SQL> conn / as sysdba
Connected.
SQL> explain plan for select * from tab;

Explained.

SQL> SELECT operation, options FROM plan_table;

OPERATION                      OPTIONS
------------------------------ ------------------------------
SELECT STATEMENT
NESTED LOOPS                   OUTER
TABLE ACCESS                   BY INDEX ROWID
INDEX                          RANGE SCAN
TABLE ACCESS                   CLUSTER
INDEX                          UNIQUE SCAN

6 rows selected.