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PART III
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These two drawbacks to the bridged solution must be weighed carefully against its advantages. Some organizations may be willing to undertake the additional work and vigilance needed to ensure that a bridge table is used correctly in exchange for the powerful analysis it enables. Unfortunately, there is one additional set of considerations that has yet to be discussed: the impact of changes on a bridged solution.
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Changes and the Hierarchy Bridge
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The final complication in the use of a hierarchy bridge table has to do with responding to changed data. It will be necessary to plan for slow changes that impact the dimension, as you are already used to doing. It will also be necessary to plan for changes that impact hierarchies themselves. Like other kinds of changes, hierarchy changes can be responded to in two ways: one that preserves the context of previously recorded facts, and one that does not. These are analogous to the type 2 and type 1 responses, respectively. Responding to type 1 changes, either to the dimension or to its hierarchy, is relatively simple. In the case of type 2 changes, however, the presence of a bridge introduces some new challenges. A type 2 change to a row in the dimension has a ripple effect, requiring type 2 changes to other members of the hierarchy.
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When changes involving a dimension with a hierarchy bridge do not require the preservation of the historic context of facts, processing is relatively simple. In the case of a dimension table, the processing of a type 1 change is identical to that presented in 3. In the case of a hierarchy change, it is a matter of updating the bridge to reflect the new relationships.
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In a bridged solution, the mechanics of the type 1 change are identical to those you have already learned. A simple update to the dimension row is all that is required. The new state of affairs is now reflected in the star, with no evidence that things ever looked any other way. After a type 1 change occurs, the bridge table can continue to be used to analyze facts by looking up or looking down from any member of the hierarchy. The same dimension rows are in place, although one has changed slightly. There is no impact on the bridge table itself, nor is there any impact on the various query techniques described earlier in this chapter.
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Type 1 Change to the Hierarchy
If a hierarchy changes, rather than a member of the hierarchy, it is only necessary to adjust the bridge table. The dimension table is unaffected. The simplest method is to delete all rows in the bridge table relating to the changed ownership, and replace them with a new set of rows. It is also possible to update affected rows, if you prefer. Referring back to Figure 10-8, suppose that Company 6 is to be sold off, but Companies 7 and 8 will be retained. In the bridge table, all rows relating to the hierarchy involving Companies 1 through 8 will be removed and replaced with a set of new rows representing the new state of affairs. If the company that purchased Company 6 is also a customer, it will also have an ownership hierarchy reflected in the bridge table. This tree, too, will require replacement.
10 Recursive Hierarchies and Bridges 245
Strictly speaking, it is not necessary to wipe out the entire hierarchy tree. It is possible to handle the change as a series of deletes, updates, and inserts as required. Many of the relationships between the companies under Company 1 remain unaltered after the sale of Company 6. These rows do not need to be replaced. All the rows relating to Company 6 will require deletion. Other rows can be updated for example, after the sale, Company 8 is one level removed from Company 5 rather than two. This piecemeal approach is workable, but prone to error. It is very rare that changes to a hierarchy are treated as type 1. If the hierarchy is significant enough to demand the extra ETL work and query complexity of a hierarchy bridge table, it is relatively certain that changes to the hierarchy will be deemed significant, calling for a more complicated type 2 response.
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