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OCP Instructors Guide for Oracle DBA Certification

Chapter 7 - Tuning and Performance

Using Explain Plan to Determine Access Paths

You add one of the two phrases below to the beginning of the statement to notify Oracle to update the plan_table with rows describing the statement’s access path:

  • "EXPLAIN PLAN INTO schema.plan_table FOR"
     

  • EXPLAIN PLAN INTO schema.plan_table SET STATEMENT_ID = ‘*******’ FOR

The second statement can be used to differentiate statements in the PLAN_TABLE if it contains rows from multiple EXPLAIN PLAN statements.

The EXPLAIN PLAN specification tells the database to insert rows into the plan_table that describe the statement’s access path. The query itself is not actually executed. The following query can be used to display the contents of the plan_table:

SELECT LPAD(' ',2*(LEVEL-1))||operation||' '||options||' '||object_name||
' '||DECODE(id, 0, 'Cost = '||position) "Query Plan"
FROM foot.plan_table
START WITH id = 0
CONNECT BY PRIOR id = parent_id

The output below is an example of an EXPLAIN PLAN "dump" using the above query:

61844 SELECT STATEMENT     Cost = 61844
 


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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