Spotlight on Oracle 10.5 - Release Notes

Interpret the Execution Plan

Interpreting a formatted execution plan requires practice and often some degree of judgment. However, the following fundamental principles guide the interpretation:

• The more heavily indented an access path is, the earlier it is executed.
• If two steps are indented at the same level, the uppermost statement is executed first.
• An access path in the execution plan may comprise a number of steps. For instance, in the execution plan below, an index access is shown as an INDEX SCAN, together with a TABLE SCAN BY ROWID.

SELECT STATEMENT

SORT ORDER BY

NESTED LOOPS

TABLE ACCESS FULL CUSTOMERS

TABLE ACCESS BY ROWID EMPLOYEES

INDEX RANGE SCAN EMPLOYEE_BOTHNAMES_IDX

Here, the indentation level of the outermost access determines the precedence of the execution. In this Explain Plan, the most heavily indented operation is the index range scan of an EMPLOYEES index. However, this operation is combined with the ROWID access of the EMPLOYEES table. Therefore, the CUSTOMERS access is the first step executed.

Related Topic

Tools | Explain Plan