How can I get the current post's index by slug?

What determines the order of the posts? Is it the ID, or the date? Why not create a function where you pass in the ID or the date of the current post, to get the next one. The SQL would be something like:

SELECT * FROM `posts` WHERE `ID` > ? ORDER BY `ID` ASC LIMIT 1

This will get you the next post in the list by ID. If your posts are ordered by date rather than ID, it would be:

SELECT * FROM `posts` WHERE `date` > ? ORDER BY `date` ASC LIMIT 1