okumin commented on code in PR #5707:
URL: https://github.com/apache/hive/pull/5707#discussion_r2026950629


##########
ql/src/test/results/clientpositive/llap/bucket_map_join_tez3.q.out:
##########
@@ -1432,7 +1432,7 @@ STAGE PLANS:
                         Map-reduce partition columns: _col0 (type: date), 
_col1 (type: decimal(38,0))
                         Statistics: Num rows: 1 Data size: 168 Basic stats: 
COMPLETE Column stats: COMPLETE
                         tag: -1
-                        auto parallelism: true
+                        auto parallelism: false

Review Comment:
   Looks good.
   
   Map 2 -> Reducer 3 -> Map 1(BMJ)



##########
ql/src/test/results/clientpositive/llap/bucket_map_join_tez3.q.out:
##########
@@ -480,7 +480,7 @@ STAGE PLANS:
                         Map-reduce partition columns: _col0 (type: date), 
_col1 (type: decimal(38,0))
                         Statistics: Num rows: 1 Data size: 168 Basic stats: 
COMPLETE Column stats: COMPLETE
                         tag: -1
-                        auto parallelism: true
+                        auto parallelism: false

Review Comment:
   Looks good.
   
   Map 2 -> Reducer 3 -> Map 1(BMJ)



##########
ql/src/java/org/apache/hadoop/hive/ql/parse/GenTezUtils.java:
##########
@@ -182,6 +187,41 @@ public static ReduceWork createReduceWork(
     return reduceWork;
   }
 
+  /**
+   * Checks if there is a Bucket Map Join (BMJ) following a hierarchy:
+   * ReduceSinkOperator (RS) -> ReduceSinkOperator (RS) -> MapJoinOperator 
(MJ).
+   * Ensures at most **two** RSOs before MJ.

Review Comment:
   I guess it happens only when there are precisely two RSOs. Given that Map 3 
-> Reducer 4 -> Reducer 5 -> Map 1(BMJ), `createReduceWork` is invoked twice.
   When we create Reducer 4, the passed RSO belongs to Map 3. The process bails 
out once we find the third RSO in Reducer 5.
   When we create Reducer 5, we find two RSOs in Reduer 4 and Reducer 5, and 
the BucketMapJoin.
   
   
   A vertex with BucketMapJoin is a mapper. So, basically, RS -> MJ doesn't 
happen.



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