Search Type 
vBulletin supports two types of search indexing. Fulltext searching uses a search index that is constructed by MySQL itself, whereas vBulletin's own search feature uses its own index.

You set the search type here:

Admin CP -> vBulletin Options -> Search Type

By default, vBulletin will use its internal indexing feature. The results of this indexing process is stored in two tables, word and postindex. This provides a fast search mechanism but can cause problems on larger forums due to the ever increasing size of these tables. Each unique word is indexed in the word table and each occurrence of the word is indexed in the postindex table. To get around the large amount of space these tables can occupy we implemented MySQL Fulltext Search. The search type screen allows you to switch between the two of these. It is a simple toggle so submitting the screen switches between the two modes.

When switching a forum to the fulltext search mode, you will want to consider emptying the indices that the default search engine built. These indices are not used by the fulltext search and consume a large portion of your database. You should be certain that you are going to permanently use the fulltext search before removing these indices since, generally, it takes a lot of time and server load to rebuild these indices. Another consideration is during any time that the fulltext option is enabled, these indices will not be updated by any new posts. Using fulltext search for an extended period of time will leave these indices stale and you may still wish to rebuild them.
Note:
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

Then restart the server and rebuild your FULLTEXT indexes. Also note particularly the remarks regarding myisamchk in the instructions following this list.

For more on Fulltext Search from MySQL please visit:
http://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.
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