Number of reducers limited to fixed 1 when updating/deleting

When updating/deleting bucketed tables an extra ReduceSink operator is created to enforce bucketing. After HIVE-22538 number of reducers limited to fixed 1 in these RS operators.

This can lead to performance degradation.

Prior HIVE-22538 multiple reducers was available such cases. The reason for limiting the number of reducers is to ensure RowId ascending order in delete delta files produced by the update/delete statements.

This is the plan of delete statement like:

DELETE FROM t1 WHERE a = 1;

TS[0]-FIL[8]-SEL[2]-RS[3]-SEL[4]-RS[5]-SEL[6]-FS[7]

RowId order is ensured by RS[3] and bucketing is enforced by RS[5]: number of reducers were limited to bucket number in the table or hive.exec.reducers.max. However RS[5] does not provide any ordering so above plan may generate unsorted deleted deltas which leads to corrupted data reads.

Prior HIVE-22538 these RS operators were merged by ReduceSinkDeduplication and the resulting RS kept the ordering and enabled multiple reducers. It could do because ReduceSinkDeduplication was prepared for ACID writes. This was removed by HIVE-22538 to get a more generic ReduceSinkDeduplication.