Hive常见问题

1.HQL是否区分大小写　　不区分

hive> select AGE from default.studeNT;  --不区分大小写，即使是表中字段

2.查看创建表过程　　show create table

hive> show create table default.student; --查看创建表过程的命令
OK
createtab_stmt
CREATE  TABLE `default.student`(
  `age` int, 
  `name` string)
ROW FORMAT DELIMITED 
  FIELDS TERMINATED BY '	'   --行内分隔符
STORED AS INPUTFORMAT 
  'org.apache.hadoop.mapred.TextInputFormat' 
OUTPUTFORMAT 
  'org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat'
LOCATION
  'hdfs://hadoop01:9000/user/hive/warehouse/student'
TBLPROPERTIES (
  'numFiles'='1', 
  'last_modified_by'='root', 
  'last_modified_time'='1439795658', 
  'COLUMN_STATS_ACCURATE'='true', 
  'transient_lastDdlTime'='1439795895', 
  'numRows'='0', 
  'totalSize'='74', 
  'rawDataSize'='0')
Time taken: 0.063 seconds, Fetched: 20 row(s)

3.hive是否支持跨数据库查询？　　支持

hive> select * from default.logs; --和普通sql一样，数据库名.数据表名

4.hive是否可以打印列名?　　set hive.cli.print.header=true;

hive> select * from default.student limit 5;
OK
1       zhangsan   --显示结果不包含列名
2       lisi
3       wangwu
4       zhaoba
5       liliuy
Time taken: 0.047 seconds, Fetched: 5 row(s)
hive> set hive.cli.print.header=true;       --设置true，即显示列名
hive> select * from default.student limit 5;
OK
student.age     student.name  --显示结果包含列名
1       zhangsan
2       lisi
3       wangwu
4       zhaoba
5       liliuy
Time taken: 0.05 seconds, Fetched: 5 row(s)

5.hive是否可以设置reduce的个数　　set mapred.reduce.tasks=5;

hive> set mapred.reduce.tasks;                       
mapred.reduce.tasks=-1 --默认reduce个数为-1
hive> insert overwrite local directory '/root/hive/a' --只能产生一个结果文件
    > row format delimited fields terminated by '	'
    > select * from test.t081901                     
    > distribute by length(country)                  
    > sort by te;  

hive> set mapred.reduce.tasks=4;      --设置4个reduce               
hive> insert overwrite local directory '/root/hive/a' --产生4个结果文件
    > row format delimited fields terminated by '	' 
    > select * from test.t081901                     
    > distribute by length(country)     --和普通的MR一样，当【reduce个数>partition个数】，只有两个(等于分区数)结果文件中有数据。             
    > sort by te; 

6.hive是否支持 exist 或 in 查询吗?　　支持

hive> select * from default.student where student.age in(3,5,29);
OK
student.age     student.name
3       wangwu
5       liliuy
Time taken: 10.217 seconds, Fetched: 2 row(s)

7.hive是否可以查询hive表的集群文件位置？　　desc formatted T_NAME

　INPUT__FILE__NAME字段可以查询出Table在Hdfs上的location

hive> desc formatted default.student; --desc formatted <T_Name>
OK
col_name        data_type       comment
# col_name              data_type               comment             
                 
age                     int                                         
name                    string                                      
                 
# Detailed Table Information             
Database:               default       --所属数据库           
Owner:                  root             --Owner        
CreateTime:             Mon Aug 17 14:24:29 SGT 2015     
LastAccessTime:         UNKNOWN                  
Protect Mode:           None                     
Retention:              0                        
Location:               hdfs://hadoop01:9000/user/hive/warehouse/student         --HDFS位置
Table Type:             MANAGED_TABLE            --内部表
Table Parameters:                
        COLUMN_STATS_ACCURATE   true                
        last_modified_by        root                
        last_modified_time      1439795658          
        numFiles                1                   
        numRows                 0                   
        rawDataSize             0                   
        totalSize               74                  
        transient_lastDdlTime   1439795895          
                 
# Storage Information            
SerDe Library:          org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe       
InputFormat:            org.apache.hadoop.mapred.TextInputFormat         
OutputFormat:           org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat   
Compressed:             No                       
Num Buckets:            -1                       
Bucket Columns:         []                       
Sort Columns:           []                       
Storage Desc Params:             
        field.delim             	                  
        serialization.format    	                  
Time taken: 0.078 seconds, Fetched: 35 row(s)

hive>  select name,age,INPUT__FILE__NAME --其中INPUT__FILE__NAME之间是两个下划线
    >  from default.student;
OK
name    age     input__file__name
zhangsan        1       hdfs://hadoop01:9000/user/hive/warehouse/student/student
lisi    2       hdfs://hadoop01:9000/user/hive/warehouse/student/student
wangwu  3       hdfs://hadoop01:9000/user/hive/warehouse/student/student
zhaoba  4       hdfs://hadoop01:9000/user/hive/warehouse/student/student
liliuy  5       hdfs://hadoop01:9000/user/hive/warehouse/student/student
jiujiu  9       hdfs://hadoop01:9000/user/hive/warehouse/student/student
shiba   18      hdfs://hadoop01:9000/user/hive/warehouse/student/student
xiyita  28      hdfs://hadoop01:9000/user/hive/warehouse/student/student
NULL    NULL    hdfs://hadoop01:9000/user/hive/warehouse/student/student
Time taken: 9.902 seconds, Fetched: 9 row(s)

8.hive是否支持本地执行模式

• 支持，当数据量小的时候，本地执行比提交到hadoop集群执行效率提升很大（大批量小作业，尽量采用本地执行模式）
• set hive.exec.mode.local.auto=true;(默认false)
• 满足如下条件才能真正使用本地模式：
• 1、job的数据数据大小必须小于参数hive.exec.mode.local.auto.inputbytes.max(默认值128M)
• 2、job的map处理的文件数小于参数hive.exec.mode.local.auto.input.files.max(默认值4)
• 3、job的reduce数必须小于等于1
• 用参数hive.mapred.local.mem来设置local mode下mapper和reducer task jvm heap size（在本地模式的最大内存量，以字节为单位，0为不限制）

hive> set hive.exec.mode.local.auto=true; --启用本地模式
hive>  select name,age,INPUT__FILE__NAME 
    >  from default.student;             
Automatically selecting local only mode for query
Total jobs = 1
Launching Job 1 out of 1
Number of reduce tasks is set to 0 since there is no reduce operator
Execution log at: /tmp/root/root_20150819122121_4a47f38a-3214-4990-a164-ce5b8a89b88e.log
Job running in-process (local Hadoop)  --本地模式的Hadoop
Hadoop job information for null: number of mappers: 0; number of reducers: 0  --0个mapper和0个reducer
2015-08-19 12:21:51,828 null map = 100%,  reduce = 0%
Ended Job = job_local531808121_0001 --从jobId也可以看出是Local_Job
Execution completed successfully
MapredLocal task succeeded  --本地MR任务完成
OK
name    age     input__file__name
zhangsan        1       hdfs://hadoop01:9000/user/hive/warehouse/student/student
lisi    2       hdfs://hadoop01:9000/user/hive/warehouse/student/student
wangwu  3       hdfs://hadoop01:9000/user/hive/warehouse/student/student
zhaoba  4       hdfs://hadoop01:9000/user/hive/warehouse/student/student
liliuy  5       hdfs://hadoop01:9000/user/hive/warehouse/student/student
jiujiu  9       hdfs://hadoop01:9000/user/hive/warehouse/student/student
shiba   18      hdfs://hadoop01:9000/user/hive/warehouse/student/student
xiyita  28      hdfs://hadoop01:9000/user/hive/warehouse/student/student
NULL    NULL    hdfs://hadoop01:9000/user/hive/warehouse/student/student
Time taken: 5.216 seconds, Fetched: 9 row(s)

hive>  select name,age,INPUT__FILE__NAME  --采用分布式执行MR任务
    >  from default.student;            
Total jobs = 1
Launching Job 1 out of 1
Number of reduce tasks is set to 0 since there is no reduce operator
Starting Job = job_1438754688191_0137,                                   --分布式MR任务的JobId
Tracking URL = http://hadoop01:8088/proxy/application_1438754688191_0137/
Kill Command = /hadoop/hadoop-2.2.0/bin/hadoop job  -kill job_1438754688191_0137 --kill hadoop job的命令
Hadoop job information for Stage-1: number of mappers: 1; number of reducers: 0 --map任务1个，reduce任务0个
2015-08-19 12:17:18,571 Stage-1 map = 0%,  reduce = 0%
2015-08-19 12:17:22,757 Stage-1 map = 100%,  reduce = 0%, Cumulative CPU 0.55 sec
MapReduce Total cumulative CPU time: 550 msec
Ended Job = job_1438754688191_0137
MapReduce Jobs Launched: 
Job 0: Map: 1   Cumulative CPU: 0.55 sec                --1个map任务
HDFS Read: 284 HDFS Write: 592                            --HDFS读取284字节，HDFS写出592字节
SUCCESS Total MapReduce CPU Time Spent: 550 msec
OK
name    age     input__file__name
zhangsan        1       hdfs://hadoop01:9000/user/hive/warehouse/student/student
lisi    2       hdfs://hadoop01:9000/user/hive/warehouse/student/student
wangwu  3       hdfs://hadoop01:9000/user/hive/warehouse/student/student
zhaoba  4       hdfs://hadoop01:9000/user/hive/warehouse/student/student
liliuy  5       hdfs://hadoop01:9000/user/hive/warehouse/student/student
jiujiu  9       hdfs://hadoop01:9000/user/hive/warehouse/student/student
shiba   18      hdfs://hadoop01:9000/user/hive/warehouse/student/student
xiyita  28      hdfs://hadoop01:9000/user/hive/warehouse/student/student
NULL    NULL    hdfs://hadoop01:9000/user/hive/warehouse/student/student
Time taken: 9.902 seconds, Fetched: 9 row(s)

9.hive是否支持自动Map-Join？

支持，设置hive.auto.convert.join=true，根据输入表的文件大小，自动将reduce端common join转为mapjoin，从而加快大表关联小表的速度。（建议开启）