Status: Patch Available
hive.groupby.skewindata makes use of rand UDF to randomly distribute grouped by keys to the reducers and hence avoids overloading a single reducer when there is a skew in data.
This random distribution of keys is buggy when the reducer fails to fetch the mapper output due to a faulty datanode or any other reason. When reducer finds that it can't fetch mapper output, it sends a signal to Application Master to reattempt the corresponding map task. The reattempted map task will now get the different random value from rand function and hence the keys that gets distributed now to the reducer will not be same as the previous run.
Steps to reproduce:
create table test(id int);
insert into test values (1),(2),(2),(3),(3),(3),(4),(4),(4),(4),(5),(5),(5),(5),(5),(6),(6),(6),(6),(6),(6),(7),(7),(7),(7),(7),(7),(7),(7),(8),(8),(8),(8),(8),(8),(8),(8),(9),(9),(9),(9),(9),(9),(9),(9),(9);
//Add a debug port for reducer
select count(1) from test group by id;
//Remove mapper's intermediate output file when map stage is completed and one out of 2 reduce tasks is completed and then continue the run. This causes 2nd reducer to send event to Application Master to rerun the map task.
The following is the expected result.
But you may get different result due to a different value returned by the rand function in the second run causing different distribution of keys.
This needs to be fixed such that the mapper distributes the same keys even if it is reattempted multiple times.