Why The data warehouse is almost always kept separate from the application databases ？

[shuijian-xu]

because:

1. Application databases are optimized to execute insert and update type queries, whereas data warehouses are optimized to execute select type queries.

2. Application databases are constantly changing, whereas data warehouses are quiet (nonvolatile).

3. Application databases have large and complex schemas whereas data warehouses are simplified, often denormalized, structures.

4. Data warehouses need historical information and this is usually missing from application databases.