by Markus Winand.

Getting an Execution Plan


Viewing an execution plan in the Oracle database involves two steps:

  1. explain plan for — saves the execution plan in the PLAN_TABLE.

  2. Format and display the execution plan.

Creating and Saving an Execution Plan

To create an execution plan, you just have to prefix the respective SQL statement with explain plan for:


          EXPLAIN PLAN FOR select * from dual
        

You can execute the explain plan for command in any development environment or SQL*Plus. It will, however, not show the plan but save it into a table named PLAN_TABLE. Starting with release 10g, this table is automatically available as a global temporary table. With previous releases, you have to create it in each schema as needed. Ask your database administrator to create it for you or to provide the create table statement from the Oracle database installation:


          $ORACLE_HOME/rdbms/admin/utlxplan.sql
        

You can execute this statement in any schema you like to create the PLAN_TABLE in this schema.

Warning

The explain plan for command does not necessarily create the same execution plan as though it would when executing the statement.

On my Own Behalf

If you like this article, you might also like my book SQL Performance Explained or my training.

Showing Execution Plans

The package DBMS_XPLAN was introduced with release 9iR2 and can format and display execution plans from the PLAN_TABLE. The following example shows how to display the last execution plan that was explained in the current database session:


          select * from table(dbms_xplan.display)
        

Once again, if that statement doesn't work out of the box, you should ask your DBA for assistance.

The query will display the execution plan as shown in the book:


          --------------------------------------------------------------
| Id | Operation         | Name | Rows | Bytes | Cost (%CPU)|.
--------------------------------------------------------------
|  0 | SELECT STATEMENT  |      |    1 |     2 |     2   (0)|.
|  1 |  TABLE ACCESS FULL| DUAL |    1 |     2 |     2   (0)|.
--------------------------------------------------------------
        

Some of the columns shown in this execution plan were removed in the book for a better fit on the page.

About the Author

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Markus Winand teaches efficient SQL—inhouse and online. He minimizes the development time using modern SQL and optimizes the runtime with smart indexing. His book entitled SQL Performance Explained has become standard reading.

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