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Thread: Difference between Top-Down and Bottom-Up Approach in Data warehouse

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    Senior Member vijay's Avatar
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    Difference between Top-Down and Bottom-Up Approach in Data warehouse

    I attended an interview recently and the interviewer asked me the question "What is the difference between top-down and bottom-up approach in data warehouse"? What are the advantages and disadvantages of these two approaches?

    Can someone please answer this question?

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    Ralph kimball proposed the bottum-up design approach of data warehouse. In bottom-up approach datamarts are created first. T datamarts are then integrated and a comprehensive data warehouse is created. As the datamarts are created first, business solutions can be answered quickly.

    Bill Inmon proposed the top-down up design approach of data warehouse. In top-down approach complete data warehouse is created first and then the datamarts are derived from the data warehouse. As the datamarts are created from a single data warehouse, all the datamarts has consistent data. It is robust against business changes. The cost of the project is very high.

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    What is the difference between top-down and bottom-up approach in data warehouse"?

    Bottom-Up Design:

    In the bottom-up design approach, the data marts are created first to provide reporting capability. A data mart addresses a single business area such as sales, Finance etc. These data marts are then integrated to build a complete data warehouse. The integration of data marts is implemented using data warehouse bus architecture. In the bus architecture, a dimension is shared between facts in two or more data marts. These dimensions are called conformed dimensions. These conformed dimensions are integrated from data marts and then data warehouse is built.

    Advantages of bottom-up design are:
    This model contains consistent data marts and these data marts can be delivered quickly.
    As the data marts are created first, reports can be generated quickly.
    The data warehouse can be extended easily to accommodate new business units. It is just creating new data marts and then integrating with other data marts.

    Disadvantages of bottom-up design are:
    The positions of the data warehouse and the data marts are reversed in the bottom-up approach design.

    Top-Down Design:

    In the top-down design approach the, data warehouse is built first. The data marts are then created from the data warehouse.

    Advantages of top-down design are:
    Provides consistent dimensional views of data across data marts, as all data marts are loaded from the data warehouse.
    This approach is robust against business changes. Creating a new data mart from the data warehouse is very easy.

    Disadvantages of top-down design are:
    This methodology is inflexible to changing departmental needs during implementation phase.
    It represents a very large project and the cost of implementing the project is significant.

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