How to diagnose a hanging MySQL UPDATE?
Just set slow_query_log = 1 (if >=5.6.1) or 
log_slow_queries = 1(if &lt;5.6.1) and 
slow_query_log_file = /path/to/logfile and long_query_time = 0 in my.cnf, then restart mysql and have a look at the logfile. I won't keep it on on production envs, such a logging is a performance killer and may result in bad things if you are on heavy load. It may also get quickly very big as long_query_time 0 results in logging everything, so "/path/to" should have plenty of free space.