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In the Structured Search paradigm, key-objects play the same role as keywords(Index term) do in the Information retrieval paradigm. While keywords are used for querying unstructured data (texts, documents, web-pages, etc.) key-objects are used for querying structured or semi-structured data, including databases, XML repositories, etc. Keywords may be viewed as rudimental key-objects consisting only of the key-object name.
Keywords allow posing queries independently of the indexed unstructured data sources - that is without knowledge of their organization, content, or number. Analogously, key-objects allow posing queries to structured data sources without knowledge of their organization (e.g., database schemes) or the number of heterogeneous sources being queried simultaneously.