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OCP Instructors Guide for Oracle DBA Certification

Chapter 7 - Tuning and Performance

Using Explain Plan to Determine Access Paths

  • Operation 3.1 – The nested loop join method is being used to join the two tables. The results from the join in Step 3.1 are used as the outer-table in Join 2.1. Oracle is only able to join two tables at a time. The rows are read again and the join column is used to find a match in the inner-table (PUBLISHER_APP)
     

  • Operation 2.1 – Once again, Oracle knows the values being passed from the nested loop join (Step 3.1). Since it is being passed values to look for matching rows, it uses the index (PK_PUBLISHER_APP) which is built on the join column. Remember, the inner-table of a nested loop join can use an index on a join column (it knows the value being passed from the outer-table).
     

  • Operation 3.2 – Oracle is using the ROWID retrieved in Step 4.1 to probe the inner-table (PK_PUBLISHER_APP).

Explain Plan Output

Because of space considerations, I am unable to cover every access path that can be selected by a particular SQL statement. In addition, the Oracle reference manuals do a pretty good (actually a very good) job of describing the various access paths a statement can take.

The Oracle8 and Oracle8i Concepts manual and the Oracle9i Database Performance Guide and Reference are the manuals to turn to when trying to learn more about Oracle access paths.   The Oracle9i Database and Performance Reference Guide is the better of the three. The manual goes into great detail on what the access path actually does, factors that affect it, why the optimizer would select it and what impact it has on performance.
 


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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