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The event trigger on thelogin event can be useful for logging user logins, for verifying the connection and assigning roles according to current circumstances, or for session data initialization. It is very important that any event trigger using thelogin event checks whether or not the database is in recovery before performing any writes. Writing to a standby server will make it inaccessible.

The following example demonstrates these options.

-- create test tables and rolesCREATE TABLE user_login_log (  "user" text,  "session_start" timestamp with time zone);CREATE ROLE day_worker;CREATE ROLE night_worker;-- the example trigger functionCREATE OR REPLACE FUNCTION init_session()  RETURNS event_trigger SECURITY DEFINER  LANGUAGE plpgsql AS$$DECLARE  hour integer = EXTRACT('hour' FROM current_time at time zone 'utc');  rec boolean;BEGIN-- 1. Forbid logging in between 2AM and 4AM.IF hour BETWEEN 2 AND 4 THEN  RAISE EXCEPTION 'Login forbidden';END IF;-- The checks below cannot be performed on standby servers so-- ensure the database is not in recovery before we perform any-- operations.SELECT pg_is_in_recovery() INTO rec;IF rec THEN  RETURN;END IF;-- 2. Assign some roles. At daytime, grant the day_worker role, else the-- night_worker role.IF hour BETWEEN 8 AND 20 THEN  EXECUTE 'REVOKE night_worker FROM ' || quote_ident(session_user);  EXECUTE 'GRANT day_worker TO ' || quote_ident(session_user);ELSE  EXECUTE 'REVOKE day_worker FROM ' || quote_ident(session_user);  EXECUTE 'GRANT night_worker TO ' || quote_ident(session_user);END IF;-- 3. Initialize user session dataCREATE TEMP TABLE session_storage (x float, y integer);ALTER TABLE session_storage OWNER TO session_user;-- 4. Log the connection timeINSERT INTO public.user_login_log VALUES (session_user, current_timestamp);END;$$;-- trigger definitionCREATE EVENT TRIGGER init_session  ON login  EXECUTE FUNCTION init_session();ALTER EVENT TRIGGER init_session ENABLE ALWAYS;