Network Data model:
Not all problems conveniently in shape a hierarchical model. If nothing besides different users may want to see the same information in a different hierarchy. For illustration an application might want to see the hierarchy ‘upside-down’ with invoice at the top and location at the bottom. Assistance for logical hierarchies (views) requires that the data management system support a general network. The competent implementation of certain relational operators (sort-merge or join) as well requires parent-child sets and so requires the full capability of the network data model.
The common statement is that if all relationships are nested one-to-many mappings then the data can be expressed as a hierarchy. If there are many-to-many mappings afterwards a network is required. To consider a specific illustration of the need for networks imagine that several locations may service the same account and that each location services several accounts Then the hierarchy introduced in the previous section would need either that locations be subsidiary to accounts and be duplicated or that the accounts record be duplicated in the hierarchy under the two locations. This will give increase to complexities about the account having two balances. A network model would permit one to construct the structure:
A network built out of two parent-child sets.
Comparison of Data models:
By utilizing ‘symbolic’ pointers (keys) one may map any network data structure into a relational structure. In that wisdom all three models are equivalent as well as the relational model is completely general. Nevertheless there are substantial differences in the style as well as convenience of the different models. Examination of specific cases habitually indicates that associative pointers (keys) cost three page faults to follow (for a multi-megabyte set) whereas following a direct pointer costs only one page fault. This performance dissimilarity explains why the equivalence of the three data models is irrelevant. If there is heavy traffic among sets then pointers must be used. (High-level languages be able to hide the use of these pointers.)
It is my bias that one must resort to the more elaborate model only when the simpler model leads to excessive complexity or to poor performance.
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