When partitions are created using queries ("insert overwrite" and "insert into") then the StatsTask updates all stats. However, when partitions are added directly through metadata-only partitions (either CLI or direct calls to Thrift Metastore) no stats are populated even if hive.stats.reliable is set to true. This puts us in a situation where we can't decide if stats are truly reliable or not.
We propose that the "fast stats" (numFiles and totalSize) which don't require a scan of the data should always be populated and be completely reliable. For now we are still excluding rowCount and rawDataSize because that will make these operations very expensive. Currently they are quick metadata-only ops.