Postgresql available connections

available_conn.sql

-- https://dba.stackexchange.com/a/161761
SELECT
     max_conn,
     used,
     res_for_super,
     max_conn-used-res_for_super res_for_normal
 FROM
     (
     SELECT
         COUNT(*) used
     FROM
         pg_stat_activity) t1,
     (
     SELECT
         setting::INT res_for_super
     FROM
         pg_settings
     WHERE
         NAME = $$superuser_reserved_connections$$) t2,
     (
     SELECT
         setting::INT max_conn
     FROM
         pg_settings
     WHERE
         NAME = $$max_connections$$) t3;