Data Warehouses: Data warehousing is defined as a process of centralized data management and retrieval. A data warehouse is a place where data is stored for archival, analysis and security purposes. A data warehouse is a central repository for all or significant parts of the data that an enterprise's various business systems collect. The term was coined by W. H. Inmon. IBM sometimes uses the term "information warehouse."
Usually a data warehouse is either a single computer or many computers (servers) tied together to create one giant computer system. Typically, a data warehouse is housed on an enterprise mainframe server. Data from various online transaction processing (OLTP) applications and other sources is selectively extracted and organized on the data warehouse database for use by analytical applications and user queries. Data warehousing emphasizes the capture of data from diverse sources for useful analysis and access. Applications of data warehouses include data mining, Web Mining, and decision support systems (DSS).
Data marts are smaller and less integrated data housings. They might be just a database on human resources records or sales data on just one division.
a) Characteristics of Data Warehousing: Data warehouse includes the following characteristics-
i) Subject Oriented: Subject oriented means that data is linked together and is organized by relationships.
ii) Time Variant: Time variant means that any data that is changed in the data warehouse can be tracked. Usually all changes to data are stamped with a time-date and with a before and after value, so that you can show the changes throughout a period of time.
iii) Non Volatile: Non volatile means that the data is never deleted or erased. This is a great way to protect your most crucial data. Because this data is retained, you can continue to use it in a later analysis.
iv) Integrated: The data is integrated, which means that a data warehouse uses data that is organizational wide instead of from just one department.
b) Advantages: The data warehouse helps the employees or end users to access and use the data for reports, analysis and decision making. Using the data in a warehouse one can locate trends, focus on relationships and understand more about the environment on which the business operates.
Data warehouses also increase the consistency of the data and allow it to be checked over and over to determine how relevant it is. Because most data warehouses are integrated, one can pull data from many different areas of the business, for instance human resources, finance, IT, accounting, etc.
c) Disadvantages: Data warehouse is time consuming to create and to keep operating. Many time the current systems become incompatible with the data. So, the hardware and software continuously need to be upgraded. Finally, security might be a huge concern, especially when the data is accessible over an open network such as the internet. In such cases the data can be viewed by the competitor or worse hacked and destroyed.