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.oid The table this constraint is on; 0 if not a table constraint.
contypid oid pg_type.oid The domain this constraint is on; 0 if not a domain constraint.
confrelid oid pg_class.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.