1、Antlr定义SQL的语法规则，完成SQL词法，语法解析，将SQL转化为抽象语法树AST Tree
2、遍历AST Tree，抽象出查询的基本组成单元QueryBlock
3、遍历QueryBlock，翻译为执行操作树OperatorTree
4、逻辑层优化器进行OperatorTree变换，合并不必要的ReduceSinkOperator，减少shuffle数据量
5、遍历OperatorTree，翻译为MapReduce任务
6、物理层优化器进行MapReduce任务的变换，生成最终的执行计划

select * from xx.xx limit 1

<property>
    <name>hive.fetch.task.conversion</name>
    <value>more</value>
    <description>
      Expects one of [none, minimal, more].
      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 incurs RS), lateral views and joins.
      0. none : disable hive.fetch.task.conversion
      1. minimal : SELECT STAR, FILTER on partition columns, LIMIT only
      2. more  : SELECT, FILTER, LIMIT only (support TABLESAMPLE and virtual columns)
    </description>
</property>

<property>
  <name>hive.fetch.task.conversion.threshold</name>
  <value>1073741824</value>
  <description>
    Input threshold for applying hive.fetch.task.conversion. If target table is native, input length
    is calculated by summation of file lengths. If it's not native, storage handler for the table
    can optionally implement org.apache.hadoop.hive.ql.metadata.InputEstimator interface.
  </description>
</property>