scaleability of failing transactions mysql

Question

I have a table that stores messages from one user to another. messages(user_id,friend_id,message,created_date). My primary key is (friend_id,created_date). This prevents duplicate messages (AFAIK) because they will fail to insert.

Right now this is ok because my code generates about 20 of these queries at a time per user and I only have one user. But if there were hundreds or thousands of users would this create a bottleneck in my database with all the failed transactions? And if what kinds of things could I do to improve the situation?

EDIT: The boiled down question is should I use the primary key constraint,check outside of mysql, or use some other mysql functionality to keep duplicates out of the database?

Answer 1

Should be fine as mysql will just do a primary key lookup internally and ignore the record (I'm assuming you're using INSERT IGNORE). If you were checking if they exist before inserting, mysql will still check again when you insert. This means if most inserts are going to succeed, then you're saving an extra check. If the vast majority of inserts were failing (not likely) then possibly the savings from not sending unnecessary data would outweigh the occasional repeated check.