I've recently had the situation where my personal development box (OSX Mojave, Rails, Postgres) has become unstable with, at times, multiple crashes per day. I logged in this morning and found myself getting errors like this:

PG::ConnectionBad (could not connect to server: No such file or directory)

I did some digging and even though Postgres appeared to be running to brew, it was a false positive due to a leftover postmaster.pd file. Here was the fix:

  1. First make sure that postgres isn't running use ps i.e. **ps auwwx grep post**.
  2. Delete the file: /usr/local/var/postgres/ i.e. rm /usr/local/var/postgres/
  3. At this point brew will, foolishly and oddly, still think Postgres is running so do do a brew restart postgres

Here's a handy Stack Overflow reference.

Note: Make absolutely sure that postgres is NOT running before you delete or you risk serious database corruption / data loss.