Bidirectional search

Structure-goal strategies—directed at the structure of a potential intermediate or potential starting material. Such a goal greatly narrows a retrosynthetic search and allows the application of bidirectional search techniques. 

Best Bidirectional Hits, Collinear Gene Partnerships, and DNA Sequence Similarity Search... 

A network model is quite similar to but more general than the hierarchical model. In a hierarchy, data have to be arranged such that one child has only one parent, and in many instances this is unrealistic. If we force the use of a hierarchical representation in such cases, data will have to be repeated at more than one location in the hierarchical model. This redundancy can create a number of problems. A record in a network model can participate in several relationships. This leads to two primary structural differences between hierarchical and network models. Some fields in the hierarchical model will become relationships in a network model. Further, the relationships in a network model are explicit and may be bidirectional. The navigation problem in a network data model can become severe. Because search of the database can start at several places in the network, there is added complexity in searching, as well. 

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