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Conceptual design of temporal aspects in data warehousing with T-ADAPT
from Dr. Michael Hahne
Chapter 4: Extension from ADAPT to T-ADAPT
It should be possible to illustrate the report requirements specified in section 2 by modelling. Therefore it must be possible to express the distinction of different versions of dimension structures. Four alternatives to the arrangement are available for implementation. The adjustment of historical volume of data is omitted because thus all requirements cannot be covered.
The remaining possibilities can be classified according to whether a hierarchy version is additionally put on at certain points of time, generally determined points of time, or whether all changes are continuously traced. These two types are described by selective time stamping and complete time stamping.
Parallel hierarchies can be used directly for the setting up of individual versions by dimension structures in ADAPT. However, the modelling of the entire hierarchical structure by levels, elements and attributes is done several times. Only the text addition with comments for the complete time stamping is available in ADAPT.
The representation of own object types for illustrating temporal properties is developed in this section in order to be able to illustrate the temporal aspects of structural changes in dimensions adequately in ADAPT. The extension of ADAPT by these objects is called T-ADAPT. The new objects which are available with T-ADAPT are arranged in table 9.
Table 9. Objects for time stamping hierarchies
A time dependence can be modelled individually for each hierarchy by those objects. The conventional representation correspondents with the neglect of any structural changes then and sets a fixed hierarchical structure ahead. The versioning can either be of a selective or of a continuous kind. A mixture within a hierarchy is impossible. Each structural change is considered at the continuous versioning. The representation of the complete versioning for the two cases of dependence on the validity time and also transaction time for the case of customer dimension of the example is represented in fig. 7. For regional customer hierarchy a versioning is shown on basis of transaction time.
Fig. 6. Complete time stamping with regard to validity time and transaction time
The temporal characteristic is attached to the hierarchy object and marked with the name of the central factor of influence. Apart from validity time as determining factor of influence for the generation of a new structural version it is also conceivable that the structure to reference dates is regarded for the hierarchy validity in each case. This view of continuous versioning is based on the transaction time.
ADAPT makes possibilities for grouping of dimension elements, which can vary from time to time, available by dimension scopes, apart from hierarchies. Demographic attributes of a customer are a typical example for this. Its form of time stamping is likewise specified in fig. 7. The dimension scopes of the demographic attributes, except sex, are time-stamped in the represented product dimension by validity time. A shortened way of writing is possible for time stamping of dimension scopes in, for instance, demographic customer characteristics. The objects in the example which represent married and single ones are bound to a time stamped object, as becomes evident from the illustration
The representation is done on basis of an appropriate object in case of selective time stamping, which indicates that only individual versions are considered by the dotted line in the symbol. The different hierarchy versions are represented as independent objects attached to the object of temporal characteristic. This is exemplary represented for the case of product dimension in fig. 9. The selective time stamping in the example is based on individual snapshots, because certain yearly end structures are additionally fixed in own versions to the up-to-date valid structure. The product dimension in the example is a suitable candidate for a selective time stamping in T-ADAPT on basis of individual snapshots, because apart from the up-to-date valid product hierarchy respective yearly end versions are still to be considered.
Fig. 7. Selective time stamping
It is possible to illustrate all forms of time stamping of hierarchies by the represented object types of modelling in T-ADAPT. Often it is asked for time stamping of attributes in addition. Here it has to be decided whether it is better to model these attributes as own dimension in the modelling phase.
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