EXPLAIN：显示查询语句的执行计划，但不运行。 <br/> explain（执行计划）基本语法： ```sql explain [extended | dependency | authorization] query ``` ```txt extended：提供执行计划关于操作的额外信息，比如文件路径 dependency：提供json格式输出，包括查询所依赖的表和分区列表 authorization：列出所有需要授权的实体，包括查询的输入输出和授权失败 ``` **查看语句的执行计划** ```sql hive(default)> explain select * from emp; ``` **查看详细执行计划** ```sql hive(default)> explain extended select * from emp; ``` 一般显示如下信息： ```sql STAGE PLANS: Stage: Stage-1 Map Reduce Map Operator Tree: //发生在job的 map 处理阶段过程 TableScan　　　　　　　 //读取表的数据 alias: emp //emp,若select * from emp e;表有别名则显示别名e Statistics: Num rows: 0 Data size: 74 Basic stats: PARTIAL Column stats: NONE Select Operator expressions: age (type: int), name (type: string) outputColumnNames: _col0, _col1 Statistics: Num rows: 0 Data size: 74 Basic stats: PARTIAL Column stats: NONE Reduce Output Operator key expressions: _col0 (type: int) sort order: + Map-reduce partition columns: _col0 (type: int) Statistics: Num rows: 0 Data size: 74 Basic stats: PARTIAL Column stats: NONE value expressions: _col0 (type: int), _col1 (type: string) Reduce Operator Tree: //发生在job的 reduce 处理阶段过程 Extract Statistics: Num rows: 0 Data size: 74 Basic stats: PARTIAL Column stats: NONE File Output Operator compressed: false Statistics: Num rows: 0 Data size: 74 Basic stats: PARTIAL Column stats: NONE table: //说明输入格式是TextInputFormat input format: org.apache.hadoop.mapred.TextInputFormat //说明输出格式是HiveIgnoreKeyTextOutputFormat output format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe Stage: Stage-0 Fetch Operator limit: -1 //--job中没有Limit ``` <br/> **通过工具生成可视化执行计划**