Restricting users to only one session each in PostgreSQL can be essential for managing system resources effectively, enforcing security policies, or complying with licensing agreements. PostgreSQL does not provide a built-in mechanism specifically designed to limit the number of sessions per user directly. However, you can achieve this functionality through a combination of database roles, connection control functions, and possibly an external connection pooling tool. Here are a few methods to restrict users to only one active session each in PostgreSQL:

1. Using a Connection Pooler

A connection pooler like PgBouncer can be configured to limit the number of connections per user. This is an indirect method but effective in managing and limiting user connections at the pooler level.

• Configure PgBouncer: In the PgBouncer configuration file, you can set the max_user_connections parameter to 1 to ensure that each user can only have one active connection at a time.

    [databases]
    mydb = host=localhost port=5432 dbname=mydb
  
    [pgbouncer]
    listen_port = 6432
    listen_addr = 127.0.0.1
    auth_type = md5
    auth_file = /etc/pgbouncer/userlist.txt
    max_user_connections = 1
  

2. Using Session-Level Triggers

You can create a session-level trigger that checks how many active connections a user has whenever a new session is initiated. If the number exceeds the limit, the new session is terminated.

• Function to Count Connections: Create a PL/pgSQL function that counts the current connections for a user and denies access if there is already an active session.

    CREATE OR REPLACE FUNCTION enforce_single_session()
    RETURNS event_trigger AS $$
    DECLARE
      v_count INTEGER;
    BEGIN
      SELECT count(*) INTO v_count FROM pg_stat_activity
      WHERE usename = current_user AND state = 'active';
  
      IF v_count > 1 THEN
        RAISE EXCEPTION 'You are already logged into another session.';
      END IF;
    END;
    $$ LANGUAGE plpgsql;
  

• Event Trigger on Session Start: Note that PostgreSQL does not support triggers directly on session start. As a workaround, you might need to enforce checks in the application layer or use a connection pooler as mentioned earlier.

3. Using External Tools

You can also use external system management tools to monitor and kill excess sessions. Scripts can be scheduled to run at short intervals to check the number of sessions per user and drop any sessions that exceed the limit.

4. Application-Level Enforcement

Adjust your application logic to ensure that each user cannot open more than one session at a time. This approach requires changes on the client side and depends on the application architecture.

Considerations and Recommendations

• Testing: Whatever method you choose, thoroughly test it in a development environment to ensure it doesn't disrupt legitimate user activities.

• User Experience: Be transparent with users about session limits to avoid confusion and frustration due to unexpectedly terminated sessions.

• Security: Always consider the security implications of any script or tool that automatically kills database sessions.

Implementing session limits in PostgreSQL usually requires a combination of configurations and external tools, as the database itself does not directly support this feature. Tailor the solution to the specific needs and infrastructure of your environment.