Getting an Oracle 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.

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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 tunes developers for high SQL performance. He also published the book SQL Performance Explained and offers in-house training as well as remote coaching at http://winand.at/

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