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Post-coordinated indexing is the opposite of pre-coordinated indexing. There are pros and cons to each method of indexing.
Terms from the index are combined during the searching, rather than before, to create an index based on the indivdual search result. Facets can be combined ad finitum following the standpoint of the user. The index is created after the information is added to the database. Post-coordinated indexing will generall increase recall but will usually decrease precision.
Post-coordinated indexing is almost always associated with computers, and usually uses some type of boolean logic and descriptors. Post-coordinated indexing allows searchers the freedom to freely combine many terms that are relevant to the search.