SQL & PL/SQL

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