Online table redefinition allows you to restructure your Oracle® table in production without making the data unavailable. You might be comfortable using temp tables to move data around, but there is a better solution.
Introduction
Staging the data and moving it around while you restructure your table makes
both the table and the data unavailable for a certain period, which is a less
than favorable situation for businesses. This is when the DBMS_REDEFINITION
package saves the day, as shown in the following image:
Purpose
Periodically, you need to modify the logical or physical structure of an Oracle table for the following reasons:
- To enhance queries or Data Manipulation Language (DML) performance
- To accommodate application changes
- To manage storage
Oracle Database provides a mechanism to make table structure modifications without significantly affecting the availability of the table, which is known as online table redefinition. Redefining tables online provides a substantial performance boost when compared to traditional methods of redefining tables.
When a table is redefined online, it is accessible to both queries and DML during most of the redefinition process. The table is locked in exclusive mode for only a short period, which is independent of the size of the table and the complexity of the redefinition. The redefinition process is completely transparent to users.
Online table redefinition requires free space that is approximately equivalent to the space currently used by the table being redefined.
There are numerous ways to reorganize a table. When downtime is a challenge,
then the DBMS_REDEFINITION package is the best option for this task.
Redefine a table online
Use the following steps to redefine a table online.
-
Choose the redefinition method, either
by keyorby rowids.By key: Select a primary key or pseudo-primary key to use for the redefinition. Pseudo-primary keys are unique keys where all the component columns contain
NOT NULLconstraints. For this method, the versions of the tables before and after redefinition should consist of the same primary key columns. This is the preferred and default method of redefinition.By rowid: Use this method if no key is available. In this method, a hidden column, named
M_ROW$$, is added to the post-redefined version of the table. This column should be dropped or marked as unused after the redefinition is complete. IfCOMPATIBLEis set to 10.2.0 or higher, the final phase of redefinition automatically sets this column as unused. You can then use theALTER TABLE ... DROP UNUSED COLUMNSstatement to drop it. You cannot use this method on index-organized tables. -
Verify that the table can be redefined online by invoking the
CAN_REDEF_TABLEprocedure. If the table is not a candidate for online redefinition, then this procedure raises an error indicating why the table cannot be redefined online. -
Create an empty interim table (in the same schema as the table to be redefined) with all of the desired logical and physical attributes.
-
It is not necessary to create the interim table with all the indexes, constraints, grants, and triggers of the table being redefined. This can be done automatically by using the
COPY_TABLE_DEPENDENTSprocedure. -
To improve performance in larger tables, you can set it in parallel by using the following commands:
ALTER SESSION force parallel dml parallel degree-of-parallelism; ALTER SESSION force parallel query parallel degree-of-parallelism; -
The
FINISH_REDEF_TABLEcommand completes the redefinition of the table. During this procedure, the original table is locked in an exclusive mode for a short time, independent of the amount of data in the original table. However,FINISH_REDEF_TABLEwaits for all the pending DML operations to commit before completing the redefinition. -
If you used
rowidsfor the redefinition and yourCOMPATIBLEinitialization parameter is set to 10.1.0 or lower, you need to drop the hidden columnM_ROW$$that was added to the redefined table. You can also set the column to “UNUSED” by using the following command:ALTER TABLE <table_name> SET UNUSED (M_ROW$$);If
COMPATIBLEis 10.2.0 or higher, this hidden column is automatically set toUNUSEDwhen the redefinition completes. You can then drop the column with theALTER TABLE ... DROP UNUSED COLUMNSstatement. Wait for any long-running queries against the interim table to complete and then drop the interim table.
Sample table redefinition
The following section shows examples of the various commands and outputs for a sample table redefinition.
Start sqlplus
The following sample demonstrates starting sqlplus:
[oracle@vm215 ~]$ sqlplus amit/amit
SQL*Plus: Release 11.2.0.3.0 Production on Sat Oct 29 05:44:44 2016
Copyright (c) 1982, 2011, Oracle. All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
Create a demo table
The following sample demonstrates creating a demo table name TEST1 under
AMIT schema.
SQL> CREATE TABLE TEST1 ( ID NUMBER(10) ,
ENAME VARCHAR2(10),
SAL NUMBER(10) ) ;
Insert bulk rows
The following sample demonstrates inserting bulk rows and setting
PPA_AGGGREGATE_TARGET in AMIT schema to max value.
SQL> INSERT INTO AMIT.TEST1 SELECT ROWNUM, 'T'|| ROWNUM,
DBMS_RANDOM.VALUE(100000, 999999) FROM DUAL CONNECT BY LEVEL < 1000000;
999999 ROWS CREATED.
SQL> COMMIT;
COMMIT COMPLETE.
Create dependent objects for testing
The following sample demonstrates creating dependent objects related to table
TEST1 rows, so that you can see what happens during an online redefinition.
View creation
SQL> CREATE VIEW TEST1_VW AS SELECT * FROM TEST1 ;
VIEW CREATED.
Sequence creation
SQL> CREATE SEQUENCE TEST_SEQ ;
SEQUENCE CREATED.
Procedure creation
CREATE OR REPLACE PROCEDURE PROC1 (P_ID IN NUMBER)
AS V_ID NUMBER ;
BEGIN
SELECT SAL
INTO V_ID
FROM TEST1
WHERE ID = P_ID;
END;
/
PROCEDURE CREATED.
DML trigger creation
SQL> CREATE OR REPLACE TRIGGER AMIT_TRIG
BEFORE INSERT OR UPDATE ON TEST1
FOR EACH ROW
DECLARE
X NUMBER;
BEGIN
SELECT COUNT(*) INTO X
FROM TEST1
WHERE ID = :NEW.ID;
IF X > 0 THEN
RAISE_APPLICATION_ERROR(-20501, 'ID' || :NEW.ID || ' ALREADY EXISTS');
END IF;
END;
/
TRIGGER CREATED.
Primary key creation
SQL> ALTER TABLE TEST1 ADD CONSTRAINT TEST1_ID_PK PRIMARY KEY (ID) ;
TABLE ALTERED.
Get the status before redefinition
SQL> COLUMN OBJECT_NAME FORMAT A20
SELECT OBJECT_NAME, OBJECT_TYPE, STATUS FROM USER_OBJECTS ORDER BY OBJECT_NAME;SQL>
OBJECT_NAME OBJECT_TYPE STATUS
-------------------- ------------------- -------
AMIT_TRIG TRIGGER VALID
PROC1 PROCEDURE VALID
TEST1 TABLE VALID
TEST1_ID_PK INDEX VALID
TEST1_VW VIEW VALID
TEST_SEQ SEQUENCE VALID
6 ROWS SELECTED.
Check table for redefinition
The following sample demonstrates checking that the table can be redefined online
using either rowids or primary key:
Using primary key
SQL> EXEC DBMS_REDEFINITION.CAN_REDEF_TABLE ('AMIT','TEST1',DBMS_REDEFINITION.CONS_USE_PK);
PL/SQL PROCEDURE SUCCESSFULLY COMPLETED.
Using rowid
SQL> EXEC DBMS_REDEFINITION.CAN_REDEF_TABLE ('AMIT','TEST1',DBMS_REDEFINITION.CONS_USE_ROWID);
PL/SQL PROCEDURE SUCCESSFULLY COMPLETED.
Create a replica of the interim table
The following sample demonstrates creating a replica of new interim table without any dependent objects:
SQL> CREATE TABLE TEST1_REORG AS SELECT * FROM TEST1 WHERE ROWNUM=5 ;
TABLE CREATED.
SQL> SELECT COUNT(*) FROM TEST1_REORG ;
COUNT(*)
----------
0
SQL> SELECT COUNT(*) FROM TEST1;
COUNT(*)
----------
999999
Connect to database
The following sample demonstrates connecting by using a privilege user to execute the table redefinition task:
[oracle@vm215 ~]$ sqlplus / as sysdba
Sql*plus: release 11.2.0.3.0 production on sat oct 29 05:16:48 2016
Copyright (c) 1982, 2011, oracle. All rights reserved.
Connected to:
Oracle database 11g enterprise edition release 11.2.0.3.0 - 64bit production
With the partitioning, olap, data mining and real application testing options
Start redefinition
The following sample demonstrates starting redefinition by using a primary key:
SQL> EXEC DBMS_REDEFINITION.START_REDEF_TABLE('AMIT','TEST1', 'TEST1_REORG');
PL/SQL PROCEDURE SUCCESSFULLY COMPLETED.
Copy dependent objects
The following sample demonstrates automatically copying dependent objects like
mview, primary key, view, sequence, and triggers. IGNORE_ERROR is set to
TRUE to avoid primary key violation with the COPY_TABLE_DEPENDENTS
command.
SQL> DECLARE
N PLS_INTEGER;
BEGIN
DBMS_REDEFINITION.COPY_TABLE_DEPENDENTS('AMIT', 'TEST1','TEST1_REORG',
DBMS_REDEFINITION.CONS_ORIG_PARAMS, TRUE, TRUE, TRUE, TRUE, N);
END;
/
PL/SQL PROCEDURE SUCCESSFULLY COMPLETED.
Check for errors
The following sample demonstrates checking for errors in
DBA_REDEFINITION_ERRORS view:
SQL> COL OBJECT_NAME FOR A25
SET LIN200 PAGES 200
COL DDL_TEXT FOR A60
SELECT OBJECT_NAME, BASE_TABLE_NAME, DDL_TXT
FROM DBA_REDEFINITION_ERRORS;
NO ROWS SELECTED
Validate both tables
The following sample demonstrates validating the row count on both tables and syncing with the interim table:
SQL> SELECT COUNT(*) FROM AMIT.TEST1_REORG ;
COUNT(*)
----------
999999
SQL> SELECT COUNT(*) FROM AMIT.TEST1 ;
COUNT(*)
----------
999999
SQL> EXEC DBMS_REDEFINITION.SYNC_INTERIM_TABLE('AMIT', 'TEST1', 'TEST1_REORG');
PL/SQL PROCEDURE SUCCESSFULLY COMPLETED.
Finish the redefinition
The following sample demonstrates finishing the redefinition:
SQL> EXEC DBMS_REDEFINITION.FINISH_REDEF_TABLE ('AMIT', 'TEST1', 'TEST1_REORG');
PL/SQL PROCEDURE SUCCESSFULLY COMPLETED.
SQL> COLUMN OBJECT_NAME FORMAT A40
SELECT OBJECT_NAME, OBJECT_TYPE, STATUS
FROM DBA_OBJECTS
WHERE OWNER='AMIT';
OBJECT_NAME OBJECT_TYPE STATUS
--------------------- ------------------- -------
TEST1_VW VIEW INVALID
TEST_SEQ SEQUENCE VALID
PROC1 PROCEDURE VALID
TEST1 TABLE VALID
TEST1_REORG TABLE VALID
TEST1_ID_PK INDEX VALID
TMP$$_TEST1_ID_PK0 INDEX VALID
TMP$$_AMIT_TRIG0 TRIGGER INVALID
AMIT_TRIG TRIGGER INVALID
9 ROWS SELECTED.
Check error and recompile the schema
The following sample demonstrates recompiling the schema with complete dependency, which is necessary because of the invalid triggers in the preceding step:
SQL> EXEC UTL_RECOMP.RECOMP_SERIAL('AMIT') ;
PL/SQL PROCEDURE SUCCESSFULLY COMPLETED.
SQL> SELECT OBJECT_NAME, OBJECT_TYPE, STATUS FROM DBA_OBJECTS WHERE OWNER='AMIT';
OBJECT_NAME OBJECT_TYPE STATUS
---------------------------------------- ------------------- -------
TEST1_VW VIEW VALID
TEST_SEQ SEQUENCE VALID
PROC1 PROCEDURE VALID
TEST1 TABLE VALID
TEST1_REORG TABLE VALID
TEST1_ID_PK INDEX VALID
TMP$$_TEST1_ID_PK0 INDEX VALID
TMP$$_AMIT_TRIG0 TRIGGER VALID
AMIT_TRIG TRIGGER VALID
9 ROWS SELECTED.
Drop the interim table
The following sample demonstrates dropping the interim table:
SQL> DROP TABLE AMIT.TEST1_REORG;
TABLE DROPPED.
Conclusion
If a table structure needs to be modified and simultaneously accessed by end
users, use DBMS_REFDEFINITION.
This feature helps reorganize the data without any downtime, thus avoiding challenges caused by downtime for customers in an online transaction processing (OLTP) environment.
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