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Data warehouse | ||
A data warehouse is a computer system designed for archiving and analyzing an organisation's historical data, such as sales, salaries, or other information from day-to-day operations. Normally, an organisation copies information from its operational systems (such as sales and human resources) to the data warehouse on a regular schedule, such as every night or every weekend; after that, management can perform complex queries and analysis (such as data mining) on the information without slowing down the operational systems. DefinitionA data warehouse is the main repository of the organisation's historical data, its corporate memory. For example, an organisation would use the information that's stored in its data warehouse to find out what day of the week they sold the most widgets in May 1992, or how employee sick leave the week before Christmas differed between While operational systems are optimized for simplicity and speed of modification (online transaction processing, or OLTP) through heavy use of database normalization and an entity-relationship model, the data warehouse is optimized for reporting and analysis (online analytical processing, or OLAP). Frequently data in Data Warehouses is heavily denormalised, summarised and/or stored in a dimension-based model but this is not always required to achieve acceptable query response times. More formally, Bill Inmon (one of the earliest and most influential practitioners) defined a data warehouse as follows: * Subject-oriented, meaning that the data in the database is organized so that all the data elements relating to the same real-world event or object are linked together; * Time-variant, meaning that the changes to the data in the database are tracked and recorded so that reports can be produced showing changes over time; * Non-volatile, meaning that data in the database is never over-written or deleted, but retained for future reporting; and, * Integrated, meaning that the database contains data from most or all of an organization's operational applications, and that this data is made consistent. Advantages of using data warehouse There are many advantages to using a data warehouse, some of them are...* Enhances end-user access to a wide variety of data. * Business decision makers can obtain various kinds of trend reports e.g. the item with the most sales in a particular area / country for the last two years. This may be helpful for future investments in a particular item. * Increases data consistency. * Increases productivity and decreases computing costs. * Is able to combine data from different sources, in one place. * It provides an infrastructure with the capability to support changes to data and to replicate the changed data back into the operational systems. Concerns in using data warehouse* Extracting, cleaning and loading data could be time consuming. But this can be made easy with the help of warehousing tools. * Data warehousing project scope might increase. * Problems with compatibility with systems already in place e.g. transaction processing system. * Providing training to end-users, who end up not using the data warehouse. * Security could develop into a serious issue, especially if the data warehouse is web accessible. Copyright 2008 - France BtoB from Wikipédia
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