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Two Alternatives for Using CTAS

It is always recommended that you resequence the table rows when performing a table reorganization with CTAS because of the huge I/O benefits. You can use the CTAS statement in one of two ways. Each of these achieves the same result, but they do it in very different ways:

* Use CTAS in conjunction with the order by clause.
* Use CTAS in conjunction with a “hint” that identifies the index to use.

The approach you choose depends on the size of the table involved, the overall processing power of your environment, and how quickly you must complete the reorganization.

The details of each CTAS approach are discussed more fully below, but in either case, when you create the new table, you can speed the process by using the Oracle nologging option (this was called unrecoverable in Oracle7). This skips the added overhead of writing to the redo log file. Of course, you cannot use the redo logs to roll forward through a nologging operation, and most DBAs take a full backup prior to using CTAS with nologging. Let’s examine the two methods and see their respective differences.


The above is an excerpt from the "Oracle9i UNIX Administration Handbook" by Oracle press, authored by Donald K. Burleson.

 

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