dengzhhu653 commented on pull request #2585:
URL: https://github.com/apache/hive/pull/2585#issuecomment-1048902780
I found something interesting, when I explain `select col1, count(distinct
col2) from partition_distinct_skew group by col1;` on master branch, the
output is following:
```
Vertices:
Map 1
Map Operator Tree:
TableScan
alias: partition_distinct_skew
Statistics: Num rows: 3 Data size: 510 Basic stats:
COMPLETE Column stats: COMPLETE
Select Operator
expressions: col1 (type: string), col2 (type: string)
outputColumnNames: col1, col2
Statistics: Num rows: 3 Data size: 510 Basic stats:
COMPLETE Column stats: COMPLETE
Group By Operator
keys: col1 (type: string), col2 (type: string)
minReductionHashAggr: 0.4
mode: hash
outputColumnNames: _col0, _col1
Statistics: Num rows: 2 Data size: 340 Basic stats:
COMPLETE Column stats: COMPLETE
Reduce Output Operator
key expressions: _col0 (type: string), _col1 (type:
string)
null sort order: zz
sort order: ++
Map-reduce partition columns: rand() (type: double)
Statistics: Num rows: 2 Data size: 340 Basic stats:
COMPLETE Column stats: COMPLETE
```
The partition column is **rand()** for this case. it's seems we have done
something to improve the skew case, though I not able to find where the cause
locates.
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