index and data conversion

[sysdba]

Hi,

Table test(col1 varchar2(32) not null,
col2 number not null,
other_columns,,,)
index i_test on test(col1, col2)
Query select ... from test
where col1 = 'this is test'
and col2 = '12345';

I do not remember exactly whether I saw an article from
AskTom.com, saying this data conversion (col2) may cause
i_test index not being used.
When I used autotrace trace only, I find the index
is used.

Even changing index on test(col2), the index
is still used.

Can someone ********?

Thank you!