Constraints at Database Level or at ETL side .

I have observed that constraints in one large datawarehouse are all maintained at ETL level and not at database ( oracle ) . Especially the primary key - foreign key relationship between fact and dimension tables.

I am new to ETL and have always believed that maintaining constraints at database level is a better design .

Can someone throw the benefits of having the constraints all maintained at ETL level ?

It's because the work ETL is doing to transform natural keys to surrogate keys ensures that all surrogate primary keys in the database exist.  Database constraints are redundant and only serve to significantly slow down loads into the database.

Often such constraints are declared in the database, but are disabled to prevent actual enforcement. Some DB optimizers use that information in developing a query plan.