CBO: Calcite Operator To Hive Operator(Calcite Return Path): Wrong result/plan in group by with hive.map.aggr=false

 set hive.cbo.returnpath.hiveop=true
 set hive.map.aggr=false

create table abcd (a int, b int, c int, d int);
LOAD DATA LOCAL INPATH '../../data/files/in4.txt' INTO TABLE abcd;

 explain select count(distinct a) from abcd group by b; 

STAGE PLANS:
  Stage: Stage-1
    Map Reduce
      Map Operator Tree:
          TableScan
            alias: abcd
            Statistics: Num rows: 19 Data size: 78 Basic stats: COMPLETE Column stats: NONE
            Select Operator
              expressions: a (type: int)
              outputColumnNames: a
              Statistics: Num rows: 19 Data size: 78 Basic stats: COMPLETE Column stats: NONE
              Reduce Output Operator
                key expressions: a (type: int), a (type: int)
                sort order: ++
                Map-reduce partition columns: a (type: int)
                Statistics: Num rows: 19 Data size: 78 Basic stats: COMPLETE Column stats: NONE
      Reduce Operator Tree:
        Group By Operator
          aggregations: count(DISTINCT KEY._col1:0._col0)
          keys: KEY._col0 (type: int)
          mode: complete
          outputColumnNames: b, $f1
          Statistics: Num rows: 9 Data size: 36 Basic stats: COMPLETE Column stats: NONE
          Select Operator
            expressions: $f1 (type: bigint)
            outputColumnNames: _o__c0
            Statistics: Num rows: 9 Data size: 36 Basic stats: COMPLETE Column stats: NONE
            File Output Operator
              compressed: false
              Statistics: Num rows: 9 Data size: 36 Basic stats: COMPLETE Column stats: NONE
              table:
                  input format: org.apache.hadoop.mapred.SequenceFileInputFormat
                  output format: org.apache.hadoop.hive.ql.io.HiveSequenceFileOutputFormat
                  serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe

 explain select count(distinct a) from abcd group by c; 

STAGE PLANS:
  Stage: Stage-1
    Map Reduce
      Map Operator Tree:
          TableScan
            alias: abcd
            Statistics: Num rows: 19 Data size: 78 Basic stats: COMPLETE Column stats: NONE
            Select Operator
              expressions: a (type: int)
              outputColumnNames: a
              Statistics: Num rows: 19 Data size: 78 Basic stats: COMPLETE Column stats: NONE
              Reduce Output Operator
                key expressions: a (type: int), a (type: int)
                sort order: ++
                Map-reduce partition columns: a (type: int)
                Statistics: Num rows: 19 Data size: 78 Basic stats: COMPLETE Column stats: NONE
      Reduce Operator Tree:
        Group By Operator
          aggregations: count(DISTINCT KEY._col1:0._col0)
          keys: KEY._col0 (type: int)
          mode: complete
          outputColumnNames: c, $f1
          Statistics: Num rows: 9 Data size: 36 Basic stats: COMPLETE Column stats: NONE
          Select Operator
            expressions: $f1 (type: bigint)
            outputColumnNames: _o__c0
            Statistics: Num rows: 9 Data size: 36 Basic stats: COMPLETE Column stats: NONE
            File Output Operator
              compressed: false
              Statistics: Num rows: 9 Data size: 36 Basic stats: COMPLETE Column stats: NONE
              table:
                  input format: org.apache.hadoop.mapred.SequenceFileInputFormat
                  output format: org.apache.hadoop.hive.ql.io.HiveSequenceFileOutputFormat
                  serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe