psql: FATAL: role "postgres" does not exist (with -h localhost option)

Question

I have Postgres installed in my local machine.

When I execute

psql -U postgres -d buzzsumo

it correctly asks for my password to the user postgres.

However, when I run

psql -U postgres -d buzzsumo -h localhost

it gives me this error message:

psql: FATAL: role "postgres" does not exist

Why is this the case, when in the first example I am connecting to my local server, which is equivalent to passing in localhost?

Answer 1

You don't say but I'm guessing you're running postgres on either on MacOS or Linux.

The first form (with no -h) connects via a "unix socket". The second form, with "-h localhost" connects via TCP/IP to localhost (IP address 127.0.0.1).

Postgres treats these two types of connections differently - or at least, it can do so. If you check the "pgsql/data/pg_hba.conf" file to see what authentication mechanisms are configured for the two types of connections.

Answer 2

Your first example connects via Unix socket, the second one via TCP/IP local loop (localhost).

For the first example, you need the connection type local in pg_hba.conf, for the second lines starting with host are relevant. You probably have defined different authentication methods for these classes of connections.