I'm not sure if the MySQL help is the best place for this question.

I'm looking to add question and answer functionality as a small addon to a website. I would like to make it so it matches existing questions (using keyword rich questions or using keyword tags) when the user types.

I'm not sure how best to achieve this (I assume some kind of jquery/ajax to trigger a script each time a character is typed?) to evoke one of the options below:
- Repeated queries to MySQL db
- Flat file search (probably not scalable enough unless I have a limited number of keywords?)
- Sphinx*

* I have just installed this for other purposes but haven't started using it yet. I assume I would need a separate config file etc. as this is unrelated to other searching on the website?

Has anyone done something like this, and have any advice on an efficient way to do this?

Although they are typing potentially full sentences it's really only going to be looking for keywords so maybe I would need to configure a lot of stopwords if I'm using sphinx on question titles?

I'm using Perl, a CentOS box and MySQL.