There are several ways to shut down the database server. Under the hood, they all reduce to sending a signal to the supervisor postgres process.
If you are using a pre-packaged version of PostgreSQL, and you used its provisions for starting the server, then you should also use its provisions for stopping the server. Consult the package-level documentation for details.
When managing the server directly, you can control the type of shutdown by sending different signals to the postgres process:
This is the Smart Shutdown mode. After receiving SIGTERM, the server disallows new connections, but lets existing sessions end their work normally. It shuts down only after all of the sessions terminate. If the server is in recovery when a smart shutdown is requested, recovery and streaming replication will be stopped only after all regular sessions have terminated.
This is the Fast Shutdown mode. The server disallows new connections and sends all existing server processes SIGTERM, which will cause them to abort their current transactions and exit promptly. It then waits for all server processes to exit and finally shuts down.
This is the Immediate Shutdown mode. The server will send SIGQUIT to all child processes and wait for them to terminate. If any do not terminate within 5 seconds, they will be sent SIGKILL. The supervisor server process exits as soon as all child processes have exited, without doing normal database shutdown processing. This will lead to recovery (by replaying the WAL log) upon next start-up. This is recommended only in emergencies.
The pg_ctl program provides a convenient interface for sending these signals to shut down the server. Alternatively, you can send the signal directly using kill on non-Windows systems. The PID of the postgres process can be found using the ps program, or from the file postmaster.pid in the data directory. For example, to do a fast shutdown:
$ kill -INT `head -1 /usr/local/pgsql/data/postmaster.pid`
It is best not to use SIGKILL to shut down the server. Doing so will prevent the server from releasing shared memory and semaphores. Furthermore, SIGKILL kills the postgres process without letting it relay the signal to its subprocesses, so it might be necessary to kill the individual subprocesses by hand as well.
To terminate an individual session while allowing other sessions to continue, use pg_terminate_backend() (see Table 9.88) or send a SIGTERM signal to the child process associated with the session.
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