PostgreSQL Connection Failure Sometimes, computers have a life of their own. You shut down your laptop and on the next boot, you are unable to connect to a PostgreSQL 12 database. Opening the installed Postgres.app, I notice it displays stale postmaster.pid file error. Postmaster.pid What the heck is the postmaster.pid file? Turns out that the postmaster.pid is a lock file used to prevent two instances of the same PostgreSQL server from running on the same data-directory.