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Fragmentation.

No matter how the original query is formulated, it is eventually converted into a graph (semantic network) which is then given to the Fragmentation Module. The output of this module is a set of small fragments called probes, a term borrowed from biology. This module is responsible for substitutions (broadening) and the computation of the probes.

Broadening is controlled by a weight associated with each possible substitution. Successive substitutions multiply the weights until a user-specified limit is reached. This prevents a combinatorial explosion.

After computing the substitutions, the resulting graphs are broken into probes. The simplest kind of probe is a single node. Each node is labeled with a type and possibly a value. Types are defined in the schema of the ontology, while values are defined in the lexicon. Roughly speaking, probes consisting of a single node correspond to the keywords of a traditional keyword-based IR system.

The next more complex probe is a pair of nodes connected by an attribute edge. Attribute edges are directed and have a label. The attribute labels are defined in the schema of the ontology. The most complex probe that is used is one having two attribute edges and two or three nodes. Note that fragments can consist of more than one node together with the edges (relationships) between them, so that any given node as well as any given relationship edge will generally occur many times in the set of all fragments.

The algorithm for fragmentation can be expressed as follows:

Each probe has an associated weight. The weight is computed using weights taken from the ontology as well as substitution weights and possibly even weights specified in some way in the original query. The number of probes can be limited by specifying a lower limit below which probes will be discarded. Larger probes have a weight larger than the sum of the weights of its constituent parts since a match with such a probe would be much more meaningful than just matches with the individual nodes.

The fact that a match with a larger probe is more meaningful than one with a smaller probe suggests that indexing with larger probes is more useful than with smaller ones, in general. However, using larger probes expands the size of the index greatly with index terms that are much less likely to be matched. One must trade-off the inclusion of index terms that have high weight but little likelihood of being matched against index terms that have lower weight but greater likelihood of being matched.

Returning to the query example in Figure 2, the fragmentation step would produce six probes: one for each node, one for each edge, and one for the whole network. This assumes no broadening of the query. There are several ways one can broaden this query. The specific language, C*, could be broadened to any parallel language. The algorithm category could be replaced with a more general category such as ``algorithm: search.'' Just using these two variants, the number of probes increases from six to twelve.

Even for a relatively small query, the fragmentation step can generate a large number of probes. However, for a fixed broadening limit, the number of probes is a constant times the size of the query. For a more precise statement of this see Theorem 1 and Corollary 2 in the Appendix.



Next: Index Structure. Up: KEYNET: Fast indexing for Previous: Ontology.


kenb@ccs.neu.edu
Fri Jan 20 22:04:00 EST 1995