@Override public RecordIdentifier getMaxKey() {
      return maxKey;
    }
If this is not the 1st file, set minKey 1 less than the start of current file
(Would not need to set minKey if we knew that there are no delta files)
{@link #advanceToMinKey()} needs this
ql/src/java/org/apache/hadoop/hive/ql/io/orc/OrcRawRecordMerger.java