作者: 大圆那些事 | 文章可以转载，请以超链接形式标明文章原始出处和作者信息
网址: http://www.cnblogs.com/panfeng412/archive/2013/04/29/hive-view-usage-and-analysis.html
本文是对Hive中逻辑视图的介绍，通过一个简单的视图例子，说明其使用方法及执行过程。
Hive 0.6版本及以上支持视图（View，详见Hive的RELEASE_NOTES.txt），Hive View具有以下特点：
1）View是逻辑视图，暂不支持物化视图（后续将在1.0.3版本以后支持）；
2）View是只读的，不支持LOAD/INSERT/ALTER。需要改变View定义，可以是用Alter View；
3）View内可能包含ORDER BY/LIMIT语句，假如一个针对View的查询也包含这些语句， 则View中的语句优先级高；
4）支持迭代View。
CDH4中自带的Hive版本为0.10.0，支持的View是逻辑视图，因此本质上来说View只是为了使用上的方便，从执行效率上来说没有区别，甚至可能因为要多一次对MetaStore元数据的操作效率略有下降（这里只是一种理论上的推测，实际可能看不出太大区别）。
下面是简单的验证过程（感兴趣的可以看下，以下过程如有问题，可以一起交流）：
1）创建一个测试表：
hive> create table test (id int, name string); OK Time taken: 0.19 seconds hive> desc test; OK id int name string Time taken: 0.16 seconds
2）创建一个View之前，使用explain命令查看创建View的命令是如何被Hive解释执行的：
hive> explain create view test_view (id, name_length) as select id, length(name) from test; OK ABSTRACT SYNTAX TREE:(TOK_CREATEVIEW (TOK_TABNAME test_view) (TOK_TABCOLNAME (TOK_TABCOL id TOK_NULL) (TOK_TABCOL name_length TOK_NULL)) (TOK_QUERY (TOK_FROM (TOK_TABREF (TOK_TABNAME test))) (TOK_INSERT (TOK_DESTINATION (TOK_DIR TOK_TMP_FILE)) (TOK_SELECT (TOK_SELEXPR (TOK_TABLE_OR_COL id)) (TOK_SELEXPR (TOK_FUNCTION length (TOK_TABLE_OR_COL name))))))) STAGE DEPENDENCIES:Stage-0 is a root stage STAGE PLANS:Stage: Stage-0Create View Operator:Create Viewif not exists: falseor replace: falsecolumns: id int, name_length intexpanded text: SELECT `id` AS `id`, `_c1` AS `name_length` FROM (select `test`.`id`, length(`test`.`name`) from `default`.`test`) `test_view`name: test_vieworiginal text: select id, length(name) from test Time taken: 0.088 seconds
可见，创建View的过程解释后并没有实际执行Map Reduce的Stage，只包含一个Create View Operator的Stage，这个阶段只是对MySQL MetaStore进行元数据操作，记录View的相关元数据而已。
3）接下来，实际创建这个View：
hive> create view test_view (id, name_length) as select id, length(name) from test; OK Time taken: 0.1 seconds
4）执行这个View之前，先explain查看实际被翻译后的执行过程：
hive> explain select name_length from test_view; OK ABSTRACT SYNTAX TREE:(TOK_QUERY (TOK_FROM (TOK_TABREF (TOK_TABNAME test_view))) (TOK_INSERT (TOK_DESTINATION (TOK_DIR TOK_TMP_FILE)) (TOK_SELECT (TOK_SELEXPR (TOK_TABLE_OR_COL name_length))))) STAGE DEPENDENCIES:Stage-1 is a root stageStage-0 is a root stage STAGE PLANS:Stage: Stage-1Map ReduceAlias -> Map Operator Tree:test_view:test_view:testTableScanalias: testSelect Operatorexpressions:expr: length(name)type: intoutputColumnNames: _col1Select Operatorexpressions:expr: _col1type: intoutputColumnNames: _col1Select Operatorexpressions:expr: _col1type: intoutputColumnNames: _col0File Output Operatorcompressed: falseGlobalTableId: 0table:input format: org.apache.hadoop.mapred.TextInputFormatoutput format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormatStage: Stage-0Fetch Operatorlimit: -1 Time taken: 0.107 seconds
可以看出，对View进行的查找过程，实际还是对原始test表进行的查询操作（分为Stage-0和Stage-1两个阶段）。
5）最后，实际对这个View执行一次查询，显示Stage-1阶段对原始表test进行了MapReduce过程：
hive> select name_length from test_view; Total MapReduce jobs = 1 Launching Job 1 out of 1 Number of reduce tasks is set to 0 since there's no reduce operator Starting Job = job_201303092253_0057, Tracking URL = http://jobtracker.host:50030/jobdetails.jsp?jobid=job_201303092253_0057 Kill Command = /usr/lib/hadoop/bin/hadoop job -kill job_201303092253_0057 Hadoop job information for Stage-1: number of mappers: 1; number of reducers: 0 2013-03-13 22:43:39,044 Stage-1 map = 0%, reduce = 0% 2013-03-13 22:43:42,074 Stage-1 map = 100%, reduce = 0%, Cumulative CPU 0.73 sec 2013-03-13 22:43:43,086 Stage-1 map = 100%, reduce = 0%, Cumulative CPU 0.73 sec 2013-03-13 22:43:44,098 Stage-1 map = 100%, reduce = 0%, Cumulative CPU 0.73 sec 2013-03-13 22:43:45,113 Stage-1 map = 100%, reduce = 100%, Cumulative CPU 0.73 sec MapReduce Total cumulative CPU time: 730 msec Ended Job = job_201303092253_0057 MapReduce Jobs Launched: Job 0: Map: 1 Cumulative CPU: 0.73 sec HDFS Read: 250 HDFS Write: 0 SUCCESS Total MapReduce CPU Time Spent: 730 msec OK Time taken: 15.793 seconds
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