By default, PostgreSQL users—also known as roles with the login option—can access the public schema. Additionally, note that the default PostgreSQL authentication policy allows users to access all databases from the localhost using peer authentication on a Linux system. Users can create database objects -tables, views, functions and so on- in the public schema of any database that they can access by default. Finally, the user can alter some settings regarding sessions such as work_mem.
The user cannot access other user objects in the public schema or create databases and schemas. However, the user can sniff data about the database objects by querying the system catalog. Unprivileged users can get information about other users, table structure, table owner, some table statistics, and so on.
The following example shows how the user test_user is able to get information about a table, which is owned by a postgres user; to simulate this situation, let's create...