@Override public RexNode visitCall(RexCall oldCall) {
      RexNode newCall = mapping.get(oldCall);
      if (newCall != null) {
        return newCall;
      } else {
        return super.visitCall(oldCall);
      }
    }
A visitor traversing row expressions and replacing calls with other expressions according
to the specified mapping.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/CorrelateProjectExtractor.java