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From "liyunzhang_intel (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HIVE-17018) Small table can not be converted to map join in TPC-DS/query.17 on 3TB data scale
Date Tue, 04 Jul 2017 07:49:00 GMT
liyunzhang_intel created HIVE-17018:
---------------------------------------

             Summary: Small table can not be converted to map join in TPC-DS/query.17 on 3TB
data scale
                 Key: HIVE-17018
                 URL: https://issues.apache.org/jira/browse/HIVE-17018
             Project: Hive
          Issue Type: Bug
            Reporter: liyunzhang_intel
            Assignee: liyunzhang_intel


 we use "hive.auto.convert.join.noconditionaltask.size" as the threshold. it means  the sum
of size for n-1 of the tables/partitions for a n-way join is smaller than it, it will be converted
to a map join. for example, A join B join C join D join E. Big table is A(100M), small tables
are B(10M),C(10M),D(10M),E(10M).  If we set hive.auto.convert.join.noconditionaltask.size=20M.
In current code, E,D,B will be converted to map join but C will not be converted to map join.
In my understanding, because hive.auto.convert.join.noconditionaltask.size can only contain
E and D, so C and B should not be converted to map join.  
Let's explain more why E can be converted to map join.
in current code, [SparkMapJoinOptimizer#getConnectedMapJoinSize|https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/optimizer/spark/SparkMapJoinOptimizer.java#L364]
calculates all the mapjoins  in the parent path and child path. The search stops when encountering
[UnionOperator or ReduceOperator|https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/optimizer/spark/SparkMapJoinOptimizer.java#L381].
Because C is not converted to map join because {{connectedMapJoinSize + totalSize) > maxSize}}
[see code|https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/optimizer/spark/SparkMapJoinOptimizer.java#L330].The
RS before the join of C remains. When calculating whether B will be converted to map join,
{{getConnectedMapJoinSize}} returns 0 as encountering [RS |https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/optimizer/spark/SparkMapJoinOptimizer.java#409]
and causes  {{connectedMapJoinSize + totalSize) < maxSize}} matches.

[~xuefuz] or [~jxiang]: can you help see the problem as you are more familiar with SparkJoinOptimizer.




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