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update facts?

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Post  topcat Wed Jan 09, 2013 4:41 pm

I have a fact table that tracks retail returns. there is a reason dimension, but often no return reason is supplied initially. the reason dimension has a "not available" record, so when the fact comes in initially, the key is set to this record. now what happens is the customer is contacted and the return reason is determined and selected in the source system. when this new data comes to the warehouse, should i insert a new fact record or update the existing? pros and cons for both, if i update, then the initial response is lost. if i insert a new fact record, then if i aggregate the returns for a given period, the amounts will be incorrect.


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Post  ngalemmo Wed Jan 09, 2013 5:44 pm

You have three options:

1. Implement a snapshot fact table, meaning rows are updated in place.
2. Implement an accumulating snapshot fact table, meaning rows are inserted and carry an effective and expiration date.
3. Maintain return reason as a separate factless fact table. Easier to load, but more challenging to use in queries.

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Post  BoxesAndLines Thu Jan 10, 2013 10:26 am

I would assume most of the business folks are not interested in tracking history for late arriving dimension data, especially for the case of Unknown to Known. If the database can take the performance hit, I would update the fact recrod.

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