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A type 3 change is not a one-time-only process. It can be repeated once, twice, or as many times as needed. Robert Davis s region, for example, may subsequently be changed again. Perhaps as a result of redrawing the region boundaries, he is redesignated as being in the Southeast. When this occurs, what was formerly in region_current (Northeast) is moved to region_previous. The new value (Southeast) is placed in region_current. A type 3 change preserves only one old version of the changed attribute. Once a second change occurs, the fact that Robert Davis was once in the East region is lost. In some cases, it may be useful to maintain multiple values. This can be done by designating three, four, or however many versions of the region column are required. Each time a change occurs, the values are all shifted among these columns. With multiple versions, the columns can be difficult to name. If the changes occur on a predictable basis, they might be named region_2009, region_2008, region_2007, and so forth. This approach has two distinct disadvantages. First, it requires the DBA to periodically change the names of columns. For example, if requirements call for the current version and the two previous versions, it is necessary at the end of 2009 year to drop the 2007 column and add one for 2010. Second, encoding the year into the column names means new queries and reports will need to be designed each year to refer to the new column names. An alternative that avoids these drawbacks is to name the columns region_current, region_last_year, and region_two_years_ago.
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The type 3 change is often misunderstood as requiring special advanced scheduling. Although developers may take advantage of the predictability of a type 3 change and handle it with a special process, this is not strictly required. As with any other slow change processing, it is possible to design an ETL process that looks for changes to the attribute in question and applies them if and when they are detected. All that is necessary is to anticipate the possibility of the type 3 change and engineer the ETL process accordingly. There may be good reasons to handle a type 3 change via a special process. In most cases, type 3 changes occur en masse. A change to the region designations, for example, may affect every customer in the dimension table. The information that drives a type 3 change may come from a separate source or involve special rules. Region designations, for example, may be driven by a spreadsheet that maps to Zip codes. A type 3 change with multiple versions may involve renaming database columns and developing new reports. Its implementation must be coordinated with these activities. The change may occur on a predicable basis, in which case it is not necessary to monitor for it as part of standard processing. For any of these reasons, it may be useful to handle the type 3 change through a separate scheduled process. This is not mandatory, but it may be convenient for processing purposes.
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When it comes to documenting a type 3 change, designers have varying preferences. In Figure 8-4, each attribute is designated as a surrogate key, as a natural key, or according to its slow change characteristic. The type 3 attribute pair is highlighted and labeled as type 3. As noted, you may prefer to designate the current member of the attribute pair as a type 1 attribute, since it behaves in exactly the same way as any other type 1 attribute. Modeling tools often impose limitations on how you can document the slow change behavior associated with an attribute. Some dimensionally aware modeling tools allow you to note slow change characteristics, but most tools will not let you highlight a pair of type 3 attributes. Naming conventions and comments may be your only route to associating them. Some modeling tools do not include the concept of a type 3 change, limiting your options to type 1 and type 2. Using these tools, you may be forced to label both members of the attribute pair as type 1. This will require some careful annotation to distinguish each attribute s purpose. You may convey the behavior of the attribute containing the previous value by noting its data source is the attribute that holds the current value.
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