barcode vb.net 2010 Flattening a Recursive Hierarchy in Software

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Flattening a Recursive Hierarchy
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A recursive hierarchy can be flattened so it is not necessary to traverse a series of relationships at query time. The hierarchy is flattened by creating new attributes that represent a fixed number of levels. The flattened hierarchy looks and behaves like an attribute hierarchy. However, it does not really solve the problems of looking up or looking down. Flattening is most successful if the levels have consistent and distinguishable meanings.
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A Flattened Hierarchy
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An instance hierarchy can be flattened so it looks more like the attribute hierarchies you have seen before. Columns are added to the dimension to represent a fixed number of hierarchy levels. For each row in the table, the values assigned to these summary columns are computed by traversing the hierarchy ahead of time, during the ETL process. These new attributes flatten the hierarchy so it will not be necessary to traverse the recursive relationship at query time. In the case of companies, for example, we can compute some extra information about each company and store it in new columns. For each company, we will identify the associated top-level company, second-level company, and so forth. These new attributes will become a part of each company s dimension record. Figure 10-4 provides an example. Ignore the shaded cells for the time being; they will be described shortly. The company dimension in Figure 10-4 is flattened into four levels. As you can see from the instance chart, the various levels of ownership have been computed for each company. For example, the last row in the chart is Company 8. You can see that its level_1_company is Company 1, its level_2_company is Company 5, and so forth. The tree diagram to the left of the table can be used to verify the accuracy of this representation.
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ORDER_FACTS day_key product_key salesrep_key company_key . . . order_dollars . . .
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COMPANY company_key company_id company_name company_type . . . level_1_company level_2_company level_3_company level_4_company
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COMPANY 1
company_ company_ key name level_1_ company level_2_ company level_3_ company level_4_ company
Company 1 Company 1 Company 1 Company 1 Company 1 Company 2 Company 1 Company 2 Company 2 Company 2 Company 3 Company 1 Company 2 Company 3 Company 3 Company 4 Company 1 Company 2 Company 4 Company 4 Company 5 Company 1 Company 5 Company 5 Company 5 Company 6 Company 1 Company 5 Company 6 Company 6 Company 7 Company 1 Company 5 Company 6 Company 7 Company 8 Company 1 Company 5 Company 6 Company 8
2 3 4 5 6
Figure 10-4 Flattening the company hierarchy
10 Recursive Hierarchies and Bridges 225
Each row still represents a single company, but it is no longer necessary to traverse a recursive relationship. Facts can be summarized at various levels in the hierarchy simply by grouping them according to the desired level. Rolling everything up to a top-level company, for example, is accomplished by grouping the facts by level_1_company. This is akin to rolling all products up to categories; the instance hierarchy is transformed into an attribute hierarchy.
Drawbacks of Flattening
Flattening an instance hierarchy poses a few problems. Though none are deal breakers, the utility of a flattened solution is limited. The importance of specific analytic requirements will determine whether flattening is sufficient.
Backfilling Is Necessary
The flattening of an instance hierarchy creates attributes to describe each row s place in a fixed number of levels. One problem with this is that not every row in the table represents the bottom of a hierarchy. For these rows, it will be necessary to decide what values to use for the lower levels. This phenomenon is clearly visible in the example. Not all companies are at the bottom of a four-level hierarchy. Company 1, for example, does not have a corresponding level_2_company, level_3_company, and so forth. So, what values will be used for these columns The usual solution for a member that is not at the bottom of the hierarchy is to backfill its own identity into the lower levels. In Figure 10-4, the shaded cells illustrate backfilled references. This allows a query to group data at any particular level, without losing transactions. So, for example, if we group all orders by level_2_company, we will not miss out on the transactions associated with Company 1. These will still be in our query results. You may choose to use a text string such as Higher Level Company in lieu of backfilling. This makes some reports easier to read but loses the association of higher-level transactions with any specific companies. Backfilling is necessary because flattening an instance hierarchy creates an artificial attribute hierarchy. Just as we could say every product has a brand, every brand has a category, we need to be able to say every company has a level 4 company, every company has a level 3 company, and so forth.
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