In text retrieval
, full text search
refers to a technique for searching a computer
. In a full text search, the search engine
examines all of the words in every stored document as it tries to match search words supplied by the user. Full-text searching techniques became common in online bibliographic databases
in the 1970s,. Most Web sites and application programs (such as word processing
software) provide full text search capabilities. Some Web search engines, such as AltaVista
employ full text search techniques, while others index only a portion of the Web pages examined by its indexing system.
When dealing with a small number of documents it is possible for the full-text search engine to directly scan the contents of the documents with each query
, a strategy called serial scanning
. This is what some rudimentary tools, such as grep
, do when searching.
However, when the number of documents to search is potentially large or the quantity of search queries to perform is substantial the problem of full text search is often divided into two tasks: indexing and searching. The indexing stage will scan the text of all the documents and build a list of search terms, often called an index, but more correctly named a concordance. In the search stage, when performing a specific query, only the index is referenced rather than the text of the original documents.
The indexer will make an entry in the index for each term or word found in a document and possibly its relative position within the document. Usually the indexer will ignore stop words, such as the English "the", which are both too common and carry too little meaning to be useful for searching. Some indexers also employ language-specific stemming on the words being indexed, so for example any of the words "drives", "drove", or "driven" will be recorded in the index under a single concept word "drive".
The precision vs. recall tradeoff
Due to the ambiguities of natural language, a full text search typically produces a retrieval list that has low precision: most of the items retrieved are irrelevant. Controlled-vocabulary searching solves this problem by tagging the documents in such a way that the ambiguities are eliminated. However, a controlled vocabulary search may have low recall: it may fail to retrieve some documents that are actually relevant to the search question. Despite the presence of many irrelevant documents in a free text search's retrieval list, a free text search may be able to locate a document that a controlled vocabulary search failed to retrieve.
The false positive problem
Free text searching is likely to retrieve many documents that are not relevant to the intended search question. Such documents are called false positives. The retrieval of irrelevant documents is often caused by the inherent ambiguity of natural language.
Certain clustering techniques based on Bayesian algorithms (similar to spam filter in gmail) can help reduce the false positive errors. So if the search term is "football", these techniques can categorize the document/data universe into say "American football", "corporate football" etc. Depending on the occurrences of words in a document, it can fall into one of the categories or more. These techniques are being extensively deployed in the e-discovery domain.
Improving the performance of full text searching
The deficiencies of free text searching have been addressed in two ways: By providing users with tools that enable them to express their search questions more precisely, and by developing new search algorithms that improve retrieval precision.
Improved querying tools
- Keywords. Document creators (or trained indexers) are asked to supply a list of words that describe the subject of the text, including synonyms of words that describe this subject. Keywords improve recall, particularly if the keyword list includes a search word that is not in the document text.
- Field-restricted search. Some search engines enable users to limit free text searches to a particular field within a stored data record, such as "Title" or "Author."
- Boolean queries. Searches that use Boolean operators (for example, "encyclopedia" AND "online" NOT "Encarta") can dramatically increase the precision of a free text search. The AND operator says, in effect, "Do not retrieve any document unless it contains both of these terms." The NOT operator says, in effect, "Do not retrieve any document that contains this word." If the retrieval list retrieves too few documents, the OR operator can be used to increase recall; consider, for example, "encyclopedia" AND "online" OR "Internet" NOT "Encarta". This search will retrieve documents about online encyclopedias that use the term "Internet" instead of "online." This increase in precision is very commonly counter-productive since it usually comes with a dramatic loss of recall.
- Phrase search. A phrase search matches only those documents that contain a specified phrase, such as "Wikipedia, the free encyclopedia."
- Concordance search. A concordance search produces an alphabetical list of all principal words that occur in a text with their immediate context.
- Proximity search. A phrase search matches only those documents that contain two or more words that are separated by a specified number of words; a search for "Wikipedia" WITHIN2 "free" would retrieve only those documents in which the words "Wikipedia" and "free" occur within two words of each other.
- Regular expression. A regular expression employs a complex but powerful querying syntax that can be used to specify retrieval conditions with precision.
- Wildcard search. A search that substitutes one or more characters in a search query for a wildcard character such as an asterisk. For example, in the search function in Microsoft Word, using the asterisk in the search query "s*n" will find "sin", "son", "sun", etc. in a text.
Improved search algorithms
Technological advances have greatly improved the performance of free text searching. For example, Google's PageRank algorithm gives more prominence to documents to which other Web pages have linked. This algorithm dramatically improves users' perception of search precision, a fact that explains its popularity among Internet users. See search engine for additional examples.
Text retrieval software
The following is a partial list of available software products whose predominant purpose is to perform full text indexing and searching. Some of these are accompanied with detailed descriptions of their theory of operation or internal algorithms, which can provide additional insight into how full text search may be accomplished.