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Thread: star schema

  1. #1
    Join Date
    Oct 2002
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    78

    Unanswered: star schema

    Hai

    Any one explan what is

    [1] Explain the definition of a "star schema."

    [2] 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
    Regards
    mohan

  2. #2
    Join Date
    Sep 2002
    Location
    UK
    Posts
    5,171
    Provided Answers: 1

    Re: star schema

    Originally posted by mohan
    Hai

    Any one explan what is

    [1] Explain the definition of a "star schema."

    [2] 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
    Regards
    mohan
    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:

    Table EMP
    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.

    The documentation on bitmapped indexes is here:

    http://technet.oracle.com/doc/oracle...schem.htm#5381

    Finally, you said "suggest a better way". A better way to do what?

  3. #3
    Join Date
    Oct 2002
    Posts
    78
    Hai andrew
    Thanks for your prompt reply.Sorry inadvertently i typed that word

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