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Oracle dbms_redefinition

Oracle Tips by Burleson Consulting

The Oracle online table reorganization package, (dbms_redefinition) is used to reorganize tables while they are accepting updates. 

- See here for a full example of dbms_redefinition

The online reorganization packages does this by creating a snapshot on the target table and applying all table changes after the table has been reorganized with the "Create table as select" command:

Here is a simple execution of an online table reorganization:

Here is a simple execution of an online table reorganization:

-- Check table can be redefined
EXEC Dbms_Redefinition.Can_Redef_Table('SCOTT', 'EMPLOYEES');
-- Create new table with CTAS
CREATE TABLE scott.employees2
SELECT empno, first_name, salary as sal
FROM employees WHERE 1=2;
-- Start Redefinition
EXEC Dbms_Redefinition.Start_Redef_Table( -
  'SCOTT', -
-- Optionally synchronize new table with interim data
EXEC dbms_redefinition.sync_interim_table( -
-- Add new keys, FKs and triggers
ALTER TABLE employees2 ADD
-- Complete redefinition
EXEC Dbms_Redefinition.Finish_Redef_Table( -
-- Remove original table which now has the name of the new table
DROP TABLE employees2;


Using Oracle dbms_redefinition

To solve the problem of doing table reorgs while the database accepts updates, Oracle9i has introduced Online Table Redefinitions using the DBMS_REDEFINITION package.

The dbms_redefinition package allows you to copy a table (using CTAS), create a snapshot on the
table, enqueue changes during the redefinition, and then re-synchronize the restructured table
with the changes that have accumulated during reorganization.

exec dbms_redefinition.abort_redef_table('PUBS','TITLES','TITLES2');

alter table titles add constraint pk_titles primary key (title_id);

exec dbms_redefinition.can_redef_table('PUBS','TITLES');

create table titles2


select * from titles;

exec dbms_redefinition.start_redef_table('PUBS','TITLES','TITLES2','title_id title_id,title
title,type type,pub_id pub_id,price price,advance advance,royalty*1.1 royalty,ytd_sales
ytd_sales,notes notes,pubdate pubdate');

exec dbms_redefinition.sync_interim_table('PUBS','TITLES','TITLES2');

exec dbms_redefinition.finish_redef_table('PUBS','TITLES','TITLES2');

If your reorganization fails, you must take special steps to make it re-start. Because the
redefinition requires creating a snapshot, you must call dbms_redefinition.abort_redef_table to
release the snapshot to re-start you procedure.

The ‘dbms_redefinition.abort_redef_table’ procedure which accepts 3 parameters (schema, original table name, holding table name), and which “pops the stack” and allows you to start over.

No database is 100% self-reliant or self-maintaining, which is a good thing for DBA job security. However, the last few major versions of Oracle have greatly increased its self-diagnostic and self-monitoring capabilities. Only database structural reorganization remains one of those tasks best left to the DBA to decide when it is appropriate to perform and when to schedule its execution. That is because data is the life blood of any modern organization, and while doing various database reorganizations, the following possibilities exist:

  • The process could blow-up mid-stream, so data may be left offline

  • The process is resource-intensive and takes significant time to execute

  • Data could be momentarily inconsistent between key steps

  • Probably advisable to consider doing a backup operation just prior to

The key point is that structural reorganizations are generally important events in any database’s life cycle. Even when a reorganization activity can theoretically be performed entirely online with little or no downtime, it is often a safer bet to perform any such activities in a controlled environment. Because the one time something that can not go wrong does, the DBA will be in a better situation to resume or recover if there are not frantic customers breathing down his neck. So schedule any reorganization event with extreme caution and over- compensation.


Now  with all that said, Oracle provides a robust and reliable package for performing many common online table level reorganizations – dbms_redefinition. Much like the dbms_metadata package, dbms_redefinition provides an almost limitless set of use cases or scenarios that it can address. Many people will probably just use the OEM graphical interface, but  here is a very common example that should fulfill as key need as well as serve as a foundation for one’s own modifications. The following are the key basic steps:

1.      Verify that the table is a candidate for online redefinition

2.      Create an interim table

3.      Enable parallel DML operations

4.      Start the redefinition process (and do not stop until step 9 is done)

5.      Copy dependent objects

6.      Check for any errors

7.      Synchronize the interim table (optional)

8.      Complete the redefinition

9.      Drop the interim table

A common question is what is happening behind the scenes here? In other words, how and what is Oracle doing? Essentially, the redefinition package is merely an API to an intelligent materialized view with a materialized view log. So a local replication of the object shows while the reorganization occurs. Then it refreshes to get up-to-date for any transaction that occurred during reorganization.

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