基础理论
这节将详细介绍 explain 的使用方法及主要参数详细介绍
HIVE出示了EXPLAIN指令来展现一个查看的执行计划,这一执行计划针对大家掌握最底层基本原理,hive 调优,清查数据倾斜等很有协助
应用英语的语法以下:
EXPLAIN [EXTENDED|CBO|AST|DEPENDENCY|AUTHORIZATION|LOCKS|VECTORIZATION|ANALYZE] query
explain 后边能够跟下列可选主要参数,留意:这好多个可选主要参数并不是 hive 每一个版本号都适用的
在 hive cli 中键入下列指令(hive 2.3.7):
- explain select sum(id) from test1;
获得結果(请一行行看了,即便不明白还要每排要看):
- STAGE DEPENDENCIES:
- Stage-1 is a root stage
- Stage-0 depends on stages: Stage-1
- STAGE PLANS:
- Stage: Stage-1
- Map Reduce
- Map Operator Tree:
- TableScan
- alias: test1
- Statistics: Num rows: 6 Data size: 75 Basic stats: COMPLETE Column stats: NONE
- Select Operator
- expressions: id (type: int)
- outputColumnNames: id
- Statistics: Num rows: 6 Data size: 75 Basic stats: COMPLETE Column stats: NONE
- Group By Operator
- aggregations: sum(id)
- mode: hash
- outputColumnNames: _col0
- Statistics: Num rows: 1 Data size: 8 Basic stats: COMPLETE Column stats: NONE
- Reduce Output Operator
- sort order:
- Statistics: Num rows: 1 Data size: 8 Basic stats: COMPLETE Column stats: NONE
- value expressions: _col0 (type: bigint)
- Reduce Operator Tree:
- Group By Operator
- aggregations: sum(VALUE._col0)
- mode: mergepartial
- outputColumnNames: _col0
- Statistics: Num rows: 1 Data size: 8 Basic stats: COMPLETE Column stats: NONE
- File Output Operator
- compressed: false
- Statistics: Num rows: 1 Data size: 8 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
- Stage: Stage-0
- Fetch Operator
- limit: -1
- Processor Tree:
- ListSink
看了以上内容有哪些体会,是否觉得都不明白,别着急,下边可能详尽解读每一个主要参数,相信你学好下边的內容以后再看 explain 的查看結果将得心应手。
一个HIVE查看被变换为一个由一个或好几个stage构成的编码序列(有向无环图DAG)。这种stage能够是MapReduce stage,还可以是承担元数据储存的stage,还可以是承担系统文件的实际操作(例如挪动和重新命名)的stage。
大家将所述結果分拆看,先从最表层逐渐,包括2个大的一部分:
首先看第一部分 stage dependencies ,包括2个 stage,Stage-1 是根stage,表明它是逐渐的stage,Stage-0 依靠 Stage-1,Stage-1实行进行后实行Stage-0。
再看第二一部分 stage plan,里边有一个 Map Reduce,一个MR的执行计划分成2个一部分:
这两个执行计划树里边包括这条sql语句的 operator:
1.map端第一个实际操作肯定是载入表,因此 便是 TableScan 表扫描仪实际操作,普遍的特性:
2.Select Operator: 选择实际操作,普遍的特性 :
3.Group By Operator:排序汇聚实际操作,普遍的特性:
4.Reduce Output Operator:輸出到reduce实际操作,普遍特性:
5.Filter Operator:过滤操作,普遍的特性:
6.Map Join Operator:join 实际操作,普遍的特性:
7.File Output Operator:文档輸出实际操作,普遍的特性
8.Fetch Operator 手机客户端读取数据实际操作,普遍的特性:
好,学得这儿再翻出上边 explain 的查看結果,是否觉得基础都看得懂了。
实践活动
这节详细介绍 explain 可以为我们在生活实践中产生什么便捷及处理大家什么蒙蔽
1. join 句子会过虑 null 的值吗?
如今,我们在hive cli 键入下列查看方案句子
- select a.id,b.user_name from test1 a join test2 b on a.id=b.id;
问:上边这条 join 句子会过虑 id 为 null 的值吗
实行下边句子:
- explain select a.id,b.user_name from test1 a join test2 b on a.id=b.id;
大家看来結果 (为了更好地融入网页页面展现,仅提取了一部分輸出信息内容):
- TableScan
- alias: a
- Statistics: Num rows: 6 Data size: 75 Basic stats: COMPLETE Column stats: NONE
- Filter Operator
- predicate: id is not null (type: boolean)
- Statistics: Num rows: 6 Data size: 75 Basic stats: COMPLETE Column stats: NONE
- Select Operator
- expressions: id (type: int)
- outputColumnNames: _col0
- Statistics: Num rows: 6 Data size: 75 Basic stats: COMPLETE Column stats: NONE
- HashTable Sink Operator
- keys:
- 0 _col0 (type: int)
- 1 _col0 (type: int)
- ...
从所述結果能够见到 predicate: id is not null 那样一行,表明 join 的时候会全自动过虑掉关系字段名为 null 值的状况,但 left join 或 full join 是不容易全自动过虑的,大伙儿能够自主试着下。
2. group by 排序句子会开展排列吗?
看下面这条sql
- select id,max(user_name) from test1 group by id;
问:group by 排序句子会开展排列吗
立即看来 explain 以后結果 (为了更好地融入网页页面展现,仅提取了一部分輸出信息内容)
- TableScan
- alias: test1
- Statistics: Num rows: 9 Data size: 108 Basic stats: COMPLETE Column stats: NONE
- Select Operator
- expressions: id (type: int), user_name (type: string)
- outputColumnNames: id, user_name
- Statistics: Num rows: 9 Data size: 108 Basic stats: COMPLETE Column stats: NONE
- Group By Operator
- aggregations: max(user_name)
- keys: id (type: int)
- mode: hash
- outputColumnNames: _col0, _col1
- Statistics: Num rows: 9 Data size: 108 Basic stats: COMPLETE Column stats: NONE
- Reduce Output Operator
- key expressions: _col0 (type: int)
- sort order:
- Map-reduce partition columns: _col0 (type: int)
- Statistics: Num rows: 9 Data size: 108 Basic stats: COMPLETE Column stats: NONE
- value expressions: _col1 (type: string)
- ...
大家看 Group By Operator,里边有 keys: id (type: int) 表明依照 id 开展排序的,再往下看也有 sort order: ,表明是依照 id 字段名开展正序排列的。
3. 哪一条sql实行高效率呢?
观查两根sql语句
- SELECT
- a.id,
- b.user_name
- FROM
- test1 a
- JOIN test2 b ON a.id = b.id
- WHERE
- a.id > 2;
- SELECT
- a.id,
- b.user_name
- FROM
- (SELECT * FROM test1 WHERE id > 2) a
- JOIN test2 b ON a.id = b.id;
这两根sql语句輸出的結果是一样的,可是哪一条sql实行高效率呢
有些人说第一条sql实行高效率,由于第二条sql有子查询,子查询会危害特性
有些人说第二条sql实行高效率,由于先过虑以后,在开展join时的总数降低了,因此 实行高效率就高了
究竟哪一条sql高效率呢,大家立即在sql语句前边再加上 explain,看下执行计划不就知道嘛
在第一条sql语句前再加上 explain,获得以下結果
- hive (default)> explain select a.id,b.user_name from test1 a join test2 b on a.id=b.id where a.id >2;
- OK
- Explain
- STAGE DEPENDENCIES:
- Stage-4 is a root stage
- Stage-3 depends on stages: Stage-4
- Stage-0 depends on stages: Stage-3
- STAGE PLANS:
- Stage: Stage-4
- Map Reduce Local Work
- Alias -> Map Local Tables:
- $hdt$_0:a
- Fetch Operator
- limit: -1
- Alias -> Map Local Operator Tree:
- $hdt$_0:a
- TableScan
- alias: a
- Statistics: Num rows: 6 Data size: 75 Basic stats: COMPLETE Column stats: NONE
- Filter Operator
- predicate: (id > 2) (type: boolean)
- Statistics: Num rows: 2 Data size: 25 Basic stats: COMPLETE Column stats: NONE
- Select Operator
- expressions: id (type: int)
- outputColumnNames: _col0
- Statistics: Num rows: 2 Data size: 25 Basic stats: COMPLETE Column stats: NONE
- HashTable Sink Operator
- keys:
- 0 _col0 (type: int)
- 1 _col0 (type: int)
- Stage: Stage-3
- Map Reduce
- Map Operator Tree:
- TableScan
- alias: b
- Statistics: Num rows: 6 Data size: 75 Basic stats: COMPLETE Column stats: NONE
- Filter Operator
- predicate: (id > 2) (type: boolean)
- Statistics: Num rows: 2 Data size: 25 Basic stats: COMPLETE Column stats: NONE
- Select Operator
- expressions: id (type: int), user_name (type: string)
- outputColumnNames: _col0, _col1
- Statistics: Num rows: 2 Data size: 25 Basic stats: COMPLETE Column stats: NONE
- Map Join Operator
- condition map:
- Inner Join 0 to 1
- keys:
- 0 _col0 (type: int)
- 1 _col0 (type: int)
- outputColumnNames: _col0, _col2
- Statistics: Num rows: 2 Data size: 27 Basic stats: COMPLETE Column stats: NONE
- Select Operator
- expressions: _col0 (type: int), _col2 (type: string)
- outputColumnNames: _col0, _col1
- Statistics: Num rows: 2 Data size: 27 Basic stats: COMPLETE Column stats: NONE
- File Output Operator
- compressed: false
- Statistics: Num rows: 2 Data size: 27 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
- Local Work:
- Map Reduce Local Work
- Stage: Stage-0
- Fetch Operator
- limit: -1
- Processor Tree:
- ListSink
在第二条sql语句前再加上 explain,获得以下結果
- hive (default)> explain select a.id,b.user_name from(select * from test1 where id>2 ) a join test2 b on a.id=b.id;
- OK
- Explain
- STAGE DEPENDENCIES:
- Stage-4 is a root stage
- Stage-3 depends on stages: Stage-4
- Stage-0 depends on stages: Stage-3
- STAGE PLANS:
- Stage: Stage-4
- Map Reduce Local Work
- Alias -> Map Local Tables:
- $hdt$_0:test1
- Fetch Operator
- limit: -1
- Alias -> Map Local Operator Tree:
- $hdt$_0:test1
- TableScan
- alias: test1
- Statistics: Num rows: 6 Data size: 75 Basic stats: COMPLETE Column stats: NONE
- Filter Operator
- predicate: (id > 2) (type: boolean)
- Statistics: Num rows: 2 Data size: 25 Basic stats: COMPLETE Column stats: NONE
- Select Operator
- expressions: id (type: int)
- outputColumnNames: _col0
- Statistics: Num rows: 2 Data size: 25 Basic stats: COMPLETE Column stats: NONE
- HashTable Sink Operator
- keys:
- 0 _col0 (type: int)
- 1 _col0 (type: int)
- Stage: Stage-3
- Map Reduce
- Map Operator Tree:
- TableScan
- alias: b
- Statistics: Num rows: 6 Data size: 75 Basic stats: COMPLETE Column stats: NONE
- Filter Operator
- predicate: (id > 2) (type: boolean)
- Statistics: Num rows: 2 Data size: 25 Basic stats: COMPLETE Column stats: NONE
- Select Operator
- expressions: id (type: int), user_name (type: string)
- outputColumnNames: _col0, _col1
- Statistics: Num rows: 2 Data size: 25 Basic stats: COMPLETE Column stats: NONE
- Map Join Operator
- condition map:
- Inner Join 0 to 1
- keys:
- 0 _col0 (type: int)
- 1 _col0 (type: int)
- outputColumnNames: _col0, _col2
- Statistics: Num rows: 2 Data size: 27 Basic stats: COMPLETE Column stats: NONE
- Select Operator
- expressions: _col0 (type: int), _col2 (type: string)
- outputColumnNames: _col0, _col1
- Statistics: Num rows: 2 Data size: 27 Basic stats: COMPLETE Column stats: NONE
- File Output Operator
- compressed: false
- Statistics: Num rows: 2 Data size: 27 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
- Local Work:
- Map Reduce Local Work
- Stage: Stage-0
- Fetch Operator
- limit: -1
- Processor Tree:
- ListSink
大伙儿有哪些发觉,除开表别称不一样,别的的执行计划彻底一样,全是先开展 where 标准过虑,在开展 join 标准关系。表明 hive 最底层会全自动帮大家开展提升,因此 这两根sql语句实行高效率是一样的。
最终
之上仅例举了3个大家生产制造中既了解又有点儿糊涂的事例,explain 也有许多别的的主要用途,如查询stage的依靠状况、清查数据倾斜、hive 调优评,朋友们能够自主试着。