About Data Warehousing

A data warehouse is the main repository of the organization's historical data, its corporate memory. For example, an organization 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 California and Quebec from 2001-2005. In other words, the data warehouse contains the raw material for management's decision support system. The critical factor leading to the use of a data warehouse is that a data analyst can perform complex queries and analysis (such as data mining) on the information without slowing down the operational systems.

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 (on line 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, once committed, the data is static, read-only, but retained for future reporting;
* 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.

A data warehouse can be a significant enabler of commercial business applications, most notably customer relationship management (CRM).

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