Indexing only a prefix of the string

[Daniel �kerud]

If I have a table:

CREATE TABLE film (

 

           title CHAR(200) NOT NULL,

           INDEX title_index (title(5))

 

);

What is the underlaying reason why it is faster to index only a prefix of the title? 
Is it always faster? even if i have a trillions of title 200 characters long? what 
should i think of when deciding of the prefixes' length? 

thanks!

---
Daniel �kerud

[ Don't underestimate the power of stupid people in large groups]