Map progress with stars per lesson. How to store/request stats sql

Question

The massive SQL request for the map is for loading the progress, not for the image. I store for every lesson result (1-3 stars). Lets say the player completed all (10) lessons of a map. To show the map with its stars, I would have to request point the highest score for each map. Not sure now if this can be done in only one SQL call. Have to check this.
Another way of doing it is storing all needed data (number of stars for each map point) in a JSON encoded string to the database. So it’s just one request. But useless for statistics, so it’s still necessary to write each lesson to “results”.
Shouldn't I just keep those stats locally so I don't have to request them from the server every time?

Answer 1

Well, been reading and mysql can handle billions of rows. So I think I read that there is some slow down in the chain
AIR - browser(php) - mysql - broswer - AIR
I know mysql processes damn fast so somewhere in that chain things are getting slowed down. I read somewhere that stored procedures in the mysql db help as they are already compiled and you can also execute various commands with one sql call. Also, helps against sql injection.
Is that right??? and where might the "slow down" come from?