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Figure 8-4 A type 3 attribute in the customer dimension
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The Type 3 Change in Action
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The mechanics of a type 3 change are illustrated in Figure 8-5. In this example, customers in the region called East are redesignated as belonging to regions called Northeast or Southeast. The top half of the diagram shows the state of affairs before the change. Several rows from the customer dimension table are shown. Notice that region_current
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Customer dimension table customer_ key 1011 1022 1302 1499 customer_ID 1140400 3305300 7733300 9900011 customer_name Davis, Robert Nguyen, Tamara Rodriguez, Jason Johnson, Sue region_ current East East West West region_ previous East East West West
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Orders fact table day_key 2322 3422
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1011 1011
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BEFORE AFTER Customer dimension table customer_ key 1011 1022 1302 1499 customer_ID 1140400 3305300 7733300 9900011 customer_name Davis, Robert Nguyen, Tamara Rodriguez, Jason Johnson, Sue region_ current Northeast Southeast West West region_ previous East East West West
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Orders fact table customer_ day_key key 1011 1011 1011 1011 2322 3422 6599 8211
Analysis with new values order_ dollars 2000 1000 1200 2000
Analysis with old values
facts in place before the change facts added after the change
Figure 8-5
A type 3 change in action
Part III
customer_ key
order_ dollars
PART III
Dimension Design
contains each customer s region East or West. There have been no changes yet, so the region_previous is initialized to contain the same values. In the fact table, two rows have been recorded for the customer identified by customer_key 1011. Tracing this back to the customer dimension, you can see that these orders were placed by Robert Davis, who is in the region East. Now suppose that the business decides to change the regional groupings. The region that was formerly known as East will be subdivided into Northeast and Southeast. For each customer who was in the East region, a type 3 change occurs. The region_current is updated with the new values. The result can be seen in the lower half of Figure 8-5. Robert Davis now has a region_current of Northeast. The two orders that Robert Davis placed before the change are still in the fact table. In addition, Robert has placed two additional orders since the change occurred. All four rows are associated with the same row in the dimension table, identified by surrogate key 1011. This solution allows you to employ either version of the region categorization in queries or reports, simply by using the appropriate column. When you want to study orders using the new scheme, you group order_dollars by region_current. This causes all the orders from Robert Davis to be rolled up under the designation of Northeast, whether they were recorded before or after the change. When you want to study orders using the old scheme, you group order_dollars by region_previous. Robert s orders now appear under the grouping for East, regardless of when they occurred. TIP A type 3 response associates the old and new values of the changed attribute with all transactions. Two dimension attributes are required. One will hold the current value; the other will hold the previous value. When the source changes, both are updated. In exchange for some additional ETL complexity, the type 3 design buys enough flexibility to meet the dual requirements outlined earlier. The new value can be used to study all transactions, whether they occurred before or after the change, and so can the old value. All that remains is to educate the users or report developers on what each column means and how to choose which one to use. Notice that the region_current attribute provides a standard type 1 response to the changed data. It always contains the most recent value; it is always overwritten when the value changes. For this reason, the region_current column can be designated as a type 1 attribute in design documentation if you prefer. You may even prefer to think of the change as a type 1/3 change to region. Designating the attributes that produce a type 3 response as a pair, however, may be more useful in communicating to ETL developers how changes should be processed. This is how the region attributes are documented in Figure 8-4. It is important to recognize that a type 3 change does not preserve the historic context of facts. Each time a type 3 change occurs, the history is restated. There is no way to know, for example, what region Robert Davis was a part of when a particular order was placed. This requires a type 2 approach. As you will see later in this chapter, type 2 and type 3 responses can be combined, in a hybrid response known as type 1/2/3. Type 3 changes are comparatively rare, but they are occasionally warranted. Redistricting and geographic realignments are the most common examples. Finance and accounting groups often designate new names or classifications for accounts on an annual basis. Such cases have the lucky distinction of occurring at predictable intervals, although this is not always the case.
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