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OCP Instructors Guide for Oracle DBA Certification

Chapter 7 - Tuning and Performance

High Water Mark and Table Scans

Oracle uses the high water mark to identify the highest amount of space used by a particular segment.  It acts as the boundary between used and unused space.  As the amount of data grows, the segment’s high water mark grows accordingly.  But as the amount of data shrinks (i.e. row deletes, updates), the high water mark is not altered to reflect the segment’s new characteristics.  The high water mark not being adjusted as the data shrinks has the tendency to create a somewhat confusing performance problem.

During a full table scan, Oracle scans all blocks up to the table’s high water mark.  This happens regardless of whether those blocks contain data or not.   Business needs often require that all rows of a table be scanned. DBAs often attempt to increase the performance of the aforementioned table scans by trying to store as little data as possible in the scanned table.  If the table is purged by SQL delete statements, the high water mark will not be moved and the table scans will continue to perform poorly.

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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