@Override public final OrcStruct nextRecord() {
      return nextRecord;
    }
@param statementId - this should be from delta_x_y_stmtId file name.  Imagine 2 load data
                   statements in 1 txn.  The stmtId will be embedded in
                   {@link RecordIdentifier#bucketId} via {@link BucketCodec} below
ql/src/java/org/apache/hadoop/hive/ql/io/orc/OrcRawRecordMerger.java