From the docs:

Your host server name, for example localhost, hostname, hostname.example.com, or the IP address. To specify a port use hostname:####; to specify a Unix socket use localhost:/path/to/socket.

So is /tmp the same as localhost:/tmp? Maybe you could even add all of the above to the description.

The docs of version 14 (here) don't state localhost:/path/to/socket but just /path/to/socket :) Also, I had to lsof $(pidof -s postgresql) to figure out the /tmp, so it was on this point I was trying to insist.

As for the other possibilities, they're all standard across all host options of all services, aren't they?

I see. But how can postgresql have its socket on /tmp itself?

It's actually /tmp/.s.PGSQL.5432, and the .s.PGSQL.5432 is automatically added by Nextcloud. Actually, I wanted to try to investigate whether that's not a security issue, as distributions usually place it in /run/postgresql/.s.PGSQL.5432, but didn't find the time to do it yet.

(BTW, if setting it to /tmp/.s.PGSQL.5432, as I did initially, then it tried to open /tmp/.s.PGSQL.5432/.s.PGSQL.5432 and failed with it)

Do you think I could word this more clearly? I was thinking the minimal explanation of how to do it with postgresql would avoid cluttering the option documentation, but maybe it deserves a sentence to explain why /tmp?

I think the docs are precise what the value should be.