Why bitmap index not working?

Question

I have a table containing 4.2M rows and 16 distinct fs_type_code. So I created a bitmap index ntr_fs_type_code_ind on the column. Then I run the query:
update /*+ INDEX_COMBINE(NTR_FS_TYPE_CODE_IND) */ ntr_CORPALL_20050801 d
set eqt_basket_id = NULL, index_weight = NULL
where
fs_type_code in ('EQGR','EQVG','EQDL')
and eqt_basket_id = equity_symbol
and index_weight = 0;
I clearly tell optimizer to use the bitmap index. But it turns out the optimizer ignore the index and still use full table scan.
When I created regular b-tree index, the same query (without hint) use index scan.
Can anybody tell me why the bitmap index not working here?
Thanks,

Answer 1

<quote>I clearly tell optimizer to use the bitmap index</quote>
You are clearly not doing it right (see bellow). But anyway …
1. For frequently modified tables (OLTP type application) you may want to rethink the applicability of bitmap indexes …
low cardinality in itself is not enough justification for using bitmap indexes.
2. Your update statement may modify a minority, a majority, or anything in between of the total number of
rows in your table … here is no one universal access method which is always better
(if there were one they wouldn’t have bothered with coding the rest).
In short, index access is not always the better way.
3. Don’t rush into hinting (because that optimizer is such a lousy piece of software) …
and if you do, make sure you do it correctly and for the right reasons.
flip@FLOP> create table t as select * from all_objects;
Table created.
flip@FLOP> insert into t select * from t;
30043 rows created.
flip@FLOP> insert into t select * from t;
60086 rows created.
flip@FLOP> insert into t select * from t;
120172 rows created.
flip@FLOP> insert into t select * from t;
240344 rows created.
flip@FLOP> create bitmap index tx on t (object_type);
Index created.
flip@FLOP> exec dbms_stats.gather_table_stats(user,'T',method_opt=>'for all indexed columns',cascade=>true)
PL/SQL procedure successfully completed.
flip@FLOP> select object_type,count(*) from t group by rollup(object_type);
OBJECT_TYPE          COUNT(*)
CONSUMER GROUP             32
DIRECTORY                  32
EVALUATION CONTEXT         16
FUNCTION                 1648
INDEX                   23152
INDEX PARTITION          2048
INDEXTYPE                 128
JAVA CLASS             163024
JAVA RESOURCE            3120
LIBRARY                   224
LOB                        16
MATERIALIZED VIEW          32
OPERATOR                  464
PACKAGE                  5488
PACKAGE BODY               32
PROCEDURE                 640
SEQUENCE                  144
SYNONYM                202512
TABLE                   18816
TABLE PARTITION           880
TRIGGER                  4768
TYPE                    10640
TYPE BODY                  16
VIEW                    42816
                       480688
flip@FLOP> set autotrace on explain
update few rows … CBO goes with the index … no hinting
flip@FLOP> update t d set object_id=object_id-1 where object_type in ('INDEX','PACKAGE','PACKAGE BODY','TABLE');
47488 rows updated.
Elapsed: 00:00:09.02
Execution Plan
   0      UPDATE STATEMENT Optimizer=CHOOSE (Cost=536 Card=47488 Bytes
          =1044736)
   1    0   UPDATE OF 'T'
   2    1     INLIST ITERATOR
   3    2       BITMAP CONVERSION (TO ROWIDS)
   4    3         BITMAP INDEX (SINGLE VALUE) OF 'TX'
update lots of rows … CBO goes with the ft … no hinting
flip@FLOP> update t d set object_id=object_id-1 where object_type in ('JAVA CLASS','SYNONYM');
365536 rows updated.
Elapsed: 00:00:25.04
Execution Plan
   0      UPDATE STATEMENT Optimizer=CHOOSE (Cost=638 Card=365536 Byte
          s=8041792)
   1    0   UPDATE OF 'T'
   2    1     TABLE ACCESS (FULL) OF 'T' (Cost=638 Card=365536 Bytes=8
          041792)
update lots of rows … wrong hint syntax … CBO goes with the ft
flip@FLOP> update /*+ index_combine(tx) */ t d set object_id=object_id-1 where object_type in ('JAVA CLASS','SYNONYM');
365536 rows updated.
Elapsed: 00:00:21.00
Execution Plan
   0      UPDATE STATEMENT Optimizer=CHOOSE (Cost=638 Card=365536 Byte
          s=8041792)
   1    0   UPDATE OF 'T'
   2    1     TABLE ACCESS (FULL) OF 'T' (Cost=638 Card=365536 Bytes=8
          041792)
update lots of rows … correct hint syntax … CBO goes with the index … but was it better than the ft?
flip@FLOP> update /*+ index_combine(d tx) */ t d set object_id=object_id-1 where object_type in ('JAVA CLASS','SYNONYM')
365536 rows updated.
Elapsed: 00:00:25.01
Execution Plan
   0      UPDATE STATEMENT Optimizer=CHOOSE (Cost=1665 Card=365536 Byt
          es=8041792)
   1    0   UPDATE OF 'T'
   2    1     INLIST ITERATOR
   3    2       BITMAP CONVERSION (TO ROWIDS)
   4    3         BITMAP INDEX (SINGLE VALUE) OF 'TX'
flip@FLOP>