There are several ways to shut down the database server. You control the type of shutdown by sending different signals to the server process.
After receiving SIGTERM, the postmaster disallows new connections, but lets existing backends end their work normally. It shuts down only after all of the backends terminate normally. This is Smart Shutdown.
The postmaster disallows new connections and sends all existing backends SIGTERM, which will cause them to abort their current transactions and exit promptly. It then waits for the backends to exit and finally shuts down. This is Fast Shutdown.
This is Immediate Shutdown, which will cause the postmaster to send a SIGQUIT to all backends and exit immediately (without properly shutting itself down). The backends 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 postmaster. This will prevent the postmaster from releasing shared memory and semaphores, which may then have to be done by manually.
$ kill -INT `head -1 /usr/local/pgsql/data/postmaster.pid`
The program pg_ctl is a shell script that provides a more convenient interface for shutting down the postmaster.
You can also shut down individual postgres processes with SIGINT and SIGTERM without killing the whole database.
Sending SIGINT will break the current transaction and close the connection, if a transaction is taking place. If a transaction is not taking place, nothing happens.
Sending SIGTERM will break the current transaction, if any, and close the connection.
I couldn\'t find these documented anywhere else. I learned about SIGINT from a newsgroup posting and SIGTERM was just a lucky guess.