A data warehouse is a database built for analysis rather than for running an application. It consolidates data from many sources into one place and is optimised for the kind of large, complex queries reporting and analytics need — the opposite of the fast, small transactions an application database is tuned for.
Keeping the two separate is deliberate: running heavy analytical queries against your live production database competes with the application for resources and slows both. The warehouse gives analysts a place to ask expensive questions without affecting the systems customers are using.
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