pg_restore: [archiver (db)] could not execute query: ERROR: schema "public" already exists

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.

In my case, the reason was that I was using pg_restore from postgresql-contrib version 11.2 to restore a dump made by pg_dump 9.6 to a PostgreSQL cluster 9.6.

After I downgraded my pg_restore back to 9.6, this schema "public" already exists error was gone, and the restoring process worked as before.

The restore includes the public schema and so does create database. So remove the schema after creating the database so that the restore can create it without error.

Note: Following assumes -h -U -p are default and PGPASSWORD or .pgpass is set

    psql << XENDX
    drop database if exists DB_NAME;
    create database DB_NAME;
    /c DB_NAME;
    drop schema if exists public;
    create schema public;
    XENDX

    pg_restore -d DB_NAME FILE_CREATED_WITH_pg_dump