kasakrisz commented on pull request #2231:
URL: https://github.com/apache/hive/pull/2231#issuecomment-829900602
Hi Marta,
Thanks for reviewing this patch.
This is what I found about distributing rows to reducers while I was
debugging:
Let's say we have the following statements:
```
create table acidtbl(a int, b int) clustered by (a) into 2 buckets stored as
orc TBLPROPERTIES ('transactional'='true');
insert ...
delete from acidtbl where a = 1 or a = 3;
```
This case the the plan of the delete statement after ReduceSinkDeDuplication
looks like:
```
TS[0]-FIL[8]-SEL[2]-RS[5]-SEL[6]-FS[7]
```
So with Tez we have a mapper: TS[0]-FIL[8]-SEL[2]-RS[5]
and have two reducers each of them has: SEL[6]-FS[7]
RS[5] has
Partition keys: GenericUDFBridge ==> UDFToInteger (Column[_col0])
Sort keys: Column[_col0]
And maxReducers: 2
where _col0 is the row_id coming from SEL[2].
UDFToInteger(<row_id_type>) extracts the bucket_id field which is going to
be used to generate a `reducesink.key` in the RS operator. This is going to be
passed to the wrapped `OutputCollector` with the row. This case this is an
`org.apache.tez.runtime.library.output.OrderedPartitionedKVOutput`. This class
is part of Tez which I'm not familiar with but I found that this is where rows
are distributed to reducers by the key coming from RS.
Hive/hadoop also has a setting
`hive.exec.reducers.max`/`mapreduce.job.reduces`. This limits the maxReducers
in RS operator. If the table has more buckets than the max reducers then
FileSink operator also distributes the rows into different files. If I
understand correctly this is done by the `multiFileSpray` functionality.
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