[efxhoy]

Typically data warehouses are OLAP databases that have much better performance than OLTP databases for large queries.

There might also be several applications in a company, each with their own database, and a need to produce reports based on combinations of data from multiple applications.

I think that in many cases your question is based on an idea that is completely right. engineers are too eager to split out applications into multiple databases and tacking on separate data warehouses. The costs of maintaining separate databases is often higher than initially thought. Especially when some of the data in the warehouse needs to go back into the application database, for example for customer facing analytics. I think many companies would be better served by considering traditional data warehousing needs directly in their main application databases and abstain from splitting out databases. Having one single ACID source of truth and paying a bit more for a single beefy database server makes a lot more sense than is commonly thought. Especially now when many customer facing products, like recommendation systems, are “data driven”. At least that’s my impression after working in the space for a while.