PostgreSQL is running locally but I cannot connect. Why?

Recently updated my machine from Mac OS X Lion (10.7.4) to Mountain Lion (10.8) and I think it borked my PostgreSQL installation. It was installed originally via Homebrew. I'm not a DBA, but hoping someone can tell me how to troubleshoot this.

I am unable to connect (but was able to before pre-Mountain Lion):

$ psql -U rails -d myapp_development
psql: could not connect to server: No such file or directory
    Is the server running locally and accepting
    connections on Unix domain socket "/var/pgsql_socket/.s.PGSQL.5432"?

But Postgres is still clearly running:

$ ps aux | grep postgres
meltemi          2010   0.0  0.0  2444124   5292   ??  Ss   Wed01PM   0:00.02 postgres: rails myapp_development [local] idle    
meltemi           562   0.0  0.0  2439312    592   ??  Ss   Wed12PM   0:02.28 postgres: stats collector process       
meltemi           561   0.0  0.0  2443228   1832   ??  Ss   Wed12PM   0:01.57 postgres: autovacuum launcher process       
meltemi           560   0.0  0.0  2443096    596   ??  Ss   Wed12PM   0:02.89 postgres: wal writer process       
meltemi           559   0.0  0.0  2443096   1072   ??  Ss   Wed12PM   0:04.01 postgres: writer process       
meltemi           466   0.0  0.0  2443096   3728   ??  S    Wed12PM   0:00.85 /usr/local/bin/postgres -D /usr/local/varpostgres -r /usr/local/var/postgres/server.log

And it's responding to queries (both to a test db and the development db) from a local Rails app

User Load (0.2ms)  SELECT "users".* FROM "users" 
  Rendered users/index.html.haml within layouts/application (1.3ms)

There appears to be no /var/pgsql_socket/ directory, let alone the /var/pgsql_socket/.s.PGSQL.5432 socket file mentioned above!?! Maybe the install of Mountain Lion wiped that out?

$ ls -l /var/ | grep pg
drwxr-x---   2 _postgres  _postgres    68 Jun 20 16:39 pgsql_socket_alt

How can I troubleshoot this?

I found that I had an extremely similar problem, namely that postgres was opening a socket in /var/pgsql_socket_alt where none of my software expects to look, but the solution to my problem was not only a problem with my $PATH.

I had to create the directory /var/pgsql_socket, chown it to myself, and set unix_socket_directory in postgresql.conf (located in /usr/local/var/postgres) to that directory, then use the pg_ctl binary in /usr/local/bin to start the right postgres server successfully (which is where $PATH comes in – make sure which pg_ctl resolves to /usr/local/bin/pg_ctl, or just always call it explicitly).

This might help other users who find this question via the /var/pgsql_socket_alt mention.

StackExchange Database Administrators Q#21587, answer score: 31