Manual:$wgDBminWordLen

Details
Minimum string length that the search engine will search on.
 * The Go function will take you to a page that is an exact match for the name.
 * The Search function will appear to search and give no results.


 * MySQL 3.x : Used to discard words that MySQL will not return any results for. Shorter values configure mysql directly.
 * MySQL 4.x and later : Ignore $wgDBminWordLen and configure mySQL directly.

To configure mySQL, see the mySQL fulltext fine-tuning documentation (4.0/4.1, 5.1). The only table with a fulltext index in wikimedia is searchindex, so after changing the mySQL configuration and restarting mySQL, issue the command.