Why need of separate data warehouse?

3 Main reasons:

1.OLTP systems require high concurrency, reliability, locking which provide good performance for short and simple OLTP queries. An OLAP query is very complex and does not require these properties. Use of OLAP query on OLTP system degrades its performance.

2.An OLAP query reads HUGE amount of data and generates the required result. The query is very complex too. Thus special primitiveshave to provided to support this kind of data access.

3.OLAP systems access historical data and not current volatile data while OLTP systems access current up-to-date data and do not need historical data.