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Adding additional columns to the bridge table may further enhance the ability to explore or describe the hierarchy. It is sometimes useful to know that a particular member happens to be at the top or bottom of the hierarchy. This can be signaled by a pair of flags in the bridge table. Since each row in the bridge table actually references two members of the dimension, four flags are required. In the case of the company example, the additional flags indicate whether the superior company is at the top or bottom of the hierarchy, and whether the subordinate company is at the top or bottom of the hierarchy. This is shown in Figure 10-19.
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10 Recursive Hierarchies and Bridges 253
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COMPANY_ HIERARCHY_ BRIDGE superior_company_key subordinate_company_key levels_removed superior_is_top superior_is_bottom subordinate_is_top subordinate_is_bottom Effective and expiration information permit point-intime analysis and aid Q.A. and troubleshooting. effective_date expiration_date Flags may aid in hierarchy analysis.
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Figure 10-19
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Embellishments to the hierarchy bridge
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The examples provided in this chapter have assumed that each node in a hierarchy has a maximum of one parent. In rare cases, it may be necessary to model a situation in which a node may have more than one parent. Bridge tables can handle this kind of situation, but it will be necessary to eliminate the levels_removed column. For example, suppose an effort is made to expand the company ownership example to accommodate shared ownership. Perhaps a pair of companies holds all the equity of a third. In this case, the third company has two parents. The bridge table can accommodate these relationships without any changes. Looking up and looking down can still be achieved by constraining on a single company, in the same ways that have already been described. The multiple parent situations introduce one potential complication. If members of a hierarchy can have multiple parents, situations can occur in which there is more than one way to relate the same two members. In order to avoid double-counting, the bridge should only contain one row linking the companies in question. The levels_removed column should be removed from the design, since it may not be possible to specify a single value for a given pair of companies.
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Multiple Hierarchies
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In some situations, there may be more than one hierarchy that applies to the members of a dimension. Departments, for example, may participate in one hierarchy for budgetary purposes and another hierarchy describing the chain of command. The best solution to this situation is to build multiple bridge tables: one for each hierarchy. For any given query or report, the appropriate bridge table is joined to the fact table and dimension tables as appropriate. This further increases the number of ways one
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Part III
While it may sound absurd that the superior company can be at the bottom, or the subordinate at the top, remember that some companies may not participate in relationships with others. Such companies will have a single row in the bridge table, in which they appear as superior and subordinate. In the case of such a row, all four flags will be set to true values.
PART III
Dimension Design
might join the tables in a particular star; it is crucial to maintain clear documentation and to carefully educate developers of queries and reports to whom access to the bridge tables will be granted. Multiple hierarchies put additional strain on the ETL process. A type 2 change to a dimension row, for example, now impacts not one but two hierarchies. A type 2 change to one of the hierarchies will affect the dimension table, and therefore the other hierarchy as well. This means both bridge tables will require new rows, even though only one reflects changed relationships. It is possible, but not recommended, to capture multiple hierarchies with a single bridge table. This can be achieved by adding a column to identify the hierarchy to which a given row pertains. In the company example, there might be a hierarchy_name column that can take on the values budgetary and chain-of-command. The disadvantage to this solution is that it will always be necessary to constrain this column for a single value. Omission of such a constraint will produce wildly inaccurate and nonsensical results.
Summary
The decision to include a hierarchy bridge is never an easy one. While it provides the most powerful and flexible analytic capability, the hierarchy bridge introduces room for error, makes the configuration of reporting software difficult, and severely complicates the ETL process. You should never make a design decision with these wide-ranging implications on your own, regardless of your role. The various points of view that should be represented in this and other design decisions are described in 18, How to Design and Document a Dimensional Model. When evaluating the option to implement a hierarchy bridge, you must fully understand the issues involved. Key points to remember include the following: An instance hierarchy is a recursive relationship within a dimension. An instance hierarchy can be flattened into a predetermined number of levels. This provides limited capability, but may be sufficient in some cases. When people want to use a point in the hierarchy as the context for studying facts, they will be looking up or looking down. Looking down aggregates facts associated with members at or below the point in question; looking up aggregates facts at or above the point in question. A hierarchy bridge table can be built to facilitate looking up or looking down. The bridge can be used in a variety of join configurations to achieve different analytic objectives. Use of a hierarchy bridge table poses a risk of double-counting when queries are not constructed properly. It is advisable to provide access to the bridge only to trained developers. The ETL process will be significantly impacted by the presence of a hierarchy bridge. Type 2 changes within the bridged dimension will have a ripple effect on other members of the hierarchy. Relationship changes will also require processing type 2 changes to hierarchy members.
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