Sql is not using index

Question

SELECT DISTINCT eval.match_student_name (a.booking_id,
                                         2059980
                                        ) "Student Name",
                NVL (b.student_email, b.email_address) email_address,
                a.booking_id "Student Registration Number",
                b.customer_name "Company Name",
                DECODE (b.pmt_confirmed,
                        'E', 'Exception',
                        'Y', 'Yes',
                        'N', 'No'
                       ) "Confirmed Commitment ",
                '1' "Edit", a.title "Class Id",
                b.exception_comments "Exception Comments",
                NVL (b.opn_member, 'N') "OPN_Member",
                NVL (b.opn_partner_name, '-') "OPN Company Name",
                a.booking_id "View Attendance",
                DECODE (eval.check_bookingid_push_gsi (a.booking_id),
                        1, apex_item.checkbox (1, a.booking_id, 'Checked'),
                        apex_item.checkbox (1, a.booking_id, 'Disabled')
                       ) "Select",
                DECODE
                   (eval.check_bookingid_push_gsi (a.booking_id),
                    1, 'Ready TO push TO GSI',
                    2, 'In Progress',
                    3, 'Complete',
                    'MANUAL'
                   ) "Booking ID Status"
           FROM ( select * from eval.eattend_student_attendance where title = 2059980 ) a,
                ( select * from eval.eattend_student_info where verified_by IS NOT NULL ) b
          WHERE
            --AND b.verified_by IS NOT NULL
        a.title = b.title
       AND a.booking_id = b.booking_id
       ORDER BY a.booking_idExecution Plan
Plan hash value: 3921177175
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
| 0 | SELECT STATEMENT | | 11 | 1870 | 353 (3)| 00:00:05 |
| 1 | SORT ORDER BY | | 11 | 1870 | 353 (3)| 00:00:05 |
| 2 | HASH UNIQUE | | 11 | 1870 | 352 (3)| 00:00:05 |
| 3 | NESTED LOOPS | | | | | |
| 4 | NESTED LOOPS | | 11 | 1870 | 351 (2)| 00:00:05 |
|* 5 | TABLE ACCESS FULL | EATTEND_STUDENT_ATTENDANCE | 11 | 176 | 318 (3)|
|* 6 | INDEX RANGE SCAN | PERF_EATTEND_STUDENT_INFO_N98 | 1 | | 2 (0)| 0
|* 7 | TABLE ACCESS BY INDEX ROWID| EATTEND_STUDENT_INFO | 1 | 154 | 3 (0)| 0
pls see the table full access of EATTEND_STUDENT_ATTENDANCE , i have a index in that table for a column being used in sql. why index not used ???
thanks
Raj

Answer 1

Because Optimizer thinks that the query will take longer when using an index.
http://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:736825544526
http://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:9422487749968
Please use:
explain plan for [your query];and then
select * from table(dbms_xplan.display);when you want an execution plan.