There are several ways to shut down the database server. You control the type of shutdown by sending different signals to the postmaster process.
SIGTERM
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 normally. This is the Smart Shutdown.
SIGINT
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 the server processes to exit and finally shuts down. This is the Fast Shutdown.
SIGQUIT
This is the Immediate Shutdown, which will cause the postmaster process to send a SIGQUIT to all child processes and exit immediately (without properly shutting itself down). The child processes likewise exit immediately upon receiving SIGQUIT. This will lead to recovery (by replaying the WAL log) upon next start-up. This is recommended only in emergencies.
Important: It is best not to use SIGKILL to shut down the server. This will prevent the server from releasing shared memory and semaphores, which may then have to be done by manually.
The PID of the postmaster process can be found using the ps program, or from the file postmaster.pid in the data directory. So for example, to do a fast shutdown:
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