properties in more detail: Atomicity: Either a transaction should be performed entirely or none of it should be performed. That is, you can t have the database performing only a part of a transaction. For example, if you issue a SQL statement that should delete 1,000 records, your entire transaction should abort (roll back) if your database crashes after the transaction deletes 999 records. Consistency: The database is supposed to ensure that it s always in a consistent state. For example, in a banking transaction that involves debits from your savings account and credits to your checking account, the database can t just credit your checking account and stop. This will lead to inconsistent data, and the consistency property of transactions ensures that the database doesn t leave data in an inconsistent state. All transactions must preserve the consistency of the database. For example, if you wish to delete a department ID from the Department table, the database shouldn t permit your action if some employees in the Employees table belong to the department you re planning on eliminating. Isolation: Isolation means that although there s concurrent access to the database by multiple transactions, each transaction must appear to be executing in isolation. The isolation property of transactions ensures that a transaction is kept from viewing changes made by another transaction before the first transaction commits. This property is upheld by the database s concurrency control mechanisms, as you ll see in the following sections. Although concurrent access is a hallmark of the relational database, isolation techniques make it appear as though users are executing transactions serially, one after another. This chapter discusses how Oracle implements concurrency control the assurance of atomicity and isolation of individual transactions in a concurrently accessed database.
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