It is based on the time consumed to fetch the result set of the select statment. If the select statment is taking time to retrieve the data it mean that it is using the full table scan and no indexes or wrong indexes has been created in table
1. using AUTOTRACE SQL> SET AUTOTRACE ON TRACEONLY SELECT * FROM emp WHERE emp_name 'ABC';
2. Using EXPLAIN PLAN SQL> EXPLAIN PLAN for SELECT * FROM emp WHERE emp_name 'ABC';
The advantage of using EXPLAIN PLAN over the AUTOTRACE is that former does not require the query to be actually run. The TRACEONLY option in AUTOTRACE just supress the query ouput but the query still runs before the explain plan is displayed.
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