barcode vb.net 2010 Do Not Resist the Ripple in Software

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Do Not Resist the Ripple
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Although steps can be taken to avoid the ripple effect, these are not worth the trouble. One approach simplifies the ETL process at the cost of adding query-time complexity; another severely hampers the analytic possibilities. Designers may be tempted to avoid the ripple effect by stamping each bridge row with effective and expiration dates. Use of these stamps, however, would require correlation with the time dimension associated with the fact table. For each transaction, the bridge table would require a pair of constraints comparing the date of the transaction with the effective and expiration dates. This places undue stress on the query resolution process and opens the solution up to the possibility of error, all to save the ETL process from having to create a few additional rows. It is clearly not a viable alternative. NOTE Though stamping bridge rows with effective and expiration dates is not an effective solution for reducing the ripple effect of a type 2 change, the technique does have value. As discussed later in this chapter, it can be used to support point-in-time analysis of the hierarchy itself, exclusive of facts. A second way to avoid the ripple is to declare type 2 changes off limits. Unfortunately, this solution limits the analytic possibilities when the bridge is not being used. In our company example, changes to headquarters_location would require type 1 treatment. Without the type 2 change, it would be impossible to group orders by location in a manner that is historically accurate. What may seem to be a third option is usually not plausible. You may have observed that the problem of double-counting in Figure 10-16 can be avoided if the query is qualified for a single version of Company E. Why not make this a simple requirement for analysis Unfortunately, this is highly impractical. Selecting one version of Company E would be easy if you knew the surrogate key values that distinguish the two versions. Unfortunately, surrogate keys are meaningless sequence numbers. The only way to distinguish the versions would be to qualify on the natural key for Company E and every type 2 attribute, which is highly impractical. Unless the dimension is time-stamped, there is no easy way to qualify for a single version of Company E. If the dimension is time-stamped, date qualifications must be added to every query that involves the bridge. It is far safer to ripple the type 2 change.
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Type 2 Changes to the Hierarchy
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Having worked through the mechanics of a type 2 change to a dimension with a hierarchy bridge, the process of responding to a change in the hierarchy itself will not be surprising. It is virtually identical.
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As before, a simple relationship change will require putting all members of the hierarchy through a type 2 change. This is necessary to properly isolate new facts from the member that has been removed, since users will be using natural keys to look up or down the hierarchy. Once again, you can skip the explanation if you want, but be sure you understand the mechanics first.
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The Mechanics of Preserving History When a Hierarchy Changes
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When the hierarchy itself changes, preservation of its history can be achieved by following the same steps used to carry out a type 2 change: 1. Create new rows in the dimension table for all members of the hierarchy that is changing. 2. Create new rows in the bridge table reflecting the new status of the hierarchy. If a change involves a member leaving one hierarchy and entering another, both hierarchies will require this processing.
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The Reason for the Ripple Effect
Once again, you might be tempted to avoid the ripple. Suppose, for example, that a company at the bottom of a hierarchy, Company H, is sold. It might appear that this company can be assigned a new key. Rows in the bridge table will still associate the old version of the company with the hierarchy but not the new version. Figure 10-17 illustrates this state of affairs graphically. While all this is true, remember that looking down and looking up require being able to identify a point in the hierarchy to start from. Since this will be a natural key, the solution in Figure 10-17 is not sufficient. Looking up from Company H, for example, will identify two bottom-level rows: H and H-1. The bridge table still contains rows that associate H with F, E, and A, and these companies are still generating transactions in the fact table. These orders will be picked up by the query, even if they took place after the change. The same would happen A in a looking-down configuration. The user cannot be expected to specify a company by its surrogate key, avoiding H but selecting H-1, so an alternative solution is required. B E Future transactions with F, E, and A can be disassociated with any version of Company H by following the standard process of putting all C D F members of the hierarchy through a type 2 change, as described earlier. New rows are added to the bridge table to reflect the post-change state of affairs. In this case, the bridge table will reflect three G H H-1 hierarchies, as shown in Figure 10-18. One is the original hierarchy, one is a new hierarchy that does Company H not include any incarnation of Company H, and the Figure 10-17 A relationship change last is the new hierarchy that Company H has joined.
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