Log only and all joins that don't use indexes

[Jonathan Robson]

On the 'Runtime Information' page in phpMyAdmin, there is a variable called 'Select_full_join' that has a description of "The number of joins that do not use indexes. If this value is not 0, you should carefully check the indexes of your tables."

Is there a way I can log only and all queries that increment this variable (the "joins that do not use indexes")?

I know how to log all queries, but I can't figure out how to log only these queries. There are dozens of websites on this server and MySQL is receiving over 200 queries per second and about 1% of them are "joins that do not use indexes." So it's very hard to track down in a log with all queries.

I'm assuming I can do this by adding some line(s) to the my.cnf file. I would prefer not to add custom options to the mysqld command used to start the server.

I'm running 5.0.81-community.