Why index is not used if oracle have to do implicit conversion?

Question

my db version: 10gR2
I created bitmap index on each of the column used in below sql.
the datatype of all three columns are VARCHAR2,
while i am using NUMBER in the query, which means oracle needs to do the implicit conversion before running the sql.
the problem is that, when I use NUMBER as below, execution plan will not use bitmap indexes at all.
but when I use STRING instead, it will make use of those bitmap indexes.
select  FI_YTD,1,2,FI_KPI_ID
  from DM_F_FI_ALL_KPI
  where
    FI_KPI_ID=4140
    and FI_PERIOD=201012
    and FI_ORG_ID=10000000;So, I wonder why? how the implicit conversion affects the access path?
Edited by: PhoenixBai on Jan 30, 2011 10:52 AM
Edited by: PhoenixBai on Jan 31, 2011 9:21 AM --added database version as 10gR2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       

Answer 1

sybrand_b wrote:
Implicit conversion in Oracle is always done like this
empno='10'
is rewritten as
to_char(empno)='10'
Now you have <function>(<indexed columnn>) = <hardcoded literal>
Oracle never puts a conversion function at the right hand side of the expresssion. NEVER.
SQL> create table emp as select * from scott.emp;
Table created.
SQL> alter table emp add constraint emp_pk primary key (empno);
Table altered.
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)
SQL> select * from emp where empno = '10'
  2
SQL> @xplan
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
Compatibility is set to 11.2.0.0.0
Plan hash value: 4024650034
| Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT            |        |     1 |    87 |     1   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| EMP    |     1 |    87 |     1   (0)| 00:00:01 |
|*  2 |   INDEX UNIQUE SCAN         | EMP_PK |     1 |       |     0   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   2 - access("EMPNO"=10)Edited by: William Robertson on Jan 30, 2011 12:56 PM - added DESCRIBE