pg_constraint
   The catalog pg_constraint stores check, primary
   key, unique, foreign key, and exclusion constraints on tables.
   (Column constraints are not treated specially.  Every column constraint is
   equivalent to some table constraint.)
   Not-null constraints are represented in the pg_attribute
   catalog, not here.
  
   User-defined constraint triggers (created with CREATE CONSTRAINT
   TRIGGER) also give rise to an entry in this table.
  
Check constraints on domains are stored here, too.
Table 51.13. pg_constraint Columns
| Column Type Description | 
|---|
| 
        Row identifier | 
| 
        Constraint name (not necessarily unique!) | 
| 
        The OID of the namespace that contains this constraint | 
| 
        
        | 
| 
        Is the constraint deferrable? | 
| 
        Is the constraint deferred by default? | 
| 
        Has the constraint been validated? Currently, can only be false for foreign keys and CHECK constraints | 
| 
        The table this constraint is on; 0 if not a table constraint | 
| 
        The domain this constraint is on; 0 if not a domain constraint | 
| 
        The index supporting this constraint, if it's a unique, primary key, foreign key, or exclusion constraint; else 0 | 
| 
        The corresponding constraint in the parent partitioned table, if this is a constraint in a partition; else 0 | 
| 
        If a foreign key, the referenced table; else 0 | 
| 
        
       Foreign key update action code:
        | 
| 
        
       Foreign key deletion action code:
        | 
| 
        
       Foreign key match type:
        | 
| 
        This constraint is defined locally for the relation. Note that a constraint can be locally defined and inherited simultaneously. | 
| 
        The number of direct inheritance ancestors this constraint has. A constraint with a nonzero number of ancestors cannot be dropped nor renamed. | 
| 
        This constraint is defined locally for the relation. It is a non-inheritable constraint. | 
| 
        If a table constraint (including foreign keys, but not constraint triggers), list of the constrained columns | 
| 
        If a foreign key, list of the referenced columns | 
| 
        If a foreign key, list of the equality operators for PK = FK comparisons | 
| 
        If a foreign key, list of the equality operators for PK = PK comparisons | 
| 
        If a foreign key, list of the equality operators for FK = FK comparisons | 
| 
        If an exclusion constraint, list of the per-column exclusion operators | 
| 
        
       If a check constraint, an internal representation of the
       expression.  (It's recommended to use
        | 
   In the case of an exclusion constraint, conkey
   is only useful for constraint elements that are simple column references.
   For other cases, a zero appears in conkey
   and the associated index must be consulted to discover the expression
   that is constrained.  (conkey thus has the
   same contents as pg_index.indkey for the
   index.)
  
    pg_class.relchecks needs to agree with the
    number of check-constraint entries found in this table for each
    relation.