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Oracle Tips by Burleson 

Using CTAS with the order by Clause

When using CTAS with the order by clause, you are directing Oracle to perform the following operations.

As we can see, the full table scan can be used with Parallel Query to speed the execution, but we still have a large disk sort following the collection of the rows. Because of the size of most tables, this sort will be done in the TEMP tablespace.

Here is an example of the SQL syntax to perform a CTAS with order by:

create table new_customer
tablespace customer_flip
storage (initial 500m
next 50m
maxextents unlimited)
parallel (degree 11)
as select * from customer
order by customer_number;

Using CTAS with order by can be very slow without the parallel clause. A parallel full table scan reads the original table quickly (in non-index order).

As we know from Oracle Parallel Query, the CTAS operation will cause Oracle to spawn to multiple background processes to service the full table scan. This often makes the order by approach faster than using the index-hint approach to CTAS. The choice to use parallel depends on the database server. If your hardware has multiple CPUs and many (perhaps hundreds of) processes, using parallel is likely to be significantly faster. However, if your hardware configuration has a relatively modest number of processes (such as the four specified in the example), the index-hint approach is likely to be faster.


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

 

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