Online Data Reorganization & Redefinition

As business become more web and e-commerce enabled, high availability computing solutions are becoming increasingly important because of the extremely high cost of application downtime. According to a recent Gartner Group study, the cost of outage for corporations involved in e-commerce can easily reach millions of dollars per hour. Therefore, database availability and manageability have become increasingly important in many of today's mission-critical environments. One way to enhance availability and manageability is to allow users full access to the database during a data reorganization operation.

The Online Reorganization & Redefinition feature in Oracle Database 11g, offers administrators unprecedented flexibility to modify table physical attributes and transform both data and table structure while allowing users full access to the database. This capability improves data availability, query performance, response time and disk space utilization, all of which are important in a mission-critical environment and it can make the application upgrade process easier, safer and faster.

When a table is redefined online, it is accessible by all read and write operations during the redefinition process. During an application upgrade, administrators can start the redefinition process, then periodically synchronize the interim table so that the interim table includes the latest changes to the original table.  This reduces the amount of time to complete the final redefinition step. Administrators can also validate and use the data in the interim table before completing the redefinition process.  This is a very useful feature for administrators, who must ensure that the upgrade process goes smoothly.Administrators then have control over when to switch from the original to the newly redefined table.  The switch process is very brief and is independent of the size of the table or the complexity of the redefinition.  The redefinition process effectively creates a new table and improves  its data block layout efficiency.

Oracle online data reorganization can be accomplished using either Oracle Enterprise Manager (OEM) or SQL*Plus command line interface. Depending on the type of online reorganization that is required, it can be performed using either the DBMS_REDEFINITION package or using the SQL CREATE/ALTER TABLE and INDEX commands as shown in the table below.

What's New in Oracle Database 11g

Fast 'add column' for default values

Instead of the default value being written to existing records, thereby accessing each row in the table, they are simply maintained in the data dictionary. This enables 'add column' to complete in sub-seconds, independent of the existing data volume, and consumes virtually no space.

Invisible Indexes

An invisible index is an alternative to making an index unusable or even to drop it. An invisible index is maintained for any DML operation but is not used by the optimizer unless you explicitly specify the index with a hint.

Invisible indexes can speed up application migration and testing. Applications often have to be modified without bringing the complete application offline. Invisible indexes enable you to leverage temporary index structures for certain operations or modules of an application without affecting the overall application. Furthermore, invisible indexes can be used to test the removal of an index without dropping it right away, thus enabling a grace period for testing in production environments.

DDL Enhancements

DDL operations can wait for a user configurable time period (DDL_LOCK_TIMEOUT), if underlying resource is busy, and is supported at system and session levels. This allows DDL to retry within the specified timeout period versus failing if a lock is not immediately obtained.

More DDL operations (add/modify constraint, add column, index create/rebuild) only require shared lock versus DML locks. This allows more DDL to complete successfully, regardless of DML workload.

DBMS_REDEFINITION Enhancements

• Additional supported table types: Tables with materialized view logs and materialized views can be redefined.
• Faster access to tables, post-redefinition: No recompilation of dependent objects (e.g. PL/SQL) is needed when redefinition does not logically affect the objects