Shreepadma Venugopalan created HIVE-4153:
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Summary: Use number of distinct values to decide whether to
perform map side aggregation
Key: HIVE-4153
URL: https://issues.apache.org/jira/browse/HIVE-4153
Project: Hive
Issue Type: Bug
Components: Query Processor
Affects Versions: 0.10.0, 0.9.0, 0.8.1, 0.8.0
Reporter: Shreepadma Venugopalan
Today, Hive decides to perform a map side aggregation by default. If the number
of unique keys in the aggregation is small, performing a map side aggregation
is beneficial. However, if the number of keys is sufficiently large, it can
lead to OOMEs. Upon encountering an OOME, hive.map.aggr has be set to false to
turn it off. Instead, we can use the number of distinct values in the group by
column along with the number of rows in the table to decide if map side
aggregation should be used.
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