If you ever have to query tables in Oracle containing million of records using "is null" in the where clause, then you will notice that your normal indexes won't kick in. They work as prescribed for "is not null", though.
Using a function-based index you can work around that issue. Take a look at the following article which might be quite helpful if you run into the issue:
http://www.dba-oracle.com/oracle_tips_null_idx.htm
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