Hive merge map files should have different bytes/mapper setting
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Key: HIVE-1118
URL: https://issues.apache.org/jira/browse/HIVE-1118
Project: Hadoop Hive
Issue Type: Improvement
Reporter: Zheng Shao
Currently, by default, we get one reducer for each 1GB of input data.
It's also true for the conditional merge job that will run if the average file
size is smaller than a threshold.
This actually makes those job very slow, because each reducer needs to consume
1GB of data.
Alternatively, we can just use that threshold to determine the number of
reducers per job (or introduce a new parameter).
Let's say the threshold is 1MB, then we only start the the merge job if the
average file size is less than 1MB, and the eventual result file size will be
around 1MB (or another small number).
This will remove the extreme cases where we have thousands of empty files, but
still make normal jobs fast enough.
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