Answer Posted / prodyot sarkar
A data mart is a collection of subject areas organized for decision support based on the needs of a given department. Finance has their data mart, marketing has theirs, sales has theirs and so on. And the data mart for marketing only faintly resembles anyone else's data mart.
Perhaps most importantly, the individual departments OWN the hardware, software, data and programs that constitute the data mart. The rights of ownership allow the departments to bypass any means of control or discipline that might coordinate the data found in the different departments.
Each department has its own interpretation of what a data mart should look like and each department's data mart is peculiar to and specific to its own needs. Typically, the database design for a data mart is built around a star-join structure that is optimal for the needs of the users found in the department. In order to shape the star join, the requirements of the users for the department must be gathered. The data mart contains only a modicum of historical information and is granular only to the point that it suits the needs of the department. The data mart is typically housed in multidimensional technology which is great for flexibility of analysis but is not optimal for large amounts of data. Data found in data marts is highly indexed.
There are two kinds of data marts--dependent and independent. A dependent data mart is one whose source is a data warehouse. An independent data mart is one whose source is the legacy applications environment. All dependent data marts are fed by the same source--the data warehouse. Each independent data mart is fed uniquely and separately by the legacy applications environment. Dependent data marts are architecturally and structurally sound. Independent data marts are unstable and architecturally unsound, at least for the long haul. The problem with independent data marts is that their deficiencies do not make themselves manifest until the organization has built multiple independent data marts.
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