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OCP Instructors Guide for Oracle DBA Certification

Chapter 5 - Oracle Database Objects

Index-only Tables

Conventional B-tree index entries are quite small since they consist of a key value and ROWID. Index-only tables index entries can become quite large since they consist of a key value and the key value’s corresponding row data. The B-tree index leaf nodes (bottom layer of the index) of an index-only table may end up storing a single row that effectively destroys the B-tree index’s dense clustering property. Oracle8 uses a Row Overflow Area to overcome the problem of large B-tree index entries. You are able to define an overflow tablespace and a threshold value (% of block size) during index-only table creation. The following DDL creates an index-only table:

CREATE TABLE store_table
(store_id VARCHAR2 (10) CONSTRAINT pk_store_table PRIMARY KEY,
store_address VARCHAR2 (200),
store_manager VARCHAR2 (50))
ORGANIZATION INDEX TABLESPACE store_table_tspace
PCTTHRESHOLD 20
OVERFLOW TABLESPACE store_table _ovflw;

The previous DDL is interpreted as follows:

  • The primary key of the store_table is the store_id column. A primary key must be specified for 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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