Hive创建索引
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索引是标准的数据库技术,hive0.7版本之后支持索引。Hive提供有限的索引功能,这不像传统的关系型数据库那样有"键(Key)"的概念,用户可以再某些列上创建索引来加速某些操作,给一个表创建的索引数据被保存在另外的表中。Hive的索引功能现在还相对较晚,提供的选项还较少。但是,索引被设计为可使用内置的可插拔的java代码来控制,用户可以扩展这个功能来满足自己的需求。当然不是说所有的查询都会受惠于Hive索引。用户可以使用EXPLAIN语法来分析HiveQL语句是否可以使用索引来提升用户查询的性能。像RDBMS中的索引一样,需要评估索引创建的是否合理,毕竟,索引需要更多的磁盘空间,并且创建维护索引也会有一定的的代价。用户必须要权衡从索引得到的好处和代价。
下面说说怎么创建索引:
1.先创建表:
- hive> create table user( id int, name string)
- > ROW FORMAT DELIMITED
- > FIELDS TERMINATED BY ‘\t‘
- > STORED AS TEXTFILE;
2.导入数据:
- hive> load data local inpath ‘/export1/tmp/wyp/row.txt‘
- > overwrite into table user;
3.创建索引之前测试:
- hive> select * from user where id =500000;
- Total MapReduce jobs = 1
- Launching Job 1 out of 1
- Number of reduce tasks is set to 0 since there‘s no reduce operator
- Cannot run job locally: Input Size (= 356888890) is larger than
- hive.exec.mode.local.auto.inputbytes.max (= 134217728)
- Starting Job = job_1384246387966_0247, Tracking URL =
- http://l-datalogm1.data.cn1:9981/proxy/application_1384246387966_0247/
- Kill Command=/home/q/hadoop/bin/hadoop job -kill job_1384246387966_0247
- Hadoop job information for Stage-1: number of mappers:2; number of reducers:0
- 2013-11-13 15:09:53,336 Stage-1 map = 0%, reduce = 0%
- 2013-11-13 15:09:59,500 Stage-1 map=50%,reduce=0%, Cumulative CPU 2.0 sec
- 2013-11-13 15:10:00,531 Stage-1 map=100%,reduce=0%, Cumulative CPU 5.63 sec
- 2013-11-13 15:10:01,560 Stage-1 map=100%,reduce=0%, Cumulative CPU 5.63 sec
- MapReduce Total cumulative CPU time: 5 seconds 630 msec
- Ended Job = job_1384246387966_0247
- MapReduce Jobs Launched:
- Job 0: Map: 2 Cumulative CPU: 5.63 sec
- HDFS Read: 361084006 HDFS Write: 357 SUCCESS
- Total MapReduce CPU Time Spent: 5 seconds 630 msec
- OK
- 500000 wyp.
- Time taken: 14.107 seconds, Fetched: 1 row(s)
一共用了14.107s
4.对user创建索引:
- hive> create index user_index on table user(id)
- > as ‘org.apache.hadoop.hive.ql.index.compact.CompactIndexHandler‘
- > with deferred rebuild
- > IN TABLE user_index_table;
- hive> alter index user_index on user rebuild;
- hive> select * from user_index_table limit 5;
- 0 hdfs://mycluster/user/hive/warehouse/table02/000000_0 [0]
- 1 hdfs://mycluster/user/hive/warehouse/table02/000000_0 [352]
- 2 hdfs://mycluster/user/hive/warehouse/table02/000000_0 [704]
- 3 hdfs://mycluster/user/hive/warehouse/table02/000000_0 [1056]
- 4 hdfs://mycluster/user/hive/warehouse/table02/000000_0 [1408]
- Time taken: 0.244 seconds, Fetched: 5 row(s)
这样就对user表创建好了一个索引。
5.对创建索引后的user再进行测试:
- hive> select * from user where id =500000;
- Total MapReduce jobs = 1
- Launching Job 1 out of 1
- Number of reduce tasks is set to 0 since there‘s no reduce operator
- Cannot run job locally: Input Size (= 356888890) is larger than
- hive.exec.mode.local.auto.inputbytes.max (= 134217728)
- Starting Job = job_1384246387966_0247, Tracking URL =
- http://l-datalogm1.data.cn1:9981/proxy/application_1384246387966_0247/
- Kill Command=/home/q/hadoop/bin/hadoop job -kill job_1384246387966_0247
- Hadoop job information for Stage-1: number of mappers:2; number of reducers:0
- 2013-11-13 15:23:12,336 Stage-1 map = 0%, reduce = 0%
- 2013-11-13 15:23:53,240 Stage-1 map=50%,reduce=0%, Cumulative CPU 2.0 sec
- 2013-11-13 15:24:00,253 Stage-1 map=100%,reduce=0%, Cumulative CPU 5.27 sec
- 2013-11-13 15:24:01,650 Stage-1 map=100%,reduce=0%, Cumulative CPU 5.27 sec
- MapReduce Total cumulative CPU time: 5 seconds 630 msec
- Ended Job = job_1384246387966_0247
- MapReduce Jobs Launched:
- Job 0: Map: 2 Cumulative CPU: 5.63 sec
- HDFS Read: 361084006 HDFS Write: 357 SUCCESS
- Total MapReduce CPU Time Spent: 5 seconds 630 msec
- OK
- 500000 wyp.
- Time taken: 13.042 seconds, Fetched: 1 row(s)
时间用了13.042s,这和没有建索引的效果差别不大。
在Hive创建索引还存在bug:如果表格的模式信息来自SerDe,Hive将不能创建索引:
- hive> CREATE INDEX employees_index
- > ON TABLE employees (country)
- > AS ‘org.apache.hadoop.hive.ql.index.compact.CompactIndexHandler‘
- > WITH DEFERRED REBUILD
- > IDXPROPERTIES (‘creator‘ = ‘me‘,‘created_at‘ = ‘some_time‘)
- > IN TABLE employees_index_table
- > COMMENT ‘Employees indexed by country and name.‘;
- FAILED: Error in metadata: java.lang.RuntimeException: \
- Check the index columns, they should appear in the table being indexed.
- FAILED: Execution Error, return code 1 from \
- org.apache.hadoop.hive.ql.exec.DDLTask
这个bug发生在Hive0.10.0、0.10.1、0.11.0,在Hive0.12.0已经修复了,详情请参见:https://issues.apache.org/jira/browse/HIVE-4251
文章来自过往记忆:http://www.iteblog.com/archives/836
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