I wanted to clean out my local PostgreSQL database today so that I could restore a database dump taken on another system, but every time I ran the psql utility, I got:

psql: could not connect to server: No such file or directory
    Is the server running locally and accepting
    connections on Unix domain socket "/tmp/.s.PGSQL.5432"?

I tried various things, including restarting Postgres several times, but nothing helped. Eventually, I thought to look in /usr/local/var/postgres/server.log, where I saw several error messages indicating that Postgres 9.5 couldn’t read data files created with 9.4. At that point, I realized that during my most recent brew update; brew upgrade, Homebrew had upgraded my in­stal­la­tion of Postgres from 9.4 to 9.5. A little in­ves­ti­ga­tion led me to this gist. Since I didn’t care about the old database files, I created a fresh /usr/local/var/postgres and ran initdb.