PostgreSQL – PSQL Connecting to Wrong Database

Since you want the backup user to connect locally from the database server. I assume the backup is running on a dedicated backup account on the OS.

Using pg_hba.conf you can specify like:

# TYPE  DATABASE USER   ADDRESS METHOD
local   all      backup_user    peer map=local_peers_backup_user

(method peer (with local/peer an address is not needed))

combine this with a pg_ident.conf like:

# MAPNAME                SYSTEM-USERNAME   PG-USERNAME
local_peers_backup_user  postgres          backup_user
local_peers_backup_user  OSBKPUSER         backup_user

This allows the os user postgres and OSBKUSER to connect to all databases in the cluster as the postgres database user named backup_user, using peer authentication. This works using sockets, on linux.

connected as your OSBKPUSER running psql suffices to get connected as backup_user, assuming that you already prepared the environment variables for the OSBKPUSER and OSBKPUSER has only 1 map. Otherwise run psql -U backup_user.

This connect method is secure, if you trust that the access to your dbserver is secured.

The error is harmless but to get rid of it, I think you need to break this restore into two commands, as in:

dropdb -U postgres mydb && \
 pg_restore --create --dbname=postgres --username=postgres pg_backup.dump

The --clean option in pg_restore doesn't look like much but actually raises non-trivial problems.

For versions up to 9.1

The combination of --create and --clean in pg_restore options used to be an error in older PG versions (up to 9.1). There is indeed some contradiction between (quoting the 9.1 manpage):

--clean Clean (drop) database objects before recreating them

and

--create Create the database before restoring into it.

Because what's the point of cleaning inside a brand-new database?

Starting from version 9.2

The combination is now accepted and the doc says this (quoting the 9.3 manpage):

--clean Clean (drop) database objects before recreating them. (This might generate some harmless error messages, if any objects were not present in the destination database.)

--create Create the database before restoring into it. If --clean is also specified, drop and recreate the target database before connecting to it.

Now having both together leads to this kind of sequence during your restore:

DROP DATABASE mydb;
...
CREATE DATABASE mydb WITH TEMPLATE = template0... [other options]
...
CREATE SCHEMA public;
...
CREATE TABLE...

There is no DROP for each individual object, only a DROP DATABASE at the beginning. If not using --create this would be the opposite.

Anyway this sequence raises the error of public schema already existing because creating mydb from template0 has imported it already (which is normal, it's the point of a template database).

I'm not sure why this case is not handled automatically by pg_restore. Maybe this would cause undesirable side-effects when an admin decides to customize template0 and/or change the purpose of public, even if we're not supposed to do that.