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Oracle Tips by Burleson 

Re-sequencing table rows to reduce disk I/O

Basically, the create table as select (CTAS) statement copies the selected portion of the table into a new table. If you select the entire table with an order by clause or an index hint, it will copy the rows in the same order as the primary index. In addition to re-sequencing the rows of the new table, the CTAS statement coalesces free space and chained rows and resets freelists, thereby providing additional performance benefits. You can also alter table parameters, such as initial extents and the number of freelists, as you create the new table. The steps in a CTAS reorganization include:

1. Define a separate tablespace (maybe with a different blocksize) to hold the reorganized table.
2. Disable all referential integrity constraints.
3. Copy the table with CTAS (using order by or an index hint)
4. Reenable all referential integrity constraints.
5. Rebuild all indexes on the new table.

The main benefit of CTAS over the other methods is speed. It is far faster to use CTAS to copy the table into a new tablespace (and then re-create all RI and indexes) than it is to use the export/import method. Using CTAS also has the added benefit of allowing the rows to be resequenced into the same order as the primary index, thereby greatly reducing I/O. Within CTAS, there are two general reorganization methods.

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


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