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OCP Instructors Guide for Oracle DBA Certification

Chapter 7 - Tuning and Performance

Data Segment Compression

Data compression in Oracle9i Release 2 is performed at the block level.  Administrators are able to compress entire tables and specific table partitions.  The COMPRESS attribute can be specified in the CREATE TABLE, CREATE TABLESPACE and CREATE TABLESPACE. PARTITION statements.  Block compression works by eliminating repeating column values. The more repeating values the columns have the greater the compression ratio becomes.  

We recently learned that compression is performed at the block level in Oracle.  This means that as the number of repetitious values per block increases so does its compression ratio.  Administrators are able to achieve higher compression ratios by sorting the rows on a column that has a poor cardinality (high number of repeating values). CREATE TABLE……SELECT * FROM…… ORDER BY low_cardinality_column can be used to ensure as many repeating values fall in the same block as possible.   Flat file data being loaded by the Oracle load utility can be sorted using an operating system sort utility to achieve the same result.        

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

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