EXPLAIN PLAN

Purpose

Use the EXPLAIN PLAN statement to determine the execution plan Oracle follows to execute a specified SQL statement. This statement inserts a row describing each step of the execution plan into a specified table. You can also issue the EXPLAIN PLAN statement as part of the SQL trace facility.

If you are using cost-based optimization, then this statement also determines the cost of executing the statement. If any domain indexes are defined on the table, then user-defined CPU and I/O costs will also be inserted.

The definition of a sample output table PLAN_TABLE is available in a SQL script on your distribution media. Your output table must have the same column names and datatypes as this table. The common name of this script is UTLXPLAN.SQL. The exact name and location depend on your operating system.

Prerequisites

To issue an EXPLAIN PLAN statement, you must have the privileges necessary to insert rows into an existing output table that you specify to hold the execution plan.

You must also have the privileges necessary to execute the SQL statement for which you are determining the execution plan. If the SQL statement accesses a view, then you must have privileges to access any tables and views on which the view is based. If the view is based on another view that is based on a table, then you must have privileges to access both the other view and its underlying table.

To examine the execution plan produced by an EXPLAIN PLAN statement, you must have the privileges necessary to query the output table.

The EXPLAIN PLAN statement is a data manipulation language (DML) statement, rather than a data definition language (DDL) statement. Therefore, Oracle does not implicitly commit the changes made by an EXPLAIN PLAN statement. If you want to keep the rows generated by an EXPLAIN PLAN statement in the output table, then you must commit the transaction containing the statement.

Syntax

explain_plan::=

Text description of explain_plan

Semantics

SET STATEMENT_ID Clause

Specify the value of the STATEMENT_ID column for the rows of the execution plan in the output table. You can then use this value to identify these rows among others in the output table. Be sure to specify a STATEMENT_ID value if your output table contains rows from many execution plans. If you omit this clause, then the STATEMENT_ID value defaults to null.

INTO table Clause

Specify the name of the output table, and optionally its schema and database. This table must exist before you use the EXPLAIN PLAN statement.

If you omit schema, then Oracle assumes the table is in your own schema.

The dblink can be a complete or partial name of a database link to a remote Oracle database where the output table is located. You can specify a remote output table only if you are using Oracle's distributed functionality. If you omit dblink, then Oracle assumes the table is on your local database.

If you omit INTO altogether, then Oracle assumes an output table named PLAN_TABLE in your own schema on your local database.

FOR statement Clause

Specify a SELECT, INSERT, UPDATE, DELETE, CREATE TABLE, CREATE INDEX, or ALTER INDEX ... REBUILD statement for which the execution plan is generated.

Notes on Using EXPLAIN PLAN

• If statement includes the parallel_clause, then the resulting execution plan will indicate parallel execution. However, EXPLAIN PLAN actually inserts the statement into the plan table, so that the parallel DML statement you submit is no longer the first DML statement in the transaction. This violates the Oracle restriction of one parallel DML statement in a single transaction, and the statement will be executed serially. To maintain parallel execution of the statements, you must commit or roll back the EXPLAIN PLAN statement, and then submit the parallel DML statement.
• To determine the execution plan for an operation on a temporary table, EXPLAIN PLAN must be run from the same session, because the data in temporary tables is session specific.

Examples

EXPLAIN PLAN Examples

The following statement determines the execution plan and cost for an UPDATE statement and inserts rows describing the execution plan into the specified plan_table table with the STATEMENT_ID value of 'Raise in Tokyo':

EXPLAIN PLAN 
    SET STATEMENT_ID = 'Raise in Tokyo' 
    INTO plan_table 
    FOR UPDATE employees 
        SET salary = salary * 1.10 
        WHERE department_id =  
           (SELECT department_id FROM departments
               WHERE location_id = 1200); 

The following SELECT statement queries the plan_table table and returns the execution plan and the cost:

SELECT LPAD(' ',2*(LEVEL-1))||operation operation, options, 
object_name, position 
    FROM plan_table 
    START WITH id = 0 AND statement_id = 'Raise in Tokyo'
    CONNECT BY PRIOR id = parent_id AND 
    statement_id = 'Raise in Tokyo'; 

The query returns this execution plan:

OPERATION            OPTIONS         OBJECT_NAME       POSITION
-------------------- --------------- --------------- ----------
UPDATE STATEMENT                                              2
 UPDATE                              EMPLOYEES                1
  TABLE ACCESS       FULL            EMPLOYEES                1
   VIEW                              index$_join$_00          1
                                     2
    HASH JOIN                                                 1
     INDEX           RANGE SCAN      DEPT_LOCATION_I          1
                                     X
     INDEX           FAST FULL SCAN  DEPT_ID_PK               2

The value in the POSITION column of the first row shows that the statement has a cost of 1.

EXPLAIN PLAN: Partitioned Example

The sample table sh.sales is partitioned on the time_id column. Partition sales_q3_2000 contains time values less than Oct. 1, 2000, and there is a local index sales_time_bix on the time_id column.

Consider the query:

EXPLAIN PLAN FOR
   SELECT * FROM sales 
      WHERE time_id BETWEEN :h AND '01-OCT-2000';

where :h represents an already declared bind variable. EXPLAIN PLAN executes this query with PLAN_TABLE as the output table. The basic execution plan, including partitioning information, is obtained with the following query:

SELECT operation, options, partition_start, partition_stop,
   partition_id FROM plan_table;