As I understand it, it's a "feature" of MySQL. As of Version 3.23.23, MySQL has support for full-text indexing and searching. FULLTEXT indexes are most often used to search natural language text, such as through newspaper articles, web page contents and so on. For this reason MySQL has added a number of features to assist this kind of searching. MySQL does not index any words less than or equal to 3 characters in length, nor does it index any words that appear in more than 50% of the rows.