MYSQL fulltext - What's the right way to get the relevance from one column? I want to make one column more valuable than others like it's been done in query bellow.This example  shows that only one fulltext index is needed And this answersuggests that three fulltext indexes are needed to do the same. 

Here is the queries from the first link:

ALTER TABLE products ADD FULLTEXT(title, sdescription, ldescription)

SELECT *,
MATCH(`title`) AGAINST ('+iphone +case +4s' IN BOOLEAN MODE) * 10 as rel1,
MATCH(`sdescription`) AGAINST ('+iphone +case +4s' IN BOOLEAN MODE) * 3 as rel2,
MATCH(`ldescription`) AGAINST ('+iphone +case +4s' IN BOOLEAN MODE) as rel3,
FROM products
WHERE MATCH (title, sdescription, ldescription) 
  AGAINST ('+iphone +case +4s' IN BOOLEAN MODE)
ORDER BY (rel1)+(rel2)+(rel3) DESC;


I'm confused how many indexes are needed for that, one or three?
I asked this question on stackoverflow but no answer yet.
MyISAM? or InnoDB?

4 FULLTEXT indexes will work in either engine.

You could find out by creating an empty table and running that SELECT against it.  It should give you an error if the set of indexes you tried is wrong.  The ALTER to add/drop indexes.