constraint_exclusion
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. 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.