Homebrew installed PostgreSQL could not connect, database files are incompatible with server Here's the error message I'm getting today when I attempt to log in to my localhost PosgreSQL databases using psql:

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 installed PostgreSQL using Homebrew and this morning I've uninstalled and reinstalled using that utility, with no change observable.  The psql client works fine, and I can still use it to connect to databases over the network.  I only connect to my local databases once every month or two, and it worked fine before, so I'm assuming this is due to some automatic update but I don't know which.  Mac OS upgraded itself to High Sierra (10.13) recently.

The same error message popped up on here in 2014 (link) but that was a case where a guy had trouble starting the server.  In my case I believe it's running -- I've used brew services start postgresql and yes, I've restarted a few times today.  However, the file mentioned in the error, /tmp/.s.PGSQL.5432, does not exist on my system.  I think this is a case of some configuration mistake.  Anyone have a clue how to fix?

Update: I tried starting the server manually (as recommended by Evan) and I think I made a breakthrough. Here's the error I get when I try to start it manually:

FATAL: database files are incompatible with server
DETAIL: The data directory was initialized by PostgreSQL version 9.6, which is not compatible with this version 10.1.


Update 2: Ultimately I solved this problem by replacing my data directory and re-initializing it with an empty database using initdb as in the answer marked "correct".  To be fair, all three answers were helpful.  I was unable to update the database from the old version to the new one because I no longer had the binaries for the older version of Postgres.
If you don't have any data, then it seems like a problem with Brew. Did you run brew upgrade, there are two answers on StackOverflow that address this, I've merged both of them together to try the best of both worlds.


Backing up

launchctl unload -w ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist
sudo mv /usr/local/var/postgres /usr/local/var/.postgres_bak;

Making sure you have the latest version

brew update
brew upgrade postgresql

Either, not both reinitialize or try to upgrade your data-dir


Nuke it (start from scratch, leaving the old back up)

initdb /usr/local/var/postgres -E utf8

Try to upgrade it

pg_upgrade -b /usr/local/Cellar/postgresql/$myOldVersion/bin/ -B /usr/local/Cellar/postgresql/$myNewVersion/bin -d /usr/local/var/.postgres_bak -D /usr/local/var/postgres


Restart server.

brew services start postgresql

Delete the backup, if the server restarts and everything works remove the backup

sudo rm -rf /usr/local/var/.postgres_bak