Why index is being ignored

Question

SQL> CREATE table t (a  VARCHAR2(10),b VARCHAR2(10))
  2  /
Table created.
SQL> BEGIN
  2    FOR i IN 1..10000
  3    LOOP
  4     INSERT INTO t VALUES (i,'A');
  5    END LOOP;
  6  END;
  7  .
SQL> /
PL/SQL procedure successfully completed.
SQL> COMMIT;
Commit complete.
SQL> CREATE INDEX t_a_idx ON t (a)
  2  /
Index created.
SQL> DESC emp
Name                                                  Null?    Type
EMPNO                                                 NOT NULL NUMBER(4)
ENAME                                                          VARCHAR2(10)
JOB                                                            VARCHAR2(9)
MGR                                                            NUMBER(4)
HIREDATE                                                       DATE
SAL                                                            NUMBER(7,2)
COMM                                                           NUMBER(7,2)
DEPTNO                                                         NUMBER(2)
GRADE                                                          NUMBER
SQL> SELECT * FROM emp WHERE empno>500
  2  /
Execution Plan
   0      SELECT STATEMENT Optimizer=ALL_ROWS (Cost=2 Card=14 Bytes=56
          0)
   1    0   TABLE ACCESS (BY INDEX ROWID) OF 'EMP' (TABLE) (Cost=2 Car
          d=14 Bytes=560)
   2    1     INDEX (RANGE SCAN) OF 'PK_EMP' (INDEX (UNIQUE)) (Cost=1
          Card=14)
Above query is not ignoring the possibility of using an index on empno column,while the same query with table t
ignoring the usage of index on 'a' column why??
SQL> SELECT *
  2    FROM t WHERE a>500
  3  /
Execution Plan
   0      SELECT STATEMENT Optimizer=ALL_ROWS (Cost=6 Card=9500 Bytes=
          133000)
   1    0   TABLE ACCESS (FULL) OF 'T' (TABLE) (Cost=6 Card=9500 Bytes
          =133000)
SQL> SELECT *
  2    FROM t WHERE a>'500'
  3  /
Execution Plan
   0      SELECT STATEMENT Optimizer=ALL_ROWS (Cost=6 Card=5552 Bytes=
          77728)
   1    0   TABLE ACCESS (FULL) OF 'T' (TABLE) (Cost=6 Card=5552 Bytes
          =77728)Khurram

Answer 1

Well, there are many issues with the test which you have posted here.
1. No statistics are collected after the table created, data inserted. Still, in your example it shows the cost, cardinality and bytes, unless you enable dynamic samplying parameter.
2. a VARCHAR2(10)
You have defined 'a' column as varchar datatype and ine one of your example you are using nubmber predicate value. Therefore, doing implicit conversion.
I have come across of this proble, like, why the hell my index ignored. Then understood the situation.
SQL> SELECT *
2 FROM t WHERE a>500
3 /
Lets take another example why index ignored,
SQL> SELECT *
2 FROM t WHERE a>'500'
3 /
column a has distinct values from 1 to 10000
formula for predicate selectity when there are no historgrams in place, as follows.
c1 > '4076' 1 - (High - Value / High - Low)
Apart from all, I dont believe in AUTOTRACE generated execution plan, it is not the real one, it is just predicated one.
If you are on >=9iR2, I strongly recommend you to use, DBMS_XPLAN.DISPLAY to get the execution plan.
Jaffar