Welcome to the world of data sourcing. This is one of the activities in data warehousing that takes a lot of time. Having been into many warehousing projects, I feel now that the most important thing in a data warehousing project is probably the data sourcing. We need to have somebody in our DW team who understands the source systems. The dimensional modelling is equally important, but if you don’t understand the source data/systems, then the model would not be correct, and the ETL spec would not be correct either. And data sourcing needs to be done early: if we don’t have the data we can’t do anything!
— Vincent Rainardi
This article is a great read – highly recommend.