The TCP/IP connection thread handles all connection requests and creates a new dedicated thread to handle the authentication and SQL query processing for each connection.

On Windows NT there is a named pipe handler thread that does the same work as the TCP/IP connection thread on named pipe connect requests.

The signal thread handles all signals. This thread also normally handles alarms and calls process_alarm() to force timeouts on connections that have been idle too long.

If mysqld is compiled with -DUSE_ALARM_THREAD, a dedicated thread that handles alarms is created. This is only used on some systems where there are problems with sigwait() or if you want to use the thr_alarm() code in your application without a dedicated signal handling thread.

If one uses the --flush_time=val option, a dedicated thread is created to flush all tables at the given interval.

Every connection has its own thread.

Each table for which INSERT DELAYED statements are issued gets its own thread.

If you use --master-host, a slave replication thread is started to read and apply updates from the master.

If the event scheduler is active, there is one thread for the scheduler, and a thread for any event currently running.