(mysql.info) fulltext-query-expansion

 
 12.7.2 Full-Text Searches with Query Expansion
 ----------------------------------------------
 
 Full-text search supports query expansion (and in particular, its
 variant `blind query expansion'). This is generally useful when a
 search phrase is too short, which often means that the user is relying
 on implied knowledge that the full-text search engine lacks. For
 example, a user searching for `database' may really mean that `MySQL',
 `Oracle', `DB2', and `RDBMS' all are phrases that should match
 `databases' and should be returned, too. This is implied knowledge.
 
 Blind query expansion (also known as automatic relevance feedback) is
 enabled by adding `WITH QUERY EXPANSION' following the search phrase.
 It works by performing the search twice, where the search phrase for the
 second search is the original search phrase concatenated with the few
 most highly relevant documents from the first search.  Thus, if one of
 these documents contains the word `databases' and the word `MySQL', the
 second search finds the documents that contain the word `MySQL' even if
 they do not contain the word `database'. The following example shows
 this difference:
 
      mysql> SELECT * FROM articles
          -> WHERE MATCH (title,body) AGAINST ('database');
      +----+-------------------+------------------------------------------+
      | id | title             | body                                     |
      +----+-------------------+------------------------------------------+
      |  5 | MySQL vs. YourSQL | In the following database comparison ... |
      |  1 | MySQL Tutorial    | DBMS stands for DataBase ...             |
      +----+-------------------+------------------------------------------+
      2 rows in set (0.00 sec)
 
      mysql> SELECT * FROM articles
          -> WHERE MATCH (title,body)
          -> AGAINST ('database' WITH QUERY EXPANSION);
      +----+-------------------+------------------------------------------+
      | id | title             | body                                     |
      +----+-------------------+------------------------------------------+
      |  1 | MySQL Tutorial    | DBMS stands for DataBase ...             |
      |  5 | MySQL vs. YourSQL | In the following database comparison ... |
      |  3 | Optimizing MySQL  | In this tutorial we will show ...        |
      +----+-------------------+------------------------------------------+
      3 rows in set (0.00 sec)
 
 Another example could be searching for books by Georges Simenon about
 Maigret, when a user is not sure how to spell `Maigret'. A search for
 `Megre and the reluctant witnesses' finds only `Maigret and the
 Reluctant Witnesses' without query expansion. A search with query
 expansion finds all books with the word `Maigret' on the second pass.
 
 * Because blind query expansion tends to increase noise
 significantly by returning non-relevant documents, it is meaningful to
 use only when a search phrase is rather short.
Info Catalog
(mysql.info) fulltext-boolean
(mysql.info) fulltext-search
(mysql.info) fulltext-stopwords
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