Function RelationInvalidatesSnapshotsOnly

src/backend/utils/cache/syscache.c:1712–1730

* Certain relations that do not have system caches send snapshot invalidation * messages in lieu of catcache messages. This is for the benefit of * GetCatalogSnapshot(), which can then reuse its existing MVCC snapshot * for scanning one of those catalogs, rather than taking a new one, if no * invalidation has been received. * * Relations that have syscaches need not (and must not) be listed

 * syscache for one of these relations, remove it from this list.
 */
bool
RelationInvalidatesSnapshotsOnly(Oid relid)
{
	switch (relid)
	{
		case DbRoleSettingRelationId:
		case DependRelationId:
		case SharedDependRelationId:
		case DescriptionRelationId:
		case SharedDescriptionRelationId:
		case SecLabelRelationId:
		case SharedSecLabelRelationId:
			return true;
		default:
			break;
	}

	return false;
}

/*
 * Test whether a relation has a system cache.