The minimum and maximum length of words to be indexed is defined by the ft_min_word_len and ft_max_word_len system variables (available as of MySQL 4.0.0). The default minimum value is four characters. The default maximum depends on your version of MySQL. If you change either value, you must rebuild your FULLTEXT indexes. For example, if you want three-character words to be searchable, you can set the ft_min_word_len variable by putting the following lines in an option file:
[mysqld]
ft_min_word_len=3
To ensure that myisamchk and the server use the same values for full-text parameters, place each one in both the [mysqld] and [myisamchk] sections of an option file:
[mysqld]
ft_min_word_len=3
[myisamchk]
ft_min_word_len=3
Then restart the server and rebuild your FULLTEXT indexes. To rebuild your indexes, you need to run the follow queries in sequence:
Emptying your search tables will speed up this process. The queries to empty the tables are:
truncate searchcore;
truncate searchcore_text;
truncate searchgroup;
truncate searchgroup_text;
Next you need to drop the old indexes off the tables. To drop the indexes you would do:
drop index text on searchcore_text;
drop index grouptitle on searchgroup_text;
Finally, you need to build new indexes. To recreate the indexes you would do:
CREATE FULLTEXT INDEX text ON searchcore_text (title, keywordtext);
CREATE FULLTEXT INDEX grouptitle ON searchgroup_text (title);
If you have a table prefix defined in your config.php you would need to add it to the beginning of each table name in every query. To rebuild your search indexes after this, you will need to go to the Update Counters section of Maintenance in your Admin CP. Rebuilding the search indexes can be a time and processor intensive process.
For more on Fulltext Search from MySQL please visit:
https://dev.mysql.com/doc/refman/5.0/en/fulltext-fine-tuning.html
You can also empty these indices in the Update Counters section of Maintenance.
You may want to optimize the postindex and word tables afterwards by going to the Repair / Optimize Tables section of Maintenance.