Understanding Oracle Explain Plan

Question:  I need some tips for understanding the Oracle explain plan.  I have been told that reading the Oracle explain plan will help me identify tuning opportunities, but the explain plan and how to use it are a mystery to me.

Answer:  In the most basic terms, the Oracle explain plan is a tool that can be used to get Oracle to tell you how it plans on executing a query.  Clearly, this makes the explain plan a valuable tool for tuning.

Once the Oracle explain plan reveals how it plans on executing the query in question, the environment can be adjusted to run the query faster.

The Oracle explain plan is a statement that returns execution plans for the requested SELECT, UPDATE, INSERT and DELETE statements.  The execution plan for any given statement shows the operations in sequence used by Oracle to run that statement.

Information returned by the Oracle explain plan includes:

• the tables referenced in the statement

• the ordering and method with which each table is accessed

• for join operations, the join method for the tables involved

• data operations such as filter or sort included in the statement.

In addition to the information listed above, the Oracle explain plan table will also contain information on:

• optimization

• partitioning

• parallel execution

The explain plan information is written to an explain plan table.  It is important to note that the explain plan table must be created before running the explain plan statement.

Once the Oracle explain plan table has been created, suspect SQL statements can be evaulated and tuned accordingly.

It is also possible to derive an Oracle explain plan using the SQL*Plus autotrace function as shown here:

SQL> SET AUTOTRACE ON
SQL> SELECT *
  2  FROM   emp e, dept d
  3  WHERE  e.deptno = d.deptno
  4  AND    e.ename  = 'SMITH';

The disadvantage of this method is that the statement has to complete its run in order to be evaluated.  If the statement is truly bad, this could involve a considerable amount of wasted time.

The preferred method would be the Oracle explain plan route.

The explain plan statement generates the execution plan for a query without executing the query itself, allowing the execution plan for poorly performing queries to be displayed without impacting the database. 

The following example shows how the explain plan statement is used to generate an execution plan:

SQL> EXPLAIN PLAN FOR  
2  SELECT *  
3  FROM   emp e, dept d  
4  WHERE  e.deptno = d.deptno  
5  AND    e.ename  = 'SMITH';

Explained.

If multiple people are accessing the same Oracle explain plan table, or a history of the Oracle execution plans is to be saved, the statement_id clause of the Oracle explain plan statement should be used.  This associates a user specified identifier with each explain plan, which can be used when retrieving the data. 

The following example shows how the statement_id is set using the Oracle explain plan statement.

SQL> EXPLAIN PLAN SET STATEMENT_ID = 'TIM' FOR  
2  SELECT *  
3  FROM   emp e, dept d  
4  WHERE  e.deptno = d.deptno  
5  AND    e.ename  = 'SMITH'; Explained.

The goals of tuning the SQL statements returned by the Oracle explain plan are fairly simple:

• Eliminate sub-optimal large-table full-table scans: Ensure that the fastest access path to the data is chosen.  The execution plan "FULL" is the key.

• Use fastest table join method: The optimizer must choose intelligently between nested loops joins, hash joins and star transformation join methods and these are displayed in the execution plan.  

• Ensure optimal table-joining order: SQL will run fastest when the first table joins deliver the smallest result set.

With Oracle 11g, SQL Plan Management evolves more fully.

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