private void preserveOperatorInfos(Operator<?> newOp, Operator<?> oldOp, OptimizeTezProcContext context) {
    newOp.setStatistics(oldOp.getStatistics());
    // linking these two operator declares that they are representing the same thing
    // currently important because statistincs are actually gather for newOp; but the lookup is done using oldOp
    context.parseContext.getContext().getPlanMapper().link(oldOp, newOp);
  }
Preserves additional information about the operator.

When an operator is replaced by a new one; some of the information of the old have to be retained.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/ConvertJoinMapJoin.java