Function ScanPgRelation

src/backend/utils/cache/relcache.c:352–416

* ScanPgRelation * * This is used by RelationBuildDesc to find a pg_class * tuple matching targetRelId. The caller must hold at least * AccessShareLock on the target relid to prevent concurrent-update * scenarios; it isn't guaranteed that all scans used to build the * relcache entry will use the same snapshot. If, for example, * an attribute were to be added after scanning pg_class

 *		and must eventually be freed with heap_freetuple.
 */
static HeapTuple
ScanPgRelation(Oid targetRelId, bool indexOK, bool force_non_historic)
{
	HeapTuple	pg_class_tuple;
	Relation	pg_class_desc;
	SysScanDesc pg_class_scan;
	ScanKeyData key[1];
	Snapshot	snapshot = NULL;

	/*
	 * If something goes wrong during backend startup, we might find ourselves
	 * trying to read pg_class before we've selected a database.  That ain't
	 * gonna work, so bail out with a useful error message.  If this happens,
	 * it probably means a relcache entry that needs to be nailed isn't.
	 */
	if (!OidIsValid(MyDatabaseId))
		elog(FATAL, "cannot read pg_class without having selected a database");

	/*
	 * form a scan key
	 */
	ScanKeyInit(&key[0],
				Anum_pg_class_oid,
				BTEqualStrategyNumber, F_OIDEQ,
				ObjectIdGetDatum(targetRelId));

	/*
	 * Open pg_class and fetch a tuple.  Force heap scan if we haven't yet
	 * built the critical relcache entries (this includes initdb and startup
	 * without a pg_internal.init file).  The caller can also force a heap
	 * scan by setting indexOK == false.
	 */
	pg_class_desc = table_open(RelationRelationId, AccessShareLock);

	/*
	 * The caller might need a tuple that's newer than the one the historic
	 * snapshot; currently the only case requiring to do so is looking up the
	 * relfilenode of non mapped system relations during decoding. That
	 * snapshot can't change in the midst of a relcache build, so there's no
	 * need to register the snapshot.
	 */
	if (force_non_historic)
		snapshot = GetNonHistoricCatalogSnapshot(
			RelationRelationId,
			DistributedTransactionContext);

	pg_class_scan = systable_beginscan(pg_class_desc, ClassOidIndexId,
									   indexOK && criticalRelcachesBuilt,
									   snapshot,
									   1, key);

	pg_class_tuple = systable_getnext(pg_class_scan);

	/*
	 * Must copy tuple before releasing buffer.
	 */
	if (HeapTupleIsValid(pg_class_tuple))
		pg_class_tuple = heap_copytuple(pg_class_tuple);