Databases Reference
In-Depth Information
The Explain Plan window is divided into three horizontal sections. The SQL
statement itself is displayed in the top section of the window. The steps that Ora-
cle uses to execute the statement are in the middle section of the window. As each
step is selected, a brief explanation of what occurs in that step is detailed in the
bottom section of the window.
In the case of the join between the EMPLOYEES and DEPARTMENTS tables in this
example, both tables are accessed with a full table scan instead of an index. This
makes sense because the query retrieves most, if not all, of the rows in both
tables. If there were a limiting condition in a WHERE clause, and the tables were
still accessed by a full table scan, then it might indicate that you are missing an
index on one or both of the tables.
You may see the terms Explain Plan and Execution Plan used interchangeably; they
mean the same thing. The SQL command
EXPLAIN PLAN
generates an execution plan.
