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Oracle Tips by Burleson |
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
http://www.rampant-books.com/book_2003_2_OCP_print.htm
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