My first Publication Agile-Data-Warehouse-Design-eBook | Página 145

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Chapter 4
Figure 4-21
Adding new
quantities, a
why/how detail
from a previous
event and an
additional repeat
story
Concentrate on
completing the event
with the when, how
many and granularity
details not
recorded
elsewhere
Thanks to its ORDER ID why detail you have the option to embellish PRODUCT
SHIPMENTS with additional order dimensions, but because these dimensions are
already well documented in the CUSTOMER ORDERS table and on the matrix,
you can, if pressed for time, add them later without stakeholders involvement. Just
make sure you let the stakeholders know you will be doing this. While you have
their attention now you should concentrate the modelstorm on capturing brand
new shipment details, especially the when, how many and granularity details not
recorded on the matrix. You also need to allow time for modelstorming the attrib-
utes of any new dimensions (the details you temporally marked [?]).
You can add any useful order dimensions to shipments but you should avoid the
how many details, such as ORDER QUANTITY or REVENUE, because the 1:M
relationship between orders and shipments would cause these measures to be
overstated when summarized; e.g., 2 partial shipments events that both record (i.e.
duplicate) the original ORDER QUANTITY of 10 units will produce a total of 20
units with double the correct order REVENUE.
Don’t copy
measures from one
event to another.
This can lead to
facts that double
count
Order measures are better left in the order event and its subsequent order fact table
at their true granularity (the order line item) rather than also stored at the shipped
line item. In Chapter 8, we cover how you would instead combine shipments with
orders to produce a single evolving order event and model the additional measures
that provides. For now, we will press on and complete the shipment event with
how details.