Hive运行原理_hive limit 10的原理、执行过程

在执行Hive语句中，难免会好奇Hive的执行原理，通过explain可以查看Hive转换后的抽象语法树和操作符树

hive> explain extended select sum(shopid) from shopt1 limit 10;

生成的语法解析树AST Tree如下所示：

ABSTRACT SYNTAX TREE:
  
TOK_QUERY
   TOK_FROM
      TOK_TABREF
         TOK_TABNAME
            shopt1    ---表名
   TOK_INSERT
      TOK_DESTINATION
         TOK_DIR
            TOK_TMP_FILE    ---所有的查询的数据会输出到HDFS的一个暂存文件中
      TOK_SELECT
         TOK_SELEXPR
            TOK_FUNCTION
               sum          ---获取使用到的函数
               TOK_TABLE_OR_COL
                  shopid    ----获取select的列
      TOK_LIMIT
         10

执行计划如下所示：

STAGE DEPENDENCIES:
  Stage-1 is a root stage
  Stage-0 depends on stages: Stage-1        ----stage-0依赖stage-1
 
STAGE PLANS:
  Stage: Stage-1
    Tez
      Edges:
        Reducer 2 <- Map 1 (SIMPLE_EDGE)
      DagName: work_20160918215757_92e7e39d-b52e-49dc-b0dd-d8807b87c7d2:1
      Vertices:
        Map 1 
            Map Operator Tree:
                TableScan        ----扫描shopt1表
                  alias: shopt1
                  Statistics: Num rows: 5 Data size: 42 Basic stats: COMPLETE Column stats: NONE
                  Select Operator  -----筛选select到的列
                    expressions: shopid (type: bigint)
                    outputColumnNames: shopid
                    Statistics: Num rows: 5 Data size: 42 Basic stats: COMPLETE Column stats: NONE
                    Group By Operator      ---分组
                      aggregations: sum(shopid)
                      mode: hash          ---hash分组
                      outputColumnNames: _col0
                      Statistics: Num rows: 1 Data size: 8 Basic stats: COMPLETE Column stats: NONE
                      Reduce Output Operator    ---Map端的Reduce过程
                        sort order: 
                        Statistics: Num rows: 1 Data size: 8 Basic stats: COMPLETE Column stats: NONE
                        value expressions: _col0 (type: bigint)
        Reducer 2 
            Reduce Operator Tree:
              Group By Operator       ----reduce端的分组合并
                aggregations: sum(VALUE._col0)
                mode: mergepartial
                outputColumnNames: _col0
                Statistics: Num rows: 1 Data size: 8 Basic stats: COMPLETE Column stats: NONE
                Select Operator
                  expressions: _col0 (type: bigint)
                  outputColumnNames: _col0
                  Statistics: Num rows: 1 Data size: 8 Basic stats: COMPLETE Column stats: NONE
                  Limit
                    Number of rows: 10
                    Statistics: Num rows: 1 Data size: 8 Basic stats: COMPLETE Column stats: NONE
                    File Output Operator    ---最终文件的输出
                      compressed: false
                      Statistics: Num rows: 1 Data size: 8 Basic stats: COMPLETE Column stats: NONE
                      table:
                          input format: org.apache.hadoop.mapred.TextInputFormat  ---输入
                          output format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat       ---输出
                          serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe   --序列化
 
  Stage: Stage-0
    Fetch Operator
      limit: 10                     ---limit 10
      Processor Tree:
        ListSink

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以下是参考美团技术博客Hive的编译过程，摘取核心执行部分share

Sql：

FROM
( 
  SELECT
    p.datekey datekey,
    p.userid userid,
    c.clienttype
  FROM
    detail.usersequence_client c
    JOIN fact.orderpayment p ON p.orderid = c.orderid
    JOIN default.user du ON du.userid = p.userid
  WHERE p.datekey = 20131118 
) base
INSERT OVERWRITE TABLE `test`.`customer_kpi`
SELECT
  base.datekey,
  base.clienttype,
  count(distinct base.userid) buyer_count
GROUP BY base.datekey, base.clienttype

1.首先对sql进行语法分析，词法分析，解析SQL生成AST Tree（抽象语法树）

2.根据AST Tree生成QueryBlock

上图是QueryBlock相关对象的类图，AST Tree生成QueryBlock的过程是一个递归的过程，先序遍历AST Tree，遇到不同的Token节点，保存到相应的属性中，主要包含以下几个过程

• TOK_QUERY => 创建QB对象，循环递归子节点
• TOK_FROM => 将表名语法部分保存到QB对象的aliasToTabs等属性中
• TOK_INSERT => 循环递归子节点
• TOK_DESTINATION => 将输出目标的语法部分保存在QBParseInfo对象的nameToDest属性中
• TOK_SELECT => 分别将查询表达式的语法部分保存在destToSelExpr、destToAggregationExprs、destToDistinctFuncExprs三个属性中
• TOK_WHERE => 将Where部分的语法保存在QBParseInfo对象的destToWhereExpr属性中

具体可参考Hive SQL的编译过程