void init(JobConf jconf, AbstractMapOperator mapOp, KeyValueReader reader) throws IOException {
    execContext = mapOp.getExecContext();
    this.mapOp = mapOp;
    if (reader instanceof KeyValueInputMerger) {
      KeyValueInputMerger kvMerger = (KeyValueInputMerger) reader;
      kvMerger.setIOCxt(execContext.getIoCxt());
    }
    this.reader = reader;
  }
Process input from tez LogicalInput and write output - for a map plan Just pump the records
through the query plan.
ql/src/java/org/apache/hadoop/hive/ql/exec/tez/MapRecordSource.java