Explain what a bitmapped index is, what it's applications would be, and why it's useful.
I would appreciate if you can suggest a better way
These are data warehousing concepts.
1) A Star Schema is where you have a main "fact" table and many "dimension" tables used to query the fact table (so the ERD would look like a star with the fact table in the middle and the dimension tables around it). For example, you might have a SALES fact table with dimension tables for REGION, MONTH, PRODUCT CATEGORY, CUSTOMER, etc.
2) A bitmapped index is a special kind of index useful in DW (not in ordinary "OLTP" databases) for columns that have a low number of distinct values (low "cardinality") - for example, Sex(M/F), Region.
It has a "row" for each possible value, with a flag for each record in the table indicating whether the column has this value. Simple example:
Row Name Sex
1, Jim, M
2, Jane, F
3, Sue, F
4, Jack, M
Bitmapped index on EMP.SEX:
Value Row1 Row2 Row3 Row4 ...
M, 1, 0, 0, 1
F, 0, 1, 1, 0
These indexes become powerful when you have several of them on different columns, and then the optimiser can combine them with a logical AND to get the matching rows.