title: PostgreSQL: when we should set constraint_exclusion off?
body: <p>From the official <a href="https://www.postgresql.org/docs/9.1/static/ddl-partitioning.html" rel="nofollow noreferrer">document</a>, it seems "off" will spend a lot of time to check the partitioned and non-partitioned table to execute the plan. In such case, why we have off mode?</p>
  
  <p>Thanks</p>