index not used question

I have an index to deposit_date column which get used in
#1 select count(*) where deposit_date =

However, this index is not used when
#2 select count(*) where deposit_date = and check_num like '%1642'
the index column is on deposit only.

Can any one shed light on this ? whyn adding check_num in where clause negate index usage(I am using oracle 9.2.0.3)

thanks ted chyn(tedchyn_at_yahoo.com)

==

#1

  1 SELECT count(*) FROM PYMTITEM PI where (PI.DEPOSIT_DATE =   2 to_date('5-18-2003', 'MM-DD-YYYY'))   3* -- and (PI.CHECK_NUM like '%1642')
SQL> /   COUNT(*)

         0

Execution Plan

   0 SELECT STATEMENT Optimizer=CHOOSE (Cost=53 Card=1 Bytes=7)    1 0 SORT (AGGREGATE)

   2    1     INDEX (RANGE SCAN) OF 'PI_IS_1_FK' (NON-UNIQUE) (Cost=53
           Card=3774 Bytes=26418)

#2

  1 SELECT count(*) FROM PYMTITEM PI where (PI.DEPOSIT_DATE =   2 to_date('5-18-2003', 'MM-DD-YYYY'))   3* and (PI.CHECK_NUM like '%1642')
SQL> /   COUNT(*)

         0

Execution Plan

   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=1022 Card=1 Bytes=22
          )

   1    0   SORT (AGGREGATE)
   2    1     TABLE ACCESS (FULL) OF 'PYMTITEM' (Cost=1022 Card=189 By
          tes=4158)