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MySQL 8.4 Reference Manual  /  ...  /  Full-Text Searches with Query Expansion

14.9.3 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 or IN NATURAL LANGUAGE MODE 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)
| id | title             | body                                     |
|  1 | MySQL Tutorial    | DBMS stands for DataBase ...             |
|  5 | MySQL vs. YourSQL | In the following database comparison ... |
2 rows in set (0.00 sec)

mysql> SELECT * FROM articles
    WHERE MATCH (title,body)
| id | title                 | body                                     |
|  5 | MySQL vs. YourSQL     | In the following database comparison ... |
|  1 | MySQL Tutorial        | DBMS stands for DataBase ...             |
|  3 | Optimizing MySQL      | In this tutorial we show ...             |
|  6 | MySQL Security        | When configured properly, MySQL ...      |
|  2 | How To Use MySQL Well | After you went through a ...             |
|  4 | 1001 MySQL Tricks     | 1. Never run mysqld as root. 2. ...      |
6 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 nonrelevant documents, use it only when a search phrase is short.