"The next most important think to remember is that holding data beyond it's useful life is a business risk."
Depends where you are ...
When I did a Bank Merger, all the data each bank had from basically day one to some current date was put into a Data Warehouse as an "active archive". Every month the current database was checked and any data longer than a certain time period was moved to that "active archive".
A data warehouse is usually one of the largest databases a company has. They cost money to maintain, back-up, restore, pushing data across networks, disk space, time to data-mine and produce reports, etc.
Good thread, nice to see different takes.
I think the biggest risk is the controls around the data in which it's input and output (GIGO). Having clean ETL's into your database / data warehouse is critical.
Having Bob the DBA running complex joins over and over again with no controls to update different pieces of the data is RISKY. Of course poor design is critical as well, knowing when to index, which tables to backup, architecture in general is critical.
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