/lib/systemd/system/postgresql.service

# systemd service for managing all PostgreSQL clusters on the system. This
# service is actually a systemd target, but we are using a service since
# targets cannot be reloaded.

[Unit]
Description=PostgreSQL RDBMS 

[Service]
Type=oneshot
ExecStart=/bin/true
ExecReload=/bin/true
RemainAfterExit=on

[Install]
WantedBy=multi-user.target

systemd, .service, /lib/systemd/system/, /etc/systemd/system/, Type=, systemctl list-units --type=service, systemctl daemon-reload, ExecStart, .timer, .mounts, .wants, .socket