Oracle® Database Advanced Application Developer's Guide 11g Release 2 (11.2) Part Number E10471-03 |
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This chapter explains how to use Oracle Flashback Technology in database applications.
Topics:
Oracle Flashback Technology is a group of Oracle Database features that let you view past states of database objects or to return database objects to a previous state without using point-in-time media recovery.
With flashback features, you can:
Perform queries that return past data
Perform queries that return metadata that shows a detailed history of changes to the database
Recover tables or rows to a previous point in time
Automatically track and archive transactional data changes
Roll back a transaction and its dependent transactions while the database remains online
Oracle Flashback features use the Automatic Undo Management (AUM) system to obtain metadata and historical data for transactions. They rely on undo data, which are records of the effects of individual transactions. For example, if a user runs an UPDATE
statement to change a salary from 1000 to 1100, then Oracle Database stores the value 1000 in the undo data.
Undo data is persistent and survives a database shutdown. By using flashback features, you can use undo data to query past data or recover from logical damage. Besides using it in flashback features, Oracle Database uses undo data to perform these actions:
Roll back active transactions
Recover terminated transactions by using database or process recovery
Provide read consistency for SQL queries
Topics:
For additional general information about flashback features, see Oracle Database Concepts
In application development, you can use these flashback features to report historical data or undo erroneous changes. (You can also use these features interactively as a database user or administrator.)
Oracle Flashback Query
Use this feature to retrieve data for an earlier time that you specify with the AS
OF
clause of the SELECT
statement. For more information, see "Using Oracle Flashback Query (SELECT AS OF)".
Oracle Flashback Version Query
Use this feature to retrieve metadata and historical data for a specific time interval (for example, to view all the rows of a table that ever existed during a given time interval). Metadata for each row version includes start and end time, type of change operation, and identity of the transaction that created the row version. To create an Oracle Flashback Version Query, use the VERSIONS
BETWEEN
clause of the SELECT
statement. For more information, see "Using Oracle Flashback Version Query".
Oracle Flashback Transaction Query
Use this feature to retrieve metadata and historical data for a given transaction or for all transactions in a given time interval. To perform an Oracle Flashback Transaction Query, select from the static data dictionary view FLASHBACK_TRANSACTION_QUERY
. For more information, see "Using Oracle Flashback Transaction Query".
Typically, you use Oracle Flashback Transaction Query with an Oracle Flashback Version Query that provides the transaction IDs for the rows of interest (see "Using Oracle Flashback Transaction Query with Oracle Flashback Version Query").
DBMS_FLASHBACK Package
Use this feature to set the internal Oracle Database clock to an earlier time so that you can examine data that was current at that time, or to roll back a transaction and its dependent transactions while the database remains online (see Flashback Transaction). For more information, see "Using DBMS_FLASHBACK Package".
Flashback Transaction
Use Flashback Transaction to roll back a transaction and its dependent transactions while the database remains online. This recovery operation uses undo data to create and run the corresponding compensating transactions that return the affected data to its original state. (Flashback Transaction is part of DBMS_FLASHBACK
package.) For more information, see "Using DBMS_FLASHBACK Package".
Flashback Data Archive (Oracle Total Recall)
Use Flashback Data Archive to automatically track and archive both regular queries and Oracle Flashback Query, ensuring SQL-level access to the versions of database objects without getting a snapshot-too-old error. For more information, see "Using Flashback Data Archive (Oracle Total Recall)".
These flashback features are primarily for data recovery. Typically, you use these features only as a database administrator.
This chapter focuses on the "Application Development Features". For more information about the database administration features, see Oracle Database Administrator's Guide and the Oracle Database Backup and Recovery User's Guide.
Oracle Flashback Table
Use this feature to restore a table to its state at a previous point in time. You can restore a table while the database is on line, undoing changes to only the specified table.
Oracle Flashback Drop
Use this feature to recover a dropped table. This feature reverses the effects of a DROP
TABLE
statement.
Oracle Flashback Database
Use this feature to quickly return the database to an earlier point in time, by undoing all of the changes that have taken place since then. This is fast, because you do not have to restore database backups.
Before you can use flashback features in your application, you or your database administrator must perform the configuration tasks described in these topics:
Configuring Your Database for Oracle Flashback Transaction Query
Enabling Oracle Flashback Operations on Specific LOB Columns
To configure your database for Automatic Undo Management (AUM), you or your database administrator must:
Create an undo tablespace with enough space to keep the required data for flashback operations.
The more often users update the data, the more space is required. The database administrator usually calculates the space requirement.
Enable AUM, as explained in Oracle Database Administrator's Guide. Set these database initialization parameters:
UNDO_MANAGEMENT
UNDO_TABLESPACE
UNDO_RETENTION
For a fixed-size undo tablespace, Oracle Database automatically tunes the system to give the undo tablespace the best possible undo retention.
For an automatically extensible undo tablespace, Oracle Database retains undo data longer than the longest query duration and the low threshold of undo retention specified by the UNDO_RETENTION
parameter.
Note:
You can queryV$UNDOSTAT
.TUNED_UNDORETENTION
to determine the amount of time for which undo is retained for the current undo tablespace. For more information about V$UNDOSTAT
, see Oracle Database Reference.Setting UNDO_RETENTION
does not guarantee that unexpired undo data is not discarded. If the system needs more space, Oracle Database can overwrite unexpired undo with more recently generated undo data.
Specify the RETENTION
GUARANTEE
clause for the undo tablespace to ensure that unexpired undo data is not discarded.
See Also:
Oracle Database Administrator's Guide for more information about creating an undo tablespace and enabling AUMTo configure your database for the Oracle Flashback Transaction Query feature, you or your database administrator must:
Ensure that Oracle Database is running with version 10.0 compatibility.
Enable supplemental logging:
ALTER DATABASE ADD SUPPLEMENTAL LOG DATA;
To configure your database for the Flashback Transaction feature, you or your database administrator must:
With the database mounted but not open, enable ARCHIVELOG
:
ALTER DATABASE ARCHIVELOG;
Open at least one archive log:
ALTER SYSTEM ARCHIVE LOG CURRENT;
If not done, enable minimal and primary key supplemental logging:
ALTER DATABASE ADD SUPPLEMENTAL LOG DATA; ALTER DATABASE ADD SUPPLEMENTAL LOG DATA (PRIMARY KEY) COLUMNS;
If you want to track foreign key dependencies, enable foreign key supplemental logging:
ALTER DATABASE ADD SUPPLEMENTAL LOG DATA (FOREIGN KEY) COLUMNS;
Note:
If you have very many foreign key constraints, enabling foreign key supplemental logging might not be worth the performance penalty.To enable flashback operations on specific LOB
columns of a table, use the ALTER
TABLE
statement with the RETENTION
option.
Because undo data for LOB
columns can be voluminous, you must define which LOB
columns to use with flashback operations.
See Also:
Oracle Database SecureFiles and Large Objects Developer's Guide to learn about LOB storage and theRETENTION
parameterYou or your database administrator must grant privileges to users, roles, or applications that must use these flashback features. For information about the GRANT
statement, see Oracle Database SQL Language Reference.
For Oracle Flashback Query and Oracle Flashback Version Query
To allow access to specific objects during queries, grant FLASHBACK
and SELECT
privileges on those objects.
To allow queries on all tables, grant the FLASHBACK
ANY
TABLE
privilege.
For Oracle Flashback Transaction Query
Grant the SELECT
ANY
TRANSACTION
privilege.
To allow execution of undo SQL code retrieved by an Oracle Flashback Transaction Query, grant SELECT
, UPDATE
, DELETE
, and INSERT
privileges for specific tables.
For DBMS_FLASHBACK Package
To allow access to the features in the DBMS_FLASHBACK
package, grant the EXECUTE
privilege on DBMS_FLASHBACK
.
For Flashback Data Archive (Oracle Total Recall)
To allow a specific user to enable Flashback Data Archive on tables, using a specific Flashback Data Archive, grant the FLASHBACK
ARCHIVE
object privilege on that Flashback Data Archive to that user. To grant the FLASHBACK
ARCHIVE
object privilege, you must either be logged on as SYSDBA
or have FLASHBACK
ARCHIVE
ADMINISTER
system privilege.
To allow execution of these statements, grant the FLASHBACK
ARCHIVE
ADMINISTER
system privilege:
CREATE
FLASHBACK
ARCHIVE
ALTER
FLASHBACK
ARCHIVE
DROP
FLASHBACK
ARCHIVE
To grant the FLASHBACK
ARCHIVE
ADMINISTER
system privilege, you must be logged on as SYSDBA
.
To create a default Flashback Data Archive, using either the CREATE
FLASHBACK
ARCHIVE
or ALTER
FLASHBACK
ARCHIVE
statement, you must be logged on as SYSDBA
.
To disable Flashback Data Archive for a table that has been enabled for Flashback Data Archive, you must either be logged on as SYSDBA
or have the FLASHBACK
ARCHIVE
ADMINISTER
system privilege.
To use Oracle Flashback Query, use a SELECT
statement with an AS
OF
clause. Oracle Flashback Query retrieves data as it existed at an earlier time. The query explicitly references a past time through a time stamp or System Change Number (SCN). It returns committed data that was current at that point in time.
Uses of Oracle Flashback Query include:
Recovering lost data or undoing incorrect, committed changes.
For example, if you mistakenly delete or update rows, and then commit them, you can immediately undo the mistake.
Comparing current data with the corresponding data at an earlier time.
For example, you can run a daily report that shows the change in data from yesterday. You can compare individual rows of table data or find intersections or unions of sets of rows.
Checking the state of transactional data at a particular time.
For example, you can verify the account balance of a certain day.
Simplifying application design by removing the need to store some kinds of temporal data.
Oracle Flashback Query lets you retrieve past data directly from the database.
Applying packaged applications, such as report generation tools, to past data.
Providing self-service error correction for an application, thereby enabling users to undo and correct their errors.
Topics:
For more information about the SELECT
AS
OF
statement, see Oracle Database SQL Language Reference.
Suppose that you discover at 12:30 PM that the row for employee Chung was deleted from the employees
table, and you know that at 9:30AM the data for Chung was correctly stored in the database. You can use Oracle Flashback Query to examine the contents of the table at 9:30 AM to find out what data was lost. If appropriate, you can restore the lost data.
Example 12-1 retrieves the state of the record for Chung
at 9:30AM, April 4, 2004:
Example 12-1 Retrieving a Lost Row with Oracle Flashback Query
SELECT * FROM employees AS OF TIMESTAMP TO_TIMESTAMP('2004-04-04 09:30:00', 'YYYY-MM-DD HH:MI:SS') WHERE last_name = 'Chung';
Example 12-2 restores Chung's information to the employees
table:
You can specify or omit the AS
OF
clause for each table and specify different times for different tables.
Note:
If a table is a Flashback Data Archive and you specify a time for it that is earlier than its creation time, the query returns zero rows for that table, rather than causing an error. (For information about Flashback Data Archives, see "Using Flashback Data Archive (Oracle Total Recall)".)You can use the AS
OF
clause in queries to perform data definition language (DDL) operations (such as creating and truncating tables) or data manipulation language (DML) statements (such as INSERT
and DELETE
) in the same session as Oracle Flashback Query.
To use the result of Oracle Flashback Query in a DDL or DML statement that affects the current state of the database, use an AS
OF
clause inside an INSERT
or CREATE
TABLE
AS
SELECT
statement.
If a possible 3-second error (maximum) is important to Oracle Flashback Query in your application, use an SCN instead of a time stamp. See "General Guidelines for Oracle Flashback Technology".
You can create a view that refers to past data by using the AS
OF
clause in the SELECT
statement that defines the view.
If you specify a relative time by subtracting from the current time on the database host, the past time is recalculated for each query. For example:
CREATE VIEW hour_ago AS SELECT * FROM employees AS OF TIMESTAMP (SYSTIMESTAMP - INTERVAL '60' MINUTE);
SYSTIMESTAMP
refers to the time zone of the database host environment.
You can use the AS
OF
clause in self-joins, or in set operations such as INTERSECT
and MINUS
, to extract or compare data from two different times.
You can store the results by preceding Oracle Flashback Query with a CREATE
TABLE
AS
SELECT
or INSERT
INTO
TABLE
SELECT
statement. For example, this query reinserts into table employees
the rows that existed an hour ago:
INSERT INTO employees (SELECT * FROM employees AS OF TIMESTAMP (SYSTIMESTAMP - INTERVAL '60' MINUTE) ) MINUS SELECT * FROM employees);
SYSTIMESTAMP
refers to the time zone of the database host environment.
Use Oracle Flashback Version Query to retrieve the different versions of specific rows that existed during a given time interval. A row version is created whenever a COMMIT
statement is executed.
Specify Oracle Flashback Version Query using the VERSIONS
BETWEEN
clause of the SELECT
statement. The syntax is:
VERSIONS {BETWEEN {SCN | TIMESTAMP} start AND end}
where start
and end
are expressions representing the start and end, respectively, of the time interval to be queried. The time interval includes (start
and end
).
Oracle Flashback Version Query returns a table with a row for each version of the row that existed at any time during the specified time interval. Each row in the table includes pseudocolumns of metadata about the row version, described in Table 12-1. This information can reveal when and how a particular change (perhaps erroneous) occurred to your database.
Table 12-1 Oracle Flashback Version Query Row Data Pseudocolumns
Pseudocolumn Name | Description |
---|---|
Starting System Change Number (SCN) or If this pseudocolumn is |
|
SCN or If this pseudocolumn is |
|
Identifier of the transaction that created the row version. |
|
Operation performed by the transaction: For user updates of an index key, Oracle Flashback Version Query might treat an |
A given row version is valid starting at its time VERSIONS_START*
up to, but not including, its time VERSIONS_END*
. That is, it is valid for any time t such that VERSIONS_START*
<= t < VERSIONS_END*
. For example, this output indicates that the salary was 10243 from September 9, 2002, included, to November 25, 2003, excluded.
VERSIONS_START_TIME VERSIONS_END_TIME SALARY ------------------- ----------------- ------ 09-SEP-2003 25-NOV-2003 10243
Here is a typical use of Oracle Flashback Version Query:
SELECT versions_startscn, versions_starttime, versions_endscn, versions_endtime, versions_xid, versions_operation, last_name, salary FROM employees VERSIONS BETWEEN TIMESTAMP TO_TIMESTAMP('2008-12-18 14:00:00', 'YYYY-MM-DD HH24:MI:SS') AND TO_TIMESTAMP('2008-12-18 17:00:00', 'YYYY-MM-DD HH24:MI:SS') WHERE first_name = 'John';
You can use VERSIONS_XID
with Oracle Flashback Transaction Query to locate this transaction's metadata, including the SQL required to undo the row change and the user responsible for the change—see "Using Oracle Flashback Transaction Query".
See Also:
Oracle Database SQL Language Reference for information about Oracle Flashback Version Query pseudocolumns and the syntax of theVERSIONS
clauseUse Oracle Flashback Transaction Query to retrieve metadata and historical data for a given transaction or for all transactions in a given time interval. Oracle Flashback Transaction Query queries the static data dictionary view FLASHBACK_TRANSACTION_QUERY
, whose columns are described in Oracle Database Reference.
The column UNDO_SQL
shows the SQL code that is the is the logical opposite of the DML operation performed by the transaction. You can usually use this code to reverse the logical steps taken during the transaction. However, there are cases where the SQL_UNDO
code is not the exact opposite of the original transaction. For example, a SQL_UNDO
INSERT
operation might not insert a row back in a table at the same ROWID
from which it was deleted.
This statement queries the FLASHBACK_TRANSACTION_QUERY
view for transaction information, including the transaction ID, the operation, the operation start and end SCNs, the user responsible for the operation, and the SQL code that shows the logical opposite of the operation:
SELECT xid, operation, start_scn, commit_scn, logon_user, undo_sql FROM flashback_transaction_query WHERE xid = HEXTORAW('000200030000002D');
This statement uses Oracle Flashback Version Query as a subquery to associate each row version with the LOGON_USER
responsible for the row data change:
SELECT xid, logon_user FROM flashback_transaction_query WHERE xid IN ( SELECT versions_xid FROM employees VERSIONS BETWEEN TIMESTAMP TO_TIMESTAMP('2003-07-18 14:00:00', 'YYYY-MM-DD HH24:MI:SS') AND TO_TIMESTAMP('2003-07-18 17:00:00', 'YYYY-MM-DD HH24:MI:SS') );
Note:
If you queryFLASHBACK_TRANSACTION_QUERY
without specifying XID
in the WHERE
clause, the query scans many unrelated rows, degrading performance.See Also:
Oracle Database Backup and Recovery User's Guide. for information about how a database administrator can use Flashback Table to restore an entire table, rather than individual rows
Oracle Database Administrator's Guide for information about how a database administrator can use Flashback Table to restore an entire table, rather than individual rows
In this example, a database administrator does this:
DROP TABLE emp; CREATE TABLE emp ( empno NUMBER PRIMARY KEY, empname VARCHAR2(16), salary NUMBER ); INSERT INTO emp (empno, empname, salary) VALUES (111, 'Mike', 555); COMMIT; DROP TABLE dept; CREATE TABLE dept ( deptno NUMBER, deptname VARCHAR2(32) ); INSERT INTO dept (deptno, deptname) VALUES (10, 'Accounting'); COMMIT;
Now emp
and dept
have one row each. In terms of row versions, each table has one version of one row. Suppose that an erroneous transaction deletes empno
111
from table emp
:
UPDATE emp SET salary = salary + 100 WHERE empno = 111; INSERT INTO dept (deptno, deptname) VALUES (20, 'Finance'); DELETE FROM emp WHERE empno = 111; COMMIT;
Next, a transaction reinserts empno
111
into the emp
table with a new employee name:
INSERT INTO emp (empno, empname, salary) VALUES (111, 'Tom', 777); UPDATE emp SET salary = salary + 100 WHERE empno = 111; UPDATE emp SET salary = salary + 50 WHERE empno = 111; COMMIT;
The database administrator detects the application error and must diagnose the problem. The database administrator issues this query to retrieve versions of the rows in the emp
table that correspond to empno 111
. The query uses Oracle Flashback Version Query pseudocolumns:
SELECT versions_xid XID, versions_startscn START_SCN, versions_endscn END_SCN, versions_operation OPERATION, empname, salary FROM emp VERSIONS BETWEEN SCN MINVALUE AND MAXVALUE WHERE empno = 111;
Results are similar to:
XID START_SCN END_SCN O EMPNAME SALARY ---------------- ---------- ---------- - ---------------- ---------- 09001100B2200000 10093466 I Tom 927 030002002B210000 10093459 D Mike 555 0800120096200000 10093375 10093459 I Mike 555 3 rows selected.
The results table rows are in descending chronological order. The third row corresponds to the version of the row in the table emp
that was inserted in the table when the table was created. The second row corresponds to the row in emp
that the erroneous transaction deleted. The first row corresponds to the version of the row in emp
that was reinserted with a new employee name.
The database administrator identifies transaction 030002002B210000
as the erroneous transaction and uses Oracle Flashback Transaction Query to audit all changes made by this transaction:
SELECT xid, start_scn, commit_scn, operation, logon_user, undo_sql
FROM flashback_transaction_query
WHERE xid = HEXTORAW('000200030000002D');
Results are similar to:
XID START_SCN COMMIT_SCN OPERATION LOGON_USER ---------------- ---------- ---------- --------- ------------------------------ UNDO_SQL -------------------------------------------------------------------------------- 030002002B210000 10093452 10093459 DELETE HR insert into "HR"."EMP"("EMPNO","EMPNAME","SALARY") values ('111','Mike','655'); 030002002B210000 10093452 10093459 INSERT HR delete from "HR"."DEPT" where ROWID = 'AAATjuAAEAAAAJrAAB'; 030002002B210000 10093452 10093459 UPDATE HR update "HR"."EMP" set "SALARY" = '555' where ROWID = 'AAATjsAAEAAAAJ7AAA'; 030002002B210000 10093452 10093459 BEGIN HR 4 rows selected.
To make the result of the next query easier to read, the database administrator uses these SQL*Plus commands:
COLUMN operation FORMAT A9 COLUMN table_name FORMAT A10 COLUMN table_owner FORMAT A11
To see the details of the erroneous transaction and all subsequent transactions, the database administrator performs this query:
SELECT xid, start_scn, commit_scn, operation, table_name, table_owner FROM flashback_transaction_query WHERE table_owner = 'HR' AND start_timestamp >= TO_TIMESTAMP ('2002-04-16 11:00:00','YYYY-MM-DD HH:MI:SS');
Results are similar to:
XID START_SCN COMMIT_SCN OPERATION TABLE_NAME TABLE_OWNER ---------------- ---------- ---------- --------- ---------- ----------- 02000E0074200000 10093435 10093446 INSERT DEPT HR 030002002B210000 10093452 10093459 DELETE EMP HR 030002002B210000 10093452 10093459 INSERT DEPT HR 030002002B210000 10093452 10093459 UPDATE EMP HR 0800120096200000 10093374 10093375 INSERT EMP HR 09001100B2200000 10093462 10093466 UPDATE EMP HR 09001100B2200000 10093462 10093466 UPDATE EMP HR 09001100B2200000 10093462 10093466 INSERT EMP HR 8 rows selected.
Note:
Because the preceding query does not specify theXID
in the WHERE
clause, it scans many unrelated rows, degrading performance.ORA_ROWSCN
is a pseudocolumn of any table that is not fixed or external. It represents the SCN of the most recent change to a given row in the current session; that is, the most recent COMMIT
operation for the row in the current session. For example:
SELECT ora_rowscn, last_name, salary FROM employees WHERE employee_id = 200;
Result:
ORA_ROWSCN LAST_NAME SALARY ---------- ------------------------- ---------- 9371092 Whalen 4288
The most recent COMMIT
operation for the row in the current session took place at approximately SCN 9371092. To convert an SCN to the corresponding TIMESTAMP
value, use the function SCN_TO_TIMESTAMP
(documented in Oracle Database SQL Language Reference).
ORA_ROWSCN
is a conservative upper bound of the latest commit time—the actual commit SCN can be somewhat earlier. ORA_ROWSCN
is more precise (closer to the actual commit SCN) for a row-dependent table (created using CREATE TABLE
with the ROWDEPENDENCIES
clause). For more information about ORA_ROWSCN
and ROWDEPENDENCIES
, see Oracle Database SQL Language Reference.
Note:
ORA_ROWSCN
is not supported for Flashback Query. Instead, use the version query pseudocolumns, which are provided explicitly for Flashback Query. For information about these pseudocolumns, see Oracle Database SQL Language Reference.Topics:
Your application examines a row of data and records the corresponding ORA_ROWSCN
as 202553. Then, your application invokes a packaged subprogram, whose implementation details you cannot see, which might or might not change the same row (and commit the change). Later, your application must update the row only if the packaged subprogram did not change it. Make the operation conditional—update the row only if ORA_ROWSCN
is still 202553, as in this equivalent interactive statement:
UPDATE employees SET salary = salary + 100 WHERE employee_id = 200 AND ora_rowscn = 202553;
If the packaged subprogram changed the row, then ORA_ROWSCN
is no longer 9371092, and the update fails.
Your application queries again to obtain the new row data and ORA_ROWSCN
. Suppose that the ORA_ROWSCN
is now 415639
. The application tries the conditional update again, using the new ORA_ROWSCN
. This time, the update succeeds, and it is committed. An interactive equivalent is:
UPDATE employees SET salary = salary + 100 WHERE employee_id = 7788 AND ora_rowscn = 415639;
When a VPD policy is added to a table, it is no longer possible to select the ORA_ROWSCN
pseudocolumn. However, because ORA_ROWSCN
is available inside the policy function, you can:
Create a function that returns a row SCN, as in Example 12-3.
In the policy predicate function, add a predicate that stores the row SCN in the context that the function uses while processing rows. For example:
||' AND f_ora_rowscn('||object_name||'.ora_rowscn) = 1'
Use the function to fetch the row. For example:
SELECT t.*, get_rowscn(t.rowid) "ORA_ROWSCN" FROM test_table t;
Example 12-3 Function that Can Return Row SCN from Table that has VPD
-- Create context that function uses while processing rows: CREATE OR REPLACE FUNCTION f_ora_rowscn (rowscn IN NUMBER) RETURN NUMBER AS BEGIN DBMS_SESSION.SET_CONTEXT('STORE_ROWSCN','ROWSCN',rowscn); RETURN 1; END; / CREATE CONTEXT store_rowscn USING f_ora_rowscn; -- Create function that returns row SCN for each row: CREATE OR REPLACE FUNCTION get_rowscn (row IN ROWID) RETURN VARCHAR2 AS BEGIN RETURN sys_context('STORE_ROWSCN','ROWSCN'); END; /
The DBMS_FLASHBACK
package provides the same functionality as Oracle Flashback Query, but Oracle Flashback Query is sometimes more convenient.
The DBMS_FLASHBACK
package acts as a time machine: you can turn back the clock, perform normal queries as if you were at that earlier time, and then return to the present. Because you can use the DBMS_FLASHBACK
package to perform queries on past data without special clauses such as AS
OF
or VERSIONS
BETWEEN
, you can reuse existing PL/SQL code to query the database at earlier times.
You must have the EXECUTE
privilege on the DBMS_FLASHBACK
package.
To use the DBMS_FLASHBACK
package in your PL/SQL code:
Specify a past time by invoking either DBMS_FLASHBACK
.ENABLE_AT_TIME
or DBMS_FLASHBACK
.ENABLE_AT_SYSTEM_CHANGE_NUMBER
.
Perform regular queries (that is, queries without special flashback-feature syntax such as AS
OF
). Do not perform DDL or DML operations.
The database is queried at the specified past time.
Return to the present time by invoking DBMS_FLASHBACK.DISABLE
.
You must invoke DBMS_FLASHBACK
.DISABLE
before invoking DBMS_FLASHBACK
.ENABLE_AT_TIME
or DBMS_FLASHBACK
.ENABLE_AT_SYSTEM_CHANGE_NUMBER
again. You cannot nest enable/disable pairs.
To use a cursor to store the results of queries, open the cursor before invoking DBMS_FLASHBACK
.DISABLE
. After storing the results and invoking DBMS_FLASHBACK
.DISABLE
, you can:
Perform INSERT
or UPDATE
operations to modify the current database state by using the stored results from the past.
Compare current data with the past data. After invoking DBMS_FLASHBACK
.DISABLE
, open a second cursor. Fetch from the first cursor to retrieve past data; fetch from the second cursor to retrieve current data. You can store the past data in a temporary table and then use set operators such as MINUS
or UNION
to contrast or combine the past and current data.
You can invoke DBMS_FLASHBACK
.GET_SYSTEM_CHANGE_NUMBER
at any time to get the current System Change Number (SCN). DBMS_FLASHBACK
.GET_SYSTEM_CHANGE_NUMBER
always returns the current SCN regardless of previous invocations of DBMS_FLASHBACK
.ENABLE
.
See Also:
Oracle Database PL/SQL Packages and Types Reference for details of the DBMS_FLASHBACK
package
The DBMS_FLASHBACK
.TRANSACTION_BACKOUT
procedure rolls back a transaction and its dependent transactions while the database remains online. This recovery operation uses undo data to create and run the compensating transactions that return the affected data to its original state.
The transactions being rolled back are subject to these restrictions:
They cannot have performed DDL operations that changed the logical structure of database tables.
They cannot use Large Object (LOB) Data Types:
BFILE
BLOB
CLOB
NCLOB
They cannot use features that LogMiner does not support.
The features that LogMiner supports depends on the value of the COMPATIBLE
initialization parameter for the database that is rolling back the transaction. The default value is the release number of the most recent major release.
Flashback Transaction inherits SQL data type support from LogMiner. Therefore, if LogMiner fails due to an unsupported SQL data type in a the transaction, Flashback Transaction fails too.
Some data types, though supported by LogMiner, do not generate undo information as part of operations that modify columns of such types. Therefore, Flashback Transaction does not support tables containing these data types. These include tables with BLOB, CLOB and XML type.
See Also:
Oracle Data Guard Concepts and Administration for information about data type and DDL support on a logical standby database
Oracle Database SQL Language Reference for information about LOB data types
Oracle Database Utilities for information about LogMiner
Oracle Database Administrator's Guide for information about the COMPATIBLE
initialization parameter
Topics:
In the context of Flashback Transaction, transaction 2 can depend on transaction 1 in any of these ways:
Transaction 1 changes a row of a table, and later transaction 2 changes the same row.
A table has a primary key constraint on column c. In a row of the table, column c has the value v. Transaction 1 deletes that row, and later transaction 2 inserts a row into the same table, assigning the value v to column c.
In table b, column b1 has a foreign key constraint on column a1 of table a. Transaction 1 changes a value in a1, and later transaction 2 changes a value in b1.
The parameters of the TRANSACTION_BACKOUT
procedure are:
Number of transactions to be backed out
List of transactions to be backed out, identified either by name or by XID
Time hint, if you identify transactions by name
Specify a time that is earlier than any transaction started.
Backout option from Table 12-2
For the syntax of the TRANSACTION_BACKOUT
procedure and detailed parameter descriptions, see Oracle Database PL/SQL Packages and Types Reference.
Table 12-2 Flashback TRANSACTION_BACKOUT Options
Option | Description |
---|---|
|
Backs out specified transactions and all dependent transactions in a post-order fashion (that is, children are backed out before parents are backed out). Without |
|
Default. Backs out specified transactions, which are expected to have no dependent transactions. First dependent transactions causes an error and appears in |
|
Backs out specified transactions, ignoring dependent transactions. Server runs undo SQL statements for specified transactions in reverse order of commit times. If no constraints break and you are satisfied with the result, you can commit the changes; otherwise, you can roll them back. |
|
Backs out changes to nonconflicting rows of the specified transactions. Database remains consistent, but transaction atomicity is lost. |
TRANSACTION_BACKOUT
analyzes the transactional dependencies, performs DML operations, and generates reports. TRANSACTION_BACKOUT
does not commit the DML operations that it performs as part of transaction backout, but it holds all the required locks on rows and tables in the right form, preventing other dependencies from entering the system. To make the transaction backout permanent, you must explicitly commit the transaction.
To see the reports that TRANSACTION_BACKOUT
generates, query the static data dictionary views *_FLASHBACK_TXN_STATE
and *_FLASHBACK_TXN_REPORT
.
The static data dictionary view *_FLASHBACK_TXN_STATE
shows whether a transaction is active or backed out. If a transaction appears in this view, it is backed out.
*_FLASHBACK_TXN_STATE
is maintained atomically for compensating transactions. If a compensating transaction is backed out, all changes that it made are also backed out, and *_FLASHBACK_TXN_STATE
reflects this. For example, if compensating transaction ct
backs out transactions t1
and t2
, then t1
and t2
appear in *_FLASHBACK_TXN_STATE
. If ct
itself is later backed out, the effects of t1
and t2
are reinstated, and t1
and t2
disappear from *_FLASHBACK_TXN_STATE
.
The static data dictionary view *_FLASHBACK_TXN_REPORT
provides a detailed report for each backed-out transaction.
A Flashback Data Archive provides the ability to track and store transactional changes to a table over its lifetime. A Flashback Data Archive is useful for compliance with record stage policies and audit reports.
A Flashback Data Archive consists of one or more tablespaces or parts thereof. You can have multiple Flashback Data Archives. If you are logged on as SYSDBA
, you can specify a default Flashback Data Archive for the system. A Flashback Data Archive is configured with retention time. Data archived in the Flashback Data Archive is retained for the retention time.
By default, flashback archiving is off for any table. You can enable flashback archiving for a table if all of these conditions are true:
You have the FLASHBACK
ARCHIVE
object privilege on the Flashback Data Archive that you want to use for that table.
The table is neither nested, clustered, temporary, remote, or external.
The table contains neither LONG
nor nested columns.
After flashback archiving is enabled for a table, you can disable it only if you either have the FLASHBACK
ARCHIVE
ADMINISTER
system privilege or you are logged on as SYSDBA
.
When choosing a Flashback Data Archive for a specific table, consider the data retention requirements for the table and the retention times of the Flashback Data Archives on which you have the FLASHBACK
ARCHIVE
object privilege.
Topics:
See Also:
http://www.oracle.com/database/total-recall.html
for more information about Oracle Total RecallCreate a Flashback Data Archive with the CREATE
FLASHBACK
ARCHIVE
statement, specifying:
Name of the Flashback Data Archive
Name of the first tablespace of the Flashback Data Archive
(Optional) Maximum amount of space that the Flashback Data Archive can use in the first tablespace
The default is unlimited. Unless your space quota on the first tablespace is also unlimited, you must specify this value; otherwise, error ORA-55621 occurs.
Retention time (number of days that Flashback Data Archive data for the table is guaranteed to be stored)
If you are logged on as SYSDBA
, you can also specify that this is the default Flashback Data Archive for the system. If you omit this option, you can still make this Flashback Data Archive the default later (see "Specifying the Default Flashback Data Archive").
Examples
Create a default Flashback Data Archive named fla1
that uses up to 10 G of tablespace tbs1
, whose data are retained for one year:
CREATE FLASHBACK ARCHIVE DEFAULT fla1 TABLESPACE tbs1 QUOTA 10G RETENTION 1 YEAR;
Create a Flashback Data Archive named fla2
that uses tablespace tbs2
, whose data are retained for two years:
CREATE FLASHBACK ARCHIVE fla2 TABLESPACE tbs2 RETENTION 2 YEAR;
For more information about the CREATE
FLASHBACK
ARCHIVE
statement, see Oracle Database SQL Language Reference.
With the ALTER
FLASHBACK
ARCHIVE
statement, you can:
Change the retention time of a Flashback Data Archive
Purge some or all of its data
Add, modify, and remove tablespaces
Note:
Removing all tablespaces of a Flashback Data Archive causes an error.If you are logged on as SYSDBA
, you can also use the ALTER
FLASHBACK
ARCHIVE
statement to make a specific file the default Flashback Data Archive for the system.
Examples
Make Flashback Data Archive fla1
the default Flashback Data Archive:
ALTER FLASHBACK ARCHIVE fla1 SET DEFAULT;
To Flashback Data Archive fla1
, add up to 5 G of tablespace tbs3
:
ALTER FLASHBACK ARCHIVE fla1 ADD TABLESPACE tbs3 QUOTA 5G;
To Flashback Data Archive fla1
, add as much of tablespace tbs4
as needed:
ALTER FLASHBACK ARCHIVE fla1 ADD TABLESPACE tbs4;
Change the maximum space that Flashback Data Archive fla1
can use in tablespace tbs3
to 20 G:
ALTER FLASHBACK ARCHIVE fla1 MODIFY TABLESPACE tbs3 QUOTA 20G;
Allow Flashback Data Archive fla1
to use as much of tablespace tbs1
as needed:
ALTER FLASHBACK ARCHIVE fla1 MODIFY TABLESPACE tbs1;
Change the retention time for Flashback Data Archive fla1
to two years:
ALTER FLASHBACK ARCHIVE fla1 MODIFY RETENTION 2 YEAR;
Remove tablespace tbs2
from Flashback Data Archive fla1
:
ALTER FLASHBACK ARCHIVE fla1 REMOVE TABLESPACE tbs2;
(Tablespace tbs2
is not dropped.)
Purge all historical data from Flashback Data Archive fla1
:
ALTER FLASHBACK ARCHIVE fla1 PURGE ALL;
Purge all historical data older than one day from Flashback Data Archive fla1
:
ALTER FLASHBACK ARCHIVE fla1 PURGE BEFORE TIMESTAMP (SYSTIMESTAMP - INTERVAL '1' DAY);
Purge all historical data older than SCN 728969 from Flashback Data Archive fla1
:
ALTER FLASHBACK ARCHIVE fla1 PURGE BEFORE SCN 728969;
For more information about the ALTER
FLASHBACK
ARCHIVE
statement, see Oracle Database SQL Language Reference.
Drop a Flashback Data Archive with the DROP
FLASHBACK
ARCHIVE
statement. Dropping a Flashback Data Archive deletes its historical data, but does not drop its tablespaces.
Example
Remove Flashback Data Archive fla1
and all its historical data, but not its tablespaces:
DROP FLASHBACK ARCHIVE fla1;
For more information about the DROP
FLASHBACK
ARCHIVE
statement, see Oracle Database SQL Language Reference.
By default, the system has no default Flashback Data Archive. If you are logged on as SYSDBA
, you can specify default Flashback Data Archive in either of these ways:
Specify the name of an existing Flashback Data Archive in the SET
DEFAULT
clause of the ALTER
FLASHBACK
ARCHIVE
statement. For example:
ALTER FLASHBACK ARCHIVE fla1 SET DEFAULT;
If fla1
does not exist, an error occurs.
Include DEFAULT
in the CREATE
FLASHBACK
ARCHIVE
statement when you create a Flashback Data Archive. For example:
CREATE FLASHBACK ARCHIVE DEFAULT fla2 TABLESPACE tbs1 QUOTA 10G RETENTION 1 YEAR;
The default Flashback Data Archive for the system is the default Flashback Data Archive for every user who does not have his or her own default Flashback Data Archive.
See Also:
Oracle Database SQL Language Reference for more information about the CREATE
FLASHBACK
ARCHIVE
statement
Oracle Database SQL Language Reference for more information about the ALTER
DATABASE
statement
By default, flashback archiving is disabled for any table. You can enable flashback archiving for a table if you have the FLASHBACK
ARCHIVE
object privilege on the Flashback Data Archive that you want to use for that table.
To enable flashback archiving for a table, include the FLASHBACK
ARCHIVE
clause in either the CREATE
TABLE
or ALTER
TABLE
statement. In the FLASHBACK
ARCHIVE
clause, you can specify the Flashback Data Archive where the historical data for the table are stored. The default is the default Flashback Data Archive for the system. If you specify a nonexistent Flashback Data Archive, an error occurs.
If you enable flashback archiving for a table, but AUM is disabled, error ORA-55614 occurs when you try to modify the table.
If a table has flashback archiving enabled, and you try to enable it again with a different Flashback Data Archive, an error occurs.
After flashback archiving is enabled for a table, you can disable it only if you either have the FLASHBACK
ARCHIVE
ADMINISTER
system privilege or you are logged on as SYSDBA
. To disable flashback archiving for a table, specify NO
FLASHBACK
ARCHIVE
in the ALTER
TABLE
statement. (It is unnecessary to specify NO
FLASHBACK
ARCHIVE
in the CREATE
TABLE
statement, because that is the default.)
See Also:
Oracle Database SQL Language Reference for more information about theFLASHBACK
ARCHIVE
clause of the CREATE
TABLE
statement, including restrictions on its useExamples
Create table employee
and store the historical data in the default Flashback Data Archive:
CREATE TABLE employee (EMPNO NUMBER(4) NOT NULL, ENAME VARCHAR2(10), JOB VARCHAR2(9), MGR NUMBER(4)) FLASHBACK ARCHIVE;
Create table employee
and store the historical data in the Flashback Data Archive fla1
:
CREATE TABLE employee (EMPNO NUMBER(4) NOT NULL, ENAME VARCHAR2(10), JOB VARCHAR2(9), MGR NUMBER(4)) FLASHBACK ARCHIVE fla1;
Enable flashback archiving for the table employee
and store the historical data in the default Flashback Data Archive:
ALTER TABLE employee FLASHBACK ARCHIVE;
Enable flashback archiving for the table employee
and store the historical data in the Flashback Data Archive fla1
:
ALTER TABLE employee FLASHBACK ARCHIVE fla1;
Disable flashback archiving for the table employee
:
ALTER TABLE employee NO FLASHBACK ARCHIVE;
Flashback Data Archive supports many DDL statements, including some that alter the table definition or move data. For example:
ALTER
TABLE
statement that does any of the following:
Adds, drops, renames, or modifies a column
Adds, drops, or renames a constraint
Drops or truncates a partition or subpartition operation
TRUNCATE
TABLE
statement
RENAME
statement that renames a table
Some DDL statements cause error ORA-55610 when used on a table enabled for Flashback Data Archive. For example:
ALTER
TABLE
statement that includes an UPGRADE
TABLE
clause, with or without an INCLUDING
DATA
clause
ALTER
TABLE
statement that moves or exchanges a partition or subpartition operation
DROP
TABLE
statement
If you must use unsupported DDL statements on a table enabled for Flashback Data Archive, use the DBMS_FLASHBACK_ARCHIVE
.DISASSOCIATE_FBA
procedure to disassociate the base table from its Flashback Data Archive. To reassociate the Flashback Data Archive with the base table afterward, use the DBMS_FLASHBACK_ARCHIVE
.REASSOCIATE_FBA
procedure.
See Also:
Oracle Database SQL Language Reference for information about the ALTER
TABLE
statement
Oracle Database SQL Language Reference for information about the TRUNCATE
TABLE
statement
Oracle Database SQL Language Reference for information about the RENAME
statement
Oracle Database SQL Language Reference for information about the DROP
TABLE
statement
Oracle Database PL/SQL Packages and Types Reference for information about the DBMS_FLASHBACK_ARCHIVE
package
Table 12-3 lists and briefly describes the static data dictionary views that you can query for information about Flashback Data Archive files.
Table 12-3 Static Data Dictionary Views for Flashback Data Archive Files
View | Description |
---|---|
|
Displays information about Flashback Data Archive files. |
|
Displays tablespaces of Flashback Data Archive files. |
|
Displays information about tables that are enabled for Data Flashback Archive files. |
See Also:
Oracle Database Reference for detailed information about *_FLASHBACK_ARCHIVE
Oracle Database Reference for detailed information about *_FLASHBACK_ARCHIVE_TS
Oracle Database Reference for detailed information about *_FLASHBACK_ARCHIVE_TABLES
Scenario: Using Flashback Data Archive to Enforce Digital Shredding
Scenario: Using Flashback Data Archive to Access Historical Data
Your company wants to "shred" (delete) historical data changes to the Taxes
table after ten years. When you create the Flashback Data Archive for Taxes
, you specify a retention time of ten years:
CREATE FLASHBACK ARCHIVE taxes_archive TABLESPACE tbs1 RETENTION 10 YEAR;
When history data from transactions on Taxes
exceeds the age of ten years, it is purged. (The Taxes
table itself, and history data from transactions less than ten years old, are not purged.)
You want to be able to retrieve the inventory of all items at the beginning of the year from the table inventory
, and to be able to retrieve the stock price for each symbol in your portfolio at the close of business on any specified day of the year from the table stock_data
.
Create a default Flashback Data Archive named fla1
that uses up to 10 G of tablespace tbs1
, whose data are retained for five years (you must be logged on as SYSDBA
):
CREATE FLASHBACK ARCHIVE DEFAULT fla1 TABLESPACE tbs1 QUOTA 10G RETENTION 5 YEAR;
Enable Flashback Data Archive for the tables inventory
and stock_data
, and store the historical data in the default Flashback Data Archive:
ALTER TABLE inventory FLASHBACK ARCHIVE; ALTER TABLE stock_data FLASHBACK ARCHIVE;
To retrieve the inventory of all items at the beginning of the year 2007, use this query:
SELECT product_number, product_name, count FROM inventory AS OF TIMESTAMP TO_TIMESTAMP ('2007-01-01 00:00:00', 'YYYY-MM-DD HH24:MI:SS');
To retrieve the stock price for each symbol in your portfolio at the close of business on July 23, 2007, use this query:
SELECT symbol, stock_price FROM stock_data AS OF TIMESTAMP TO_TIMESTAMP ('2007-07-23 16:00:00', 'YYYY-MM-DD HH24:MI:SS') WHERE symbol IN my_portfolio;
You want users to be able to generate reports from the table investments
, for data stored in the past five years.
Create a default Flashback Data Archive named fla2
that uses up to 20 G of tablespace tbs1
, whose data are retained for five years (you must be logged on as SYSDBA
):
CREATE FLASHBACK ARCHIVE DEFAULT fla2 TABLESPACE tbs1 QUOTA 20G RETENTION 5 YEAR;
Enable Flashback Data Archive for the table investments
, and store the historical data in the default Flashback Data Archive:
ALTER TABLE investments FLASHBACK ARCHIVE;
Lisa wants a report on the performance of her investments at the close of business on December 31, 2006. She uses this query:
SELECT * FROM investments AS OF TIMESTAMP TO_TIMESTAMP ('2006-12-31 16:00:00', 'YYYY-MM-DD HH24:MI:SS') WHERE name = 'LISA';
A medical insurance company must audit a medical clinic. The medical insurance company has its claims in the table Billings
, and creates a default Flashback Data Archive named fla4
that uses up to 100 G of tablespace tbs1
, whose data are retained for 10 years:
CREATE FLASHBACK ARCHIVE DEFAULT fla4 TABLESPACE tbs1 QUOTA 100G RETENTION 10 YEAR;
The company enables Flashback Data Archive for the table Billings
, and stores the historical data in the default Flashback Data Archive:
ALTER TABLE Billings FLASHBACK ARCHIVE;
On May 1, 2007, clients were charged the wrong amounts for some diagnoses and tests. To see the records as of May 1, 2007, the company uses this query:
SELECT date_billed, amount_billed, patient_name, claim_Id, test_costs, diagnosis FROM Billings AS OF TIMESTAMP TO_TIMESTAMP('2007-05-01 00:00:00', 'YYYY-MM-DD HH24:MI:SS');
An end user recovers from erroneous transactions that were previously committed in the database. The undo data for the erroneous transactions is no longer available, but because the required historical information is available in the Flashback Data Archive, Flashback Query works seamlessly.
Lisa manages a software development group whose product sales are doing well. On November 3, 2007, she decides to give all her level-three employees who have more than two years of experience a salary increase of 10% and a promotion to level four. Lisa asks her HR representative, Bob, to make the changes.
Using the HR web application, Bob updates the employee
table to give Lisa's level-three employees a 10% raise and a promotion to level four. Then Bob finishes his work for the day and leaves for home, unaware that he omitted the requirement of two years of experience in his transaction. A few days later, Lisa checks to see if Bob has done the updates and finds that everyone in the group was given a raise! She calls Bob immediately and asks him to correct the error.
At first, Bob thinks he cannot return the employee table to its prior state without going to the backups. Then he remembers that the employee
table has Flashback Data Archive enabled.
First, he verifies that no other transaction modified the employee
table after his: The commit time stamp from the transaction query corresponds to Bob's transaction, two days ago.
Next, Bob uses these statements to return the employee
table to the way it was before his erroneous change:
DELETE EMPLOYEE WHERE MANAGER = 'LISA JOHNSON'; INSERT INTO EMPLOYEE SELECT * FROM EMPLOYEE AS OF TIMESTAMP (SYSTIMESTAMP - INTERVAL '2' DAY) WHERE MANAGER = 'LISA JOHNSON';
Bob then reexecutes the update that Lisa had requested.
Use the DBMS_FLASHBACK
.ENABLE
and DBMS_FLASHBACK
.DISABLE
procedures around SQL code that you do not control, or when you want to use the same past time for several consecutive queries.
Use Oracle Flashback Query, Oracle Flashback Version Query, or Oracle Flashback Transaction Query for SQL code that you write, for convenience. An Oracle Flashback Query, for example, is flexible enough to do comparisons and store results in a single query.
To obtain an SCN to use later with a flashback feature, use DBMS_FLASHBACK
.GET_SYSTEM_CHANGE_NUMBER
.
To compute or retrieve a past time to use in a query, use a function return value as a time stamp or SCN argument. For example, add or subtract an INTERVAL
value to the value of the SYSTIMESTAMP
function.
Use Oracle Flashback Query, Oracle Flashback Version Query, and Oracle Flashback Transaction Query locally or remotely. An example of a remote Oracle Flashback Query is:
(SELECT * FROM employees@some_remote_host AS OF TIMESTAMP (SYSTIMESTAMP - INTERVAL '60' MINUTE);
To ensure database consistency, always perform a COMMIT
or ROLLBACK
operation before querying past data.
Remember that all flashback processing uses the current session settings, such as national language and character set, not the settings that were in effect at the time being queried.
Remember that DDLs that alter the structure of a table (such as drop/modify column, move table, drop partition, truncate table/partition, and add constraint) invalidate any existing undo data for the table. If you try to retrieve data from a time before such a DDL executed, error ORA-01466 occurs. DDL operations that alter the storage attributes of a table (such as PCTFREE
, INITRANS
, and MAXTRANS
) do not invalidate undo data.
To query past data at a precise time, use an SCN. If you use a time stamp, the actual time queried might be up to 3 seconds earlier than the time you specify. Oracle Database uses SCNs internally and maps them to time stamps at a granularity of 3 seconds.
For example, suppose that the SCN values 1000 and 1005 are mapped to the time stamps 8:41 AM and 8:46 AM, respectively. A query for a time between 8:41:00 and 8:45:59 AM is mapped to SCN 1000; an Oracle Flashback Query for 8:46 AM is mapped to SCN 1005. Therefore, if you specify a time that is slightly after a DDL operation (such as a table creation) Oracle Database might use an SCN that is just before the DDL operation, causing error ORA-01466.
You cannot retrieve past data from a dynamic performance (V$
) view. A query on such a view always returns current data.
You can perform queries on past data in static data dictionary views, such as *_TABLES
.
Use the DBMS_STATS
package to generate statistics for all tables involved in an Oracle Flashback Query. Keep the statistics current. Oracle Flashback Query uses the cost-based optimizer, which relies on these statistics.
Minimize the amount of undo data that must be accessed. Use queries to select small sets of past data using indexes, not to scan entire tables. If you must scan a full table, add a parallel hint to the query.
The performance cost in I/O is the cost of paging in data and undo blocks that are not in the buffer cache. The performance cost in CPU use is the cost of applying undo information to affected data blocks. When operating on changes in the recent past, flashback operations are CPU-bound.
For Oracle Flashback Version Query, use index structures. Oracle Database keeps undo data for index changes and data changes. Performance of index lookup-based Oracle Flashback Version Query is an order of magnitude faster than the full table scans that are otherwise needed.
In an Oracle Flashback Transaction Query, the xid
column is of the type RAW(8)
. To take advantage of the index built on the xid
column, use the HEXTORAW
conversion function: HEXTORAW(xid)
.
A Oracle Flashback Query against a materialized view does not take advantage of query rewrite optimization.