Hive基础知识（二一）：优化篇-执行计划与Fetch抓取

1. 执行计划（Explain）

1）基本语法

EXPLAIN [EXTENDED | DEPENDENCY | AUTHORIZATION] query

2）案例实操

（1）查看下面这条语句的执行计划

没有生成 MR 任务的

hive (default)> explain select * from emp;
OK
Explain
STAGE DEPENDENCIES:
  Stage-0 is a root stage
​
STAGE PLANS:
  Stage: Stage-0
    Fetch Operator
      limit: -1
      Processor Tree:
        TableScan
          alias: emp
          Statistics: Num rows: 1 Data size: 6460 Basic stats: COMPLETE Column stats: NONE
          Select Operator
            expressions: empno (type: int), ename (type: string), job (type: string), mgr (type: int), hiredate (type: string), sal (type: double), comm (type: double), deptno (type: int)
            outputColumnNames: _col0, _col1, _col2, _col3, _col4, _col5, _col6, _col7
            Statistics: Num rows: 1 Data size: 6460 Basic stats: COMPLETE Column stats: NONE
            ListSink
​
Time taken: 14.564 seconds, Fetched: 17 row(s)

有生成 MR 任务的

hive (default)> explain select deptno, avg(sal) avg_sal fr