pg_constraint

The pg_constraint system catalog table stores check and foreign key 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 table. Check constraints on domains are stored here, too.

Table 1. pg_catalog.pg_constraint

column	type	references	description
conname	name		Constraint name
connamespace	oid	pg_namespace.oid	The OID of the namespace (schema) that contains this constraint.
contype	char		c = check constraint, f = foreign key constraint.
condeferrable	boolean		Is the constraint deferrable?
condeferred	boolean		Is the constraint deferred by default?
conrelid	oid	pg_class_internal.oid	The table this constraint is on; 0 if not a table constraint.
conrootrelid	oid		Root OID of the table this constraint is on;
contypid	oid	pg_type.oid	The domain this constraint is on; 0 if not a domain constraint.
confrelid	oid	pg_class_internal.oid	If a foreign key, the referenced table; else 0.
confupdtype	char		Foreign key update action code.
confdeltype	char		Foreign key deletion action code.
confmatchtype	char		Foreign key match type.
conkey	smallint[]	pg_attribute.attnum	If a table constraint, list of columns which the constraint constrains.
confkey	smallint[]	pg_attribute.attnum	If a foreign key, list of the referenced columns.
conbin	text		If a check constraint, an internal representation of the expression.
consrc	text		If a check constraint, a human-readable representation of the expression. This is not updated when referenced objects change; for example, it won’t track renaming of columns. Rather than relying on this field, it is best to use pg_get_constraintdef() to extract the definition of a check constraint.