Dennes Torres talks about whether a constraint is trustworthy:
If the check constraint is trustable, it can be used by the query optimizer. For example, if the check constraint avoid values below 100 in a field and a query for 50 arrives, the query optimizer uses the check constraint to stop the query.
The query optimizer can only use the check constraint if it’s trustable, otherwise it could exist in the table records with values below 100, according to our example, and the query would loose these records.
Dennes then goes on to show how you can have non-trustworthy constraints and how to fix the issue.