可以用explain查看执行计划
比如
explain select deptno `dept`,year(hiredate) `year`,sum(sal)from tb_empgroup by deptno, year(hiredate);
1 可以先看有几个stage
比如这个例子有2个
+------------------------------------+|Explain |+------------------------------------+|STAGE DEPENDENCIES: || Stage-1 is a root stage || Stage-0 depends on stages: Stage-1|+------------------------------------+
stage 0 依赖于stage1,就是说先执行stage1,再执行stage 0
1查看stage1的map阶段
可以看出map阶段主要做了
表的扫描表数据量的统计检索的字段 就是expressions那块aggregations
+-------------------------------------------------------------------------------------------------+|Explain|+-------------------------------------------------------------------------------------------------+| Map Reduce ||Map Operator Tree: ||TableScan || alias: tb_emp || Statistics: Num rows: 6 Data size: 718 Basic stats: COMPLETE Column stats: NONE|| Select Operator|| expressions: deptno (type: int), year(hiredate) (type: int), sal (type: float)|| outputColumnNames: _col0, _col1, _col2|| Statistics: Num rows: 6 Data size: 718 Basic stats: COMPLETE Column stats: NONE || Group By Operator||aggregations: sum(_col2) ||keys: _col0 (type: int), _col1 (type: int) ||mode: hash ||outputColumnNames: _col0, _col1, _col2 ||Statistics: Num rows: 6 Data size: 718 Basic stats: COMPLETE Column stats: NONE ||Reduce Output Operator || key expressions: _col0 (type: int), _col1 (type: int)|| sort order: ++|| Map-reduce partition columns: _col0 (type: int), _col1 (type: int) || Statistics: Num rows: 6 Data size: 718 Basic stats: COMPLETE Column stats: NONE|| value expressions: _col2 (type: double)|+-------------------------------------------------------------------------------------------------+
3看reduce阶段
确定输入与输出格式+-------------------------------------------------------------------------------------------+|Explain |+-------------------------------------------------------------------------------------------+|Reduce Operator Tree: || Group By Operator||aggregations: sum(VALUE._col0) ||keys: KEY._col0 (type: int), KEY._col1 (type: int) ||mode: mergepartial ||outputColumnNames: _col0, _col1, _col2 ||Statistics: Num rows: 3 Data size: 359 Basic stats: COMPLETE Column stats: NONE ||File Output Operator || compressed: false || Statistics: Num rows: 3 Data size: 359 Basic stats: COMPLETE Column stats: NONE|| table: ||input format: org.apache.hadoop.mapred.SequenceFileInputFormat ||output format: org.apache.hadoop.hive.ql.io.HiveSequenceFileOutputFormat ||serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe |+-------------------------------------------------------------------------------------------+
参考
Hive实验5：查看Hql执行计划及关键步骤说明_heroicpoem的专栏-CSDN博客_hive查看执行计划
LanguageManual Explain - Apache Hive - Apache Software Foundation
