Function get_relation_keys

src/backend/utils/cache/lsyscache.c:4141–4200

* get_relation_keys * Return a list of relation keys */

 *	  Return a list of relation keys
 */
List *
get_relation_keys(Oid relid)
{
	List *keys = NIL;

	// lookup unique constraints for relation from the catalog table
	ScanKeyData skey[1];

	Relation rel = table_open(ConstraintRelationId, AccessShareLock);
	SysScanDesc scan;
	HeapTuple	htup;

	ScanKeyInit(&skey[0], Anum_pg_constraint_conrelid, BTEqualStrategyNumber, F_OIDEQ, relid);
	scan = systable_beginscan(rel, ConstraintRelidTypidNameIndexId, true,
							  NULL, 1, skey);

	while (HeapTupleIsValid(htup = systable_getnext(scan)))
	{
		Form_pg_constraint contuple = (Form_pg_constraint) GETSTRUCT(htup);

		// skip non-unique constraints
		if (CONSTRAINT_UNIQUE != contuple->contype &&
			CONSTRAINT_PRIMARY != contuple->contype)
		{
			continue;
		}

		// skip the constraint if deferrable
		if (contuple->condeferrable)
		{
			continue;
		}

		// store key set in an array
		List *key = NIL;
		
		bool null = false;
		Datum dat = 
				heap_getattr(htup, Anum_pg_constraint_conkey, RelationGetDescr(rel), &null);
		
		Datum *dats = NULL;
		int numKeys = 0;

		// extract key elements
		deconstruct_array(DatumGetArrayTypeP(dat), INT2OID, 2, true, 's', &dats, NULL, &numKeys);
			
		for (int i = 0; i < numKeys; i++)
		{
			int16 key_elem =  DatumGetInt16(dats[i]);
			key = lappend_int(key, key_elem);
		}
		
		keys = lappend(keys, key);
	}

	systable_endscan(scan);
	table_close(rel, AccessShareLock);