Retrospection？

[shuijian-xu]

Each component of the model that is subject to change will be evaluated to assess the way in which past (historical) values should be treated. When we refer to a component we mean:

Entity: a set of circumstances or a dimension (e.g., customer details or product dimension)

Relationship: for example, a hierarchy

Attribute: for example, the customer's address

Each component will then be given a classification. This is called the retrospection[1] of the component.

[shuijian-xu]

Retrospection has three possible values:

[1] Retrospection means literally “looking back into the past.”

1. True

2. False

3. Permanent

[shuijian-xu]

True retrospection means that the object will reflect the past faithfully. It enables queries to return temporal subsets of the data reflecting the partitioning of historical values. Each dimension, relationship, and attribute value will, in effect, have a lifespan that describes the existence of the object. An object may have a discontinuous lifespan, that is, many periods of activity, punctuated by periods of inactivity. True retrospection is the most accurate portrayal of the life of a data warehouse object.

[shuijian-xu]

False retrospection means that the view of history will be altered when the object's value changes. In simple terms, when changes occur, the old values will be overwritten and are, therefore, lost. It is as though the old values had never existed.

[shuijian-xu]

Permanent retrospection means that the value of the object will not change over time.

[shuijian-xu]

Retrospection = true for entities means that the lifespan of the entity consists of one or more time intervals. A single customer may not have a single, continuous lifespan. It is also true of other entities such as the wine dimension. A wine may be available for intervals of time spanning many years, or the entire lifespan may be punctuated by periods when the wine is not available. An example of this would be the Beaujolais Nouveau, which, for some reason, is very popular when first available in November each year but must be consumed quickly, as it soon deteriorates. As this wine is not available for 10 months out of each year, it is reasonable to say that the lifespan of this wine is discontinuous.

[shuijian-xu]

Retrospection = false for entities means that the current state only, of the existence of the entities, is recorded. An example from the Wine Club would be the supplier dimension. There may be a need to be able to distinguish between current suppliers and previous suppliers. There is no requirement to record the intervals of time when a supplier was actually supplying wine to the Wine Club as distinct from the intervals of time when they were not.

[shuijian-xu]

Retrospection = permanent for entities means that the entity exists forever. The concept of existence does not, therefore, apply. An example from the Wine Club would be the region dimension. Regions, which represent the wine-growing areas of the world, are unlikely to disappear once created.

[shuijian-xu]

Retrospection = true for relationships means that the lifespan of each relationship must be recorded and kept so that the results from queries will faithfully reflect history. An example of this in the Wine Club is the relationship between customer and sales area. If a customer moves from one sales area to another, it is important that the previous relationships of that customer with sales areas are retained.

[shuijian-xu]

Retrospection = true for relationships means that the lifespan of each relationship must be recorded and kept so that the results from queries will faithfully reflect history. An example of this in the Wine Club is the relationship between customer and sales area. If a customer moves from one sales area to another, it is important that the previous relationships of that customer with sales areas are retained.

[shuijian-xu]

Retrospection = permanent for relationships means that the relationship is never expected to change. No change procedures have to be considered. An example of this kind of relationship, in the Wine Club, is the relationship between a wine and a growing region. A wine is produced in a region. A particular wine will always be produced in the same region, so it is reasonable to take the view that this relationship will not change.

[shuijian-xu]

Retrospection = true for attributes means that we need to record faithfully the values associated with the attribute over time. An example of this is the cost of a bottle of wine. As this cost changes over time, we need to record the new cost price without losing any of the previous cost prices.

[shuijian-xu]

Retrospection = false for attributes means that only the latest value for the attribute should be kept. When the value of the attribute changes, the new value replaces the old value such that the old value is lost permanently. An example of this in the Wine Club is the alcohol by volume (ABV) value for the wine. If the ABV changes, then the previous ABV is replaced by the new ABV.

[shuijian-xu]

Retrospection = permanent for attributes means that the value is not expected to change at all. Changes to the values of these type of attributes do not have to be considered. An example of this type of attribute in the Wine Club is the hobby name. Once a hobby has been given a name, it is never expected to change.