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 45-12. pg_constraint Columns
|conname||name||Constraint name (not necessarily unique!)|
|connamespace||oid||pg_namespace.oid||The OID of the namespace that contains this constraint|
|contype||char||c = check constraint, f = foreign key constraint, p = primary key constraint, u = unique constraint, t = constraint trigger, x = exclusion constraint|
|condeferrable||bool||Is the constraint deferrable?|
|condeferred||bool||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|
|conindid||oid||pg_class.oid||The index supporting this constraint, if it's a unique, primary key, foreign key, or exclusion constraint; else 0|
|confrelid||oid||pg_class.oid||If a foreign key, the referenced table; else 0|
|confupdtype||char||Foreign key update action code: a = no action, r = restrict, c = cascade, n = set null, d = set default|
|confdeltype||char||Foreign key deletion action code: a = no action, r = restrict, c = cascade, n = set null, d = set default|
|confmatchtype||char||Foreign key match type: f = full, p = partial, u = simple (unspecified)|
|conislocal||bool||This constraint is defined locally for the relation. Note that a constraint can be locally defined and inherited simultaneously.|
|coninhcount||int4||The number of direct inheritance ancestors this constraint has. A constraint with a nonzero number of ancestors cannot be dropped nor renamed.|
|conkey||int2||pg_attribute.attnum||If a table constraint (including foreign keys, but not constraint triggers), list of the constrained columns|
|confkey||int2||pg_attribute.attnum||If a foreign key, list of the referenced columns|
|conpfeqop||oid||pg_operator.oid||If a foreign key, list of the equality operators for PK = FK comparisons|
|conppeqop||oid||pg_operator.oid||If a foreign key, list of the equality operators for PK = PK comparisons|
|conffeqop||oid||pg_operator.oid||If a foreign key, list of the equality operators for FK = FK comparisons|
|conexclop||oid||pg_operator.oid||If an exclusion constraint, list of the per-column exclusion operators|
|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|
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.)
Note: consrc is not updated when referenced objects change; for example, it won't track renaming of columns. Rather than relying on this field, it's best to use
pg_get_constraintdef()to extract the definition of a check constraint.
Note: pg_class.relchecks needs to agree with the number of check-constraint entries found in this table for each relation. Also, pg_class.relhasexclusion must be true if there are any exclusion-constraint entries for the relation.