Yin Huai created HIVE-4827:
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Summary: Merge a Map-only job to its following MapReduce job with
multiple inputs
Key: HIVE-4827
URL: https://issues.apache.org/jira/browse/HIVE-4827
Project: Hive
Issue Type: Improvement
Reporter: Yin Huai
Assignee: Yin Huai
When hive.optimize.mapjoin.mapreduce is on, CommonJoinResolver can attach a
Map-only job (MapJoin) to its following MapReduce job. But this merge only
happens when the MapReduce job has a single input. With Correlation Optimizer
(HIVE-2206), it is possible that the MapReduce job can have multiple inputs
(for multiple operation paths). It is desired to improve CommonJoinResolver to
merge a Map-only job to the corresponding Map task of the MapReduce job.
Example:
{code:sql}
SELECT tmp1.key, count(*)
FROM (SELECT x1.key2 AS key
FROM bigTable1 x1 JOIN smallTable1 y1 ON (x1.key1 = y1.key1)
GROUP BY x1.key2) tmp1
JOIN (SELECT x2.key2 AS key
FROM bigTable2 x2 JOIN smallTable2 y2 ON (x2.key1 = y2.key1)
GROUP BY x2.key2) tmp2
ON (tmp1.key = tmp2.key)
GROUP BY tmp1.key;
{\code}
In this query, join operations inside tmp1 and tmp2 will be converted to two
MapJoins. With Correlation Optimizer, aggregations in tmp1, tmp2, and join of
tmp1 and tmp2, and the last aggregation will be executed in the same MapReduce
job (Reduce side). Since this MapReduce job has two inputs, right now,
CommonJoinResolver cannot attach two MapJoins to the Map side of a MapReduce
job.
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