fetchtask直接获取数据，而mapreduce将调用mapreduce作业

<property>
  <name>hive.fetch.task.conversion</name>
  <value>minimal</value>
  <description>
    Some select queries can be converted to single FETCH task 
    minimizing latency.Currently the query should be single 
    sourced not having any subquery and should not have
    any aggregations or distincts (which incurrs RS), 
    lateral views and joins.
    1. minimal : SELECT STAR, FILTER on partition columns, LIMIT only
    2. more    : SELECT, FILTER, LIMIT only (+TABLESAMPLE, virtual columns)
  </description>
</property>

还有另一个参数 hive.fetch.task.conversion.threshold 0.10-0.13中的默认值是-1，>0.14是1g（1073741824），这表示如果表大小大于1g，则使用mapreduce而不是fetch task
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