If you are using PostgreSQL 8.3 or earlier versions, and you are using the database's table inheritance feature to partition your data, you'll need to turn this parameter on, or on partition. The reasons for that are covered in Chapter 15, Partitioning Data. Starting in 8.4, constraint_exclusion defaults to a new smarter setting named partition that will do the right thing in most situations without it ever needing to be adjusted.
On PostgreSQL 10.0, allowed values of constraint_exclusion are on (examine constraints for all tables), off (never examine constraints), and partition (examine constraints only for inheritance child tables and UNION ALL subqueries). partition is the default setting. It is often used with inheritance ...
