Correlation between columns in the same table always causes problems if those columns appear together in your where clause; the issue is not restricted to indexes. Sometimes you will be able to work around the problem by using the optimizer_dynamic_sampling parameter, or the dynamic_sampling hint (both of which appeared at some stage in 9i) to instruct Oracle to take a run-time sample of 32 or more blocks fromcritical tables to see what fraction of the rows matches your where clause.
/*+ index(t1) dynamic_sampling(t1 1) */
ind_pad = rpad('x',40)
and n1 = 2
and n2 = 2
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