We have a ERP system that uses a DB in postgres, each table has many rows and we want to build data warehouse (data warehousing). Also we want to add BI using ETL
where would begin? would use schemes or two separate databases?
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I would suggest that you first need to do some study. Your questions suggest that you are unclear on the issues needed for building a data warehouse.
Read books such as Ralph Kimball's "Data Warehouse Toolkit" as a starting place to understanding the issues you are facing.
Likewise, you search through some of the online resources to learn the parts and processes. For example: http://www.learndatamodeling.com/dw_concepts.php