Index on difference of two columns

[Marty -]

Hello,

I have a table where is stored position of a word in a text and i would like to create an index on difference between positions of every two words in the same text.

This index would increase speed of my queries, because I have a condition in my sql statement like this: WHERE tab1.position - tab2.position < 15 (generally a difference; tab1 and tab2 are actually the same table that is described below)

table looks like this:

id int auto_increment,
word varchar(30),
position smallint,
id_article smallint,
primary key(id)

Thank you for any reply