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  create table using CTAS tips

Oracle Database Tips by Donald BurlesonMarch 31, 2015

Question:  I
need to understand how to create table using CTAS.  Can you show an example a create table using CTAS?

Answer:  The create table using CTAS command is very straightforward:

The Create table as select (CTAS) statement can be used to change storage parameters for a table (INITIAL, NEXT, FREELISTS) and also change the physical sequence of the table rows. Create table as select (CTAS) has the following syntax:

create table
   xxx_new
tablespace
   new_tablespace_name
storage
   (initial new_initial next new_next freelists new_freelist_number )
as
select
   *
from
   xxx
order by
   primary_index_key_values;


There are several way to execute CTAS to reorganize table; many of the options depend upon the version of Oracle and the particular configuration of the Oracle database.

Create table with Parallel CTAS

Running a create table as select (CTAS) in parallel can dramatically speed up SAP table reorganization. As a rule of thumb, the parallel option is used only on SAP database servers that have multiple CPUs (for example, SMP processor CPUs), but there will be some performance improvement when invoking parallelism, even on a uni-processor CPU.

Note that the UNRECOVERABLE clause can be used in conjunction with the parallel clause, or you can run UNRECOVERABLE CTAS without using parallelism. Here is an example of a parallel CTAS:

create table
vbap_sorted
tablespace
vbap_copy
storage (
initial 500m
next 50m
maxextents unlimited
)
parallel (degree 4)
as
select *
from
sapr3.vbap
order by
mandt,
vbeln,
posnr;


Create table with CTAS using INDEX hint

This is an excellent way of reorganizing a table to physically re-sequence the rows in the table. It is commonly known that Oracle deliberately omitted the ORDER BY clause in their implementation of CREATE TABLE AS SELECT. This is because of Oracle's early philosophy (pre-release 7.3.4 on AIX) that the physical sequence of rows within a table should not matter to the performance of the system. Unfortunately, this is not the case. As any DB2 professional is aware, "clustering" the rows in a table in the same order as the primary key index can greatly improve the performance of the queries.

Note: Some releases of Oracle prior to 7.3.4 may support ORDER BY with CTAS, but for AIX 7.3.3 and before generate a syntax error when ORDER BY is used with CTAS. Oracle does allow the use of the INDEX "hint" to request an ordered copy of the table.

Here is an example of INDEX hint with CTAS.

create table vbap_sorted
tablespace vbap_copy
storage (initial 500m
next 50m
freelists 30
maxextents unlimited
)
as
select /*+ index(vbap vbap___0) */
*
from
sapr3.vbap
;

See these related notes on create table with CTAS:

 

   
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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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