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Oracle Tips by Burleson |
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 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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