Consider a persistent class of MyObjects with a bitmap index on Property1, and then have a subclass of MySpecialObjects.

If I use

select * from MySpecialObjects where Property1 = 'Something'

I only want the items that are MySpecialObjects and not any other MyObjects. The extent index makes this possible. It ANDs the MySpecialObjects extent index with the Property1 index.

A bonus, is it also makes select count(*) very fast, even with tables of over 50,000,000,000 rows