Data warehouse

A data warehouse is a database built for analysis rather than for running an application. It holds data consolidated from several operational systems, organised for questions that scan large numbers of rows.

It exists because the two workloads conflict. An application database is tuned to read and write single records very quickly; a question like "revenue by region by month for three years" reads millions of rows and, run against production, competes with the customers using it.

The separation also fixes the version of the truth. When finance, sales and operations each report from their own system, the numbers disagree; a warehouse is where the definitions are reconciled once, which is more of a governance benefit than a technical one.