[Hive] multi group by statement is not optimized
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Key: HADOOP-4139
URL: https://issues.apache.org/jira/browse/HADOOP-4139
Project: Hadoop Core
Issue Type: Bug
Reporter: Namit Jain
Assignee: Namit Jain
A simple multi-group by statement is not optimized. A simple statement like:
FROM SRC
INSERT OVERWRITE TABLE DEST1 SELECT SRC.key, count(distinct
SUBSTR(SRC.value,4)) GROUP BY SRC.key
INSERT OVERWRITE TABLE DEST2 SELECT SRC.key, count(distinct
SUBSTR(SRC.value,4)) GROUP BY SRC.key;
results in making 2 copies of the data (SRC). Instead, the data can be first
partially aggregated on the distinct value and then aggregated.
The first step can be common to all group bys.
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