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https://issues.apache.org/jira/browse/HIVE-17018?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16083058#comment-16083058
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liyunzhang_intel commented on HIVE-17018:
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[~csun]:
{quote}A better way might be to have a separate config just for HoS, and maybe
a limit on small table memory per executor.{quote}
what I confused is how to do this? the original code is to calculate whether
the total mapjoin size in the same stage exceed the threshold or not. Now
create a new configure about calculating the threshold of all small tables
according to the spark.executor.memory? If the total size of small tables in
the same stage bigger than the spark.executor.memory, then not allow these
small tables into the same stage but
{{hive.auto.convert.join.nonconditionaltask.size}} is for caculating the total
size of mapjoin size of small tables in the query?
> Small table is converted to map join even the total size of small tables
> exceeds the threshold(hive.auto.convert.join.noconditionaltask.size)
> ---------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: HIVE-17018
> URL: https://issues.apache.org/jira/browse/HIVE-17018
> Project: Hive
> Issue Type: Bug
> Reporter: liyunzhang_intel
> Assignee: liyunzhang_intel
> Attachments: HIVE-17018_data_init.q, HIVE-17018.q, t3.txt
>
>
> 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 whether this is a bug or not as you
> are more familiar with SparkJoinOptimizer.
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