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OCP Instructors Guide for Oracle DBA Certification

Chapter 5 - Oracle Database Objects

Space Utilization for Parallel Table Creates

CREATE TABLE test2 AS SELECT * FROM test1 STORAGE (INITIAL 2047M NEXT 500M... PARALLEL 4;

The above statement would create a table that has 4 extents with each extent being 2047M in size.

Index-only Tables

Oracle8 allowed you to define an index-only table (see Figure 6) that keeps data sorted on the primary key.  Oracle stores the table rows in a B-tree index built on the primary key. The B-tree index contains both the key value and the key value’s corresponding row data. 

Image 6: Index-only tables.

 

Index-only tables increase the performance of SQL statements by providing faster access to table data for queries that use the key value for exact value searches.  Index-only tables also increase performance by eliminating the index to table I/O associated with conventional B-Tree index structures. 

The first implementation of index-only tables contained many restrictions on their implementation and usage. One example is that secondary indexes were not allowed to be built on index-only tables in Oracle8. Oracle removed restrictions and enhanced index-only functionality with each subsequent release. It is important that the DBA check the Oracle administrator’s guide for their release to determine how to correctly administer index-only tables.
 


The above text is an excerpt from:


OCP Instructors Guide for Oracle DBA Certification
A Study Guide to Advanced Oracle Certified Professional Database
Administration Techniques

ISBN 0-9744355-3-8

by Christopher T. Foot
 

http://www.rampant-books.com/book_2003_2_OCP_print.htm


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