索引是标准的数据库技术，hive 0.7版本之后支持索引。hive索引采用的不是'one size fites all'的索引实现方式，而是提供插入式接口，并且提供一个具体的索引实现作为参考。Hive的Index接口如下：
public interface HiveIndexHandler extends Configurable { /** * Determines whether this handler implements indexes by creating an index * table. * * @return true if index creation implies creation of an index table in Hive; * false if the index representation is not stored in a Hive table */ boolean usesIndexTable(); /** * Requests that the handler validate an index definition and fill in * additional information about its stored representation. * @throw HiveException if the index definition is invalid with respect to * either the base table or the supplied index table definition */ void analyzeIndexDefinition( org.apache.hadoop.hive.metastore.api.Table baseTable, org.apache.hadoop.hive.metastore.api.Index index, org.apache.hadoop.hive.metastore.api.Table indexTable) throws HiveException; /** * Requests that the handler generate a plan for building the index; the plan * should read the base table and write out the index representation. */ List<Task<?>> generateIndexBuildTaskList( org.apache.hadoop.hive.ql.metadata.Table baseTbl, org.apache.hadoop.hive.metastore.api.Index index, List<Partition> indexTblPartitions, List<Partition> baseTblPartitions, org.apache.hadoop.hive.ql.metadata.Table indexTbl, Set<ReadEntity> inputs, Set<WriteEntity> outputs) throws HiveException; }
创建索引的时候，Hive首先调用接口的usesIndexTable方法，判断索引是否是已Hive Table的方式存储（默认的实现是存储在Hive中的）。然后调用analyzeIndexDefinition分析索引创建语句是否合法，如果没有问题将在元数据标IDXS中添加索引表，否则抛出异常。如果索引创建语句中使用with deferred rebuild，在执行alter index xxx_index on xxx rebuild时将调用generateIndexBuildTaskList获取Index的MapReduce，并执行为索引填充数据。
下面是借鉴别人设计的测试索引的例子：
首先生成测试数据：
#! /bin/bash #generating 350M raw data. i=0 while [ $i -ne 1000000 ] do echo -e "$i\tA decade ago, many were predicting that Cooke, a New York City prodigy, would become a basketball shoe pitchman and would flaunt his wares and skills at All-Star weekends like the recent aerial show in Orlando, Fla. There was a time, however fleeting, when he was more heralded, or perhaps merely hyped, than any other high school player in America." i=$(($i+1)) done
创建测试表：
hive> create table table01( id int, name string)
> ROW FORMAT DELIMITED
> FIELDS TERMINATED BY '\t';
OK
Time taken: 0.371 seconds
hive> load data local inpath '/home/hadoop/hive_index_test/dual.txt' overwrite into table table01;
Copying data from file:/home/hadoop/hive_index_test/dual.txt
Copying file: file:/home/hadoop/hive_index_test/dual.txt
Loading data to table default.table01
Deleted hdfs://localhost:9000/user/hive/warehouse/table01
OK
Time taken: 13.492 seconds
hive> create table table02 as select id,name as text from table01;
Total MapReduce jobs = 2
Launching Job 1 out of 2
Number of reduce tasks is set to 0 since there's no reduce operator
Starting Job = job_201301221042_0006, Tracking URL = http://localhost:50030/jobdetails.jsp?jobid=job_201301221042_0006
Kill Command = /usr/local/hadoop/bin/../bin/hadoop job -Dmapred.job.tracker=localhost:9001 -kill job_201301221042_0006
2013-01-22 11:21:19,639 Stage-1 map = 0%, reduce = 0%
2013-01-22 11:21:25,678 Stage-1 map = 33%, reduce = 0%
2013-01-22 11:21:37,754 Stage-1 map = 67%, reduce = 0%
2013-01-22 11:21:43,788 Stage-1 map = 100%, reduce = 0%
2013-01-22 11:21:46,828 Stage-1 map = 100%, reduce = 100%
Ended Job = job_201301221042_0006
Ended Job = -663277165, job is filtered out (removed at runtime).
Moving data to: hdfs://localhost:9000/tmp/hive-hadoop/hive_2013-01-22_11-21-13_661_2061036951988537032/-ext-10001
Moving data to: hdfs://localhost:9000/user/hive/warehouse/table02
1000000 Rows loaded to hdfs://localhost:9000/tmp/hive-hadoop/hive_2013-01-22_11-21-13_661_2061036951988537032/-ext-10000
OK
Time taken: 33.904 seconds
hive> dfs -ls /user/hive/warehouse/table02;
Found 6 items
-rw-r--r-- 3 hadoop supergroup 67109134 2013-01-22 11:21 /user/hive/warehouse/table02/000000_0
-rw-r--r-- 3 hadoop supergroup 67108860 2013-01-22 11:21 /user/hive/warehouse/table02/000001_0
-rw-r--r-- 3 hadoop supergroup 67108860 2013-01-22 11:21 /user/hive/warehouse/table02/000002_0
-rw-r--r-- 3 hadoop supergroup 67108860 2013-01-22 11:21 /user/hive/warehouse/table02/000003_0
-rw-r--r-- 3 hadoop supergroup 67108860 2013-01-22 11:21 /user/hive/warehouse/table02/000004_0
-rw-r--r-- 3 hadoop supergroup 21344316 2013-01-22 11:21 /user/hive/warehouse/table02/000005_0
hive> select * from table02 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
Starting Job = job_201301221042_0007, Tracking URL = http://localhost:50030/jobdetails.jsp?jobid=job_201301221042_0007
Kill Command = /usr/local/hadoop/bin/../bin/hadoop job -Dmapred.job.tracker=localhost:9001 -kill job_201301221042_0007
2013-01-22 11:22:26,865 Stage-1 map = 0%, reduce = 0%
2013-01-22 11:22:28,884 Stage-1 map = 33%, reduce = 0%
2013-01-22 11:22:31,905 Stage-1 map = 67%, reduce = 0%
2013-01-22 11:22:34,921 Stage-1 map = 100%, reduce = 0%
2013-01-22 11:22:37,943 Stage-1 map = 100%, reduce = 100%
Ended Job = job_201301221042_0007
OK
500000 A decade ago, many were predicting that Cooke, a New York City prodigy, would become a basketball shoe pitchman and would flaunt his wares and skills at All-Star weekends like the recent aerial show in Orlando, Fla. There was a time, however fleeting, when he was more heralded, or perhaps merely hyped, than any other high school player in America.
Time taken: 18.551 seconds
创建索引：
hive> create index table02_index on table table02(id)
> as 'org.apache.hadoop.hive.ql.index.compact.CompactIndexHandler'
> with deferred rebuild;
OK
Time taken: 0.503 seconds
填充索引数据：
hive> alter index table02_index on table02 rebuild;
Total MapReduce jobs = 1
Launching Job 1 out of 1
Number of reduce tasks not specified. Estimated from input data size: 1
In order to change the average load for a reducer (in bytes):
set hive.exec.reducers.bytes.per.reducer=<number>
In order to limit the maximum number of reducers:
set hive.exec.reducers.max=<number>
In order to set a constant number of reducers:
set mapred.reduce.tasks=<number>
Starting Job = job_201301221042_0008, Tracking URL = http://localhost:50030/jobdetails.jsp?jobid=job_201301221042_0008
Kill Command = /usr/local/hadoop/bin/../bin/hadoop job -Dmapred.job.tracker=localhost:9001 -kill job_201301221042_0008
2013-01-22 11:23:56,870 Stage-1 map = 0%, reduce = 0%
2013-01-22 11:24:02,902 Stage-1 map = 33%, reduce = 0%
2013-01-22 11:24:08,929 Stage-1 map = 67%, reduce = 0%
2013-01-22 11:24:11,944 Stage-1 map = 67%, reduce = 11%
2013-01-22 11:24:14,966 Stage-1 map = 100%, reduce = 11%
2013-01-22 11:24:21,007 Stage-1 map = 100%, reduce = 22%
2013-01-22 11:24:27,043 Stage-1 map = 100%, reduce = 67%
2013-01-22 11:24:30,056 Stage-1 map = 100%, reduce = 86%
2013-01-22 11:24:33,089 Stage-1 map = 100%, reduce = 100%
Ended Job = job_201301221042_0008
Loading data to table default.default__table02_table02_index__
Deleted hdfs://localhost:9000/user/hive/warehouse/default__table02_table02_index__
Table default.default__table02_table02_index__ stats: [num_partitions: 0, num_files: 1, num_rows: 0, total_size: 74701985]
OK
Time taken: 61.203 seconds
hive> dfs -ls /user/hive/warehouse/default*;
Found 1 items
-rw-r--r-- 3 hadoop supergroup 74701985 2013-01-22 11:24 /user/hive/warehouse/default__table02_table02_index__/000000_0
可以看到索引内存储的数据：
hive> select * from default__table02_table02_index__ limit 3;
OK
0 hdfs://localhost:9000/user/hive/warehouse/table02/000000_0 [0]
1 hdfs://localhost:9000/user/hive/warehouse/table02/000000_0 [352]
2 hdfs://localhost:9000/user/hive/warehouse/table02/000000_0 [704]
Time taken: 0.156 seconds
自己做一个索引文件测试：
hive> SET hive.input.format=org.apache.hadoop.hive.ql.io.HiveInputFormat;
hive> Insert overwrite directory "/tmp/table02_index_data" select `_bucketname`, `_offsets` from default__table02_table02_index__ where id =500000;
Total MapReduce jobs = 2
Launching Job 1 out of 2
Number of reduce tasks is set to 0 since there's no reduce operator
Starting Job = job_201301221042_0009, Tracking URL = http://localhost:50030/jobdetails.jsp?jobid=job_201301221042_0009
Kill Command = /usr/local/hadoop/bin/../bin/hadoop job -Dmapred.job.tracker=localhost:9001 -kill job_201301221042_0009
2013-01-22 11:30:23,859 Stage-1 map = 0%, reduce = 0%
2013-01-22 11:30:26,872 Stage-1 map = 100%, reduce = 0%
2013-01-22 11:30:29,904 Stage-1 map = 100%, reduce = 100%
Ended Job = job_201301221042_0009
Ended Job = -489547412, job is filtered out (removed at runtime).
Launching Job 2 out of 2
Number of reduce tasks is set to 0 since there's no reduce operator
Starting Job = job_201301221042_0010, Tracking URL = http://localhost:50030/jobdetails.jsp?jobid=job_201301221042_0010
Kill Command = /usr/local/hadoop/bin/../bin/hadoop job -Dmapred.job.tracker=localhost:9001 -kill job_201301221042_0010
2013-01-22 11:30:35,861 Stage-2 map = 0%, reduce = 0%
2013-01-22 11:30:38,882 Stage-2 map = 100%, reduce = 0%
2013-01-22 11:30:41,907 Stage-2 map = 100%, reduce = 100%
Ended Job = job_201301221042_0010
Moving data to: /tmp/table02_index_data
1 Rows loaded to /tmp/table02_index_data
OK
Time taken: 25.173 seconds
hive> select * from table02 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
Starting Job = job_201301221042_0011, Tracking URL = http://localhost:50030/jobdetails.jsp?jobid=job_201301221042_0011
Kill Command = /usr/local/hadoop/bin/../bin/hadoop job -Dmapred.job.tracker=localhost:9001 -kill job_201301221042_0011
2013-01-22 11:31:06,055 Stage-1 map = 0%, reduce = 0%
2013-01-22 11:31:09,066 Stage-1 map = 33%, reduce = 0%
2013-01-22 11:31:12,083 Stage-1 map = 67%, reduce = 0%
2013-01-22 11:31:15,102 Stage-1 map = 100%, reduce = 0%
2013-01-22 11:31:18,127 Stage-1 map = 100%, reduce = 100%
Ended Job = job_201301221042_0011
OK
500000 A decade ago, many were predicting that Cooke, a New York City prodigy, would become a basketball shoe pitchman and would flaunt his wares and skills at All-Star weekends like the recent aerial show in Orlando, Fla. There was a time, however fleeting, when he was more heralded, or perhaps merely hyped, than any other high school player in America.
Time taken: 17.533 seconds
hive> Set hive.index.compact.file=/tmp/table02_index_data;
hive> Set hive.optimize.index.filter=false;
hive> Set hive.input.format=org.apache.hadoop.hive.ql.index.compact.HiveCompactIndexInputFormat;
hive> select * from table02 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
Starting Job = job_201301221042_0012, Tracking URL = http://localhost:50030/jobdetails.jsp?jobid=job_201301221042_0012
Kill Command = /usr/local/hadoop/bin/../bin/hadoop job -Dmapred.job.tracker=localhost:9001 -kill job_201301221042_0012
2013-01-22 11:32:14,929 Stage-1 map = 0%, reduce = 0%
2013-01-22 11:32:17,942 Stage-1 map = 100%, reduce = 0%
2013-01-22 11:32:20,968 Stage-1 map = 100%, reduce = 100%
Ended Job = job_201301221042_0012
OK
500000 A decade ago, many were predicting that Cooke, a New York City prodigy, would become a basketball shoe pitchman and would flaunt his wares and skills at All-Star weekends like the recent aerial show in Orlando, Fla. There was a time, however fleeting, when he was more heralded, or perhaps merely hyped, than any other high school player in America.
Time taken: 11.222 seconds
总结：索引表的基本包含几列：1. 源表的索引列；2. _bucketname hdfs中文件地址 3. 索引列在hdfs文件中的偏移量。原理是通过记录索引列在HDFS中的偏移量，精准获取数据，避免全表扫描。
参考资料：http://blog.csdn.net/liwei_1988/article/details/7319030
