If a user is looking for information in a document set, the system searches the corresponding index rather than the documents themselves. An index is a data structure that allows you to search for information efficiently. Searching in an index is considerably faster than searching directly in documents.
Below is a simplified example of how an index can be structured. The example shows an inverted index. This is used for an exact search. The inverted index consists of a list of terms that are extracted from documents. Each term references the document it appears in.
bus: document_1, document_5, document_4
bush: document_16, document_21
business: document_16, document_21
busy: document_16, document_21