22 MySQL internals

This chapter will describe a lot of things that you need to know when working on the MySQL code.

22.1 MySQL threads

The MySQL server creates the the following threads:

• The TCP/IP connection thread handles all connect requests and creates a new dedicated thread to handle the authentication and and SQL query processing for the connection.
• On NT there is 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 has been idle too long.
• If compiled with -DUSE_ALARM_THREAD, a dedicated thread that handles alarms is created; This is only used one some system where there is some problems with sigwait() or if one wants to use the thr_alarm() code in ones one application without a dedicates signal handling thread.
• If one uses the --flush-time option, a dedicated thread is created to flush all tables at the given interval.
• Every connection has it's own thread.
• Every different table on which one uses INSERT DELAYED gets its own thread.

mysqladmin processlist only shows the connection and INSERT DELAYED threads.

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