Tuning Oracle queries with null values

Today I tested two queries in Oracle database. I noticed that when I execute query there column value is null, oracle engine directly use full access to table. I created index like:

create index MY_IDX on MY_TABLE (COLUMN_CAN_BE_NULL, -1);

and I have to use hint to use that index. Automatically oracle used full access, even when I counted statistic for this table.
However when it used index like this, it took much more then full access.

I have to dwell on a subject....