Transaction control and isolation levels in Oracle

1. Transaction control and isolation levels in Oracle Evgeniya Kotzeva Vereo Technologies www.vereo.bg academy.devbg.org www.devbg.org

2. Contents • Transactioncontrol • Data Concurrency and Consistency in a Multiuser Environment • Locking

3. Database Transaction A database transaction consists of one of thefollowing: • DML statements which constitute one consistentchange to the data • One DDL statement • One DCL statement

4. Oracle Transaction Types

5. A transaction begins with the first executable SQL statement. A transaction ends with one of the following events: A COMMIT or ROLLBACK statement is issued A DDL or DCL statement executes (automatic commit) The user exits iSQL*Plus The system crashes Transaction boundaries

6. Advantages of COMMITand ROLLBACK With COMMIT and ROLLBACK statements, you can: • Ensure data consistency • Preview data changes before making changes permanent • Group logically related operations

7. Controlling transaction

8. Before COMMIT generated rollback segment records in buffers in the SGA generated redo log entries in the redo log buffer of the SGA. The changes have been made to the database buffers of the SGA. After COMMIT The internal transaction table for the associated rollback segment records updated with SCN LGWR writes SGA redo log entries to the online redo log file Oracle releases locks Oracle marks the transaction complete. COMMIT transaction

9. ROLLBACK Oracle undoes all transaction changes using the undo tablespace or rollback segments Oracle releases all the transaction’s locks of data The transaction ends ROLLBACK to SAVEPOINT Oracle rolls back only the statements run after the savepoint. Oracle preserves the specified savepoint, but all savepoints that were established after the specified one are lost Oracle releases all table and row locks acquired since that savepoint ROLLBACK transaction

10. State of the DataBefore COMMIT or ROLLBACK • The previous state of the datacan be recovered. • The current usercan reviewthe results of theDML operations by using the SELECT statement. • Other userscan not viewthe results of the DML statements by the current user. • The affected rowsare locked • Other userscannot changethe data within the affected rows.

11. State of the Data after COMMIT • Data changes are made permanent in the database. • The previous state of the data is permanently lost. • All users can view the results. • Locks on the affected rows are released; those rows are available for other users to manipulate. • All savepoints are erased.

12. Distributed database • Distributed transaction is a transaction that includes one or more statements that update data on two or more distinct nodes of a distributed database • A two-phase commit mechanism guarantees the data consistent in all nodes.

13. Autonomous transactions • Autonomous transactions are independent transactions that can be called from within another transaction • An autonomous transaction lets you leave the context of the calling transaction • You can call autonomous transactions from within a PL/SQL block by using the pragma AUTONOMOUS_TRANSACTION.

14. Data Concurrency and Consistency • Data concurrency means that many users can access data at the same time. • Data consistency means that each user sees a consistent view of the data, including visible changes made by the user’s own transactions and transactions of other users.

15. The isolation models prevents • Dirty reads • Nonrepeatable (fuzzy) reads • Phantom reads

16. Isolation levels (SQL92) controls

17. Oracle isolation levels

18. Multiversion Concurrency Control • Statement-level read consistency The data returned by a single query comes from a single point in time — the time that the query began • Transaction-level read consistency When a transaction executes in serializable mode, all data accesses reflect the state of the database as of the time the transaction began

19. Transactions and Read Consistency

20. “Snapshot too old” When commit or rollback has been executed, the pre-images can be overwritten even if they are needed to provide a read-consistent view to another query. "Snapshot too old" simply means that pre-images which the query needs to maintain a read-consistent view have been overwritten.

21. Common recommendations Common recommendations to reduce the possibility of "snapshot too old" are: • Keep transactions as fast as possible • Increase the size/number of rollback segments • Do not specify an OPTIMAL size for your rollback segments. • Increase the size of UNDO_RETENTION parameter (amount of committed undo information to retain in the database) • Avoid executing long-running queries when transactions which update the table are also executing.

22. Set the Isolation Level You can set the isolation level of a transaction by usingone of these statements at the beginning of a transaction: • SET TRANSACTION ISOLATION LEVELREAD COMMITTED; • SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; • SET TRANSACTION ISOLATION LEVELREAD ONLY;

23. Serializable Transaction Failure

24. Modes of Locking • Exclusive lock The mode prevents the associates resource from being shared • Share lock The mode allows the associated resource to be shared, depending on the operations involved

25. Deadlock

26. Types of Locks

27. Table Locks • RS: row share • RX: row exclusive • S: share • SRX: share row exclusive • X: exclusive

28. Data Lock Escalation • A transaction holds exclusive row locks for all rows inserted, updated, or deleted within the transaction. Because row locks are acquired at the highest degree of restrictiveness, no lock conversion is required or performed. • Oracle automatically converts a table lock of lower restrictiveness to one of higher restrictiveness as appropriate

29. MS SQL Isolation Levels

30. DB2 Isolation Levels Isolation levels • Repeatable Read (RR) • Read Stability (RS) • Cursor Stability (CS) • Uncommitted Read (UR) Levels of locking • Tablespace • Table • Row

31. Transactions in Oracle Въпроси?