Explain Plan Gives 0 Cost.

All,

Why is this happening? V7.0.16 and V7.1.3

SQL> delete from plan_table
  2 /

6 rows deleted.

SQL> explain plan
  2 for
  3 select a.username, a.user_id, b.profile   4 from a, b
  5 where a.user_id = b.user_id
  6 and b.created < sysdate
  7 and b.profile != 'DEFAULT'
  8 /

Explained.

SQL> select id, parent_id, position
  2 from plan_table;

        ID PARENT_ID POSITION
---------- ---------- ----------

	 0                              <<<<< Null here corresponds to Cost = 0
	 1	    0	       0
	 2	    1	       1
	 3	    2	       1
	 4	    1	       2
	 5	    4	       1

6 rows selected. Received on Fri Sep 22 1995 - 00:00:00 CEST