Avoid cache lookups for internal PostgreSQL tables when checking for Citus tables

PostgreSQL starts assigning user OIDs from FirstNormalObjectId onward,
so any relation with an OID below that is a system object and cannot be a Citus table.
This change introduces an early bailout for such relationIds,
eliminating unnecessary calls to BuildCitusTableCache.

This optimization is particularly beneficial in performance-critical workflows—such
as rapid connect -> query-catalog_table -> disconnect cycles—by avoiding the
expensive cache build and lookup.
Since BuildCitusTableCache operates under a lock, which can lead to contention
among concurrent processes (and, in some cases, distributed deadlocks),
bypassing it for internal tables improves both performance and stability.

src/backend/distributed/metadata/metadata_cache.c

@@ -660,6 +660,18 @@ GetTableTypeName(Oid tableId)
 bool
 IsCitusTable(Oid relationId)
 {
+	/*
+	 * PostgreSQL's OID generator assigns user operation OIDs starting
+	 * from FirstNormalObjectId. This means no user object can have
+	 * an OID lower than FirstNormalObjectId. Therefore, if the
+	 * relationId is less than FirstNormalObjectId
+	 * (i.e. in PostgreSQL's reserved range), we can immediately
+	 * return false, since such objects cannot be Citus tables.
+	 */
+	if (relationId < FirstNormalObjectId)
+	{
+		return false;
+	}
 	return LookupCitusTableCacheEntry(relationId) != NULL;
 }