public Path getMergeInputDirName() {
    Path root = getFinalDirName();
    if (isMmTable()) {
      return new Path(root, AcidUtils.deltaSubdir(tableWriteId, tableWriteId, statementId));
    } else {
      return root;
    }
  }
getFinalDirName that takes into account MM, but not DP, LB or buckets.
ql/src/java/org/apache/hadoop/hive/ql/plan/FileSinkDesc.java