Hive的Explain命令

Hive的Explain命令，用于显示SQL查询的执行计划。

Hive查询被转化成序列阶段（这是一个有向无环图）。这些阶段可能是mapper/reducer阶段，或者是Metastore或文件系统的操作，如移动和重命名的阶段。

例子 

hive> explain [extended] --加上关键字extended,则显示抽象语法树 > select * from student > cluster by age;

 1.查询的抽象语法树(没用,可以忽略)

2.执行计划计划的不同阶段之间的依赖关系(很重要)

OK STAGE DEPENDENCIES: Stage-1 is a root stage Stage-0 is a root stage

3.每个场景的描述

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: student //表名为student,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 
 Time taken: 0.085 seconds, Fetched: 36 row(s)