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Hive笔记

cluster by vs order by vs sort by

  • ORDER BY x: guarantees global ordering, but does this by pushing all data through just one reducer. This is basically unacceptable for large datasets. You end up one sorted file as output.

  • SORT BY x: orders data at each of N reducers, but each reducer can receive overlapping ranges of data. You end up with N or more sorted files with overlapping ranges.

  • DISTRIBUTE BY x: ensures each of N reducers gets non-overlapping ranges of x, but doesn’t sort the output of each reducer. You end up with N or unsorted files with non-overlapping ranges.

  • CLUSTER BY x: ensures each of N reducers gets non-overlapping ranges, then sorts by those ranges at the reducers. This gives you global ordering, and is the same as doing (DISTRIBUTE BY x and SORT BY x). You end up with N or more sorted files with non-overlapping ranges.

Hive NULL处理

默认情况下,每行数据在被输入script前,列会被转换为 \t 分隔的string,所有 NULL 值会被转换为字符串 \N,目的是为了与空string(长度为0)区分开。script输出一行数据时,标准输出会将输出是为 \t 分隔的列,同时 \N 被转换为 NULL

因此Hive的空值处理分为:

  • NULL\N

    HIVE底层是用\N代替NULL进行保存的,可以通过alter table name SET SERDEPROPERTIES('serialization.null.format' = '\N'); 来更改底层以什么样的字符来存储NULL。

    查询空值字段可用:

    1
    a is null 或 a = '\\N'

    相应的在script在判断是否为 NULL 时,是需要判断!= '\N'

  • 空string

    '' 表示长度为0的 string,但此时字段并非为 NULL,而是一个长度为0的 string。

    查询空string可用:

    1
    a = '' 或 length(a)=0

    在使用outer join的时候尤其要注意以上区别,要想清楚究竟是要判断字段为 NULL,还是判断string为空。

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    select case when t.idfa != '' and t.idfa != null then t.idfa end from (select 'B5BF3011-A8A0-46A2-B8C3-0A1976FDD4BE' as idfa) as t;
    select case when t.idfa != '' and t.idfa != NULL then t.idfa end from (select 'B5BF3011-A8A0-46A2-B8C3-0A1976FDD4BE' as idfa) as t;
    select case when t.idfamd5 != null then '1' else '2' end from (select '\N' as idfamd5) as t;
    select case when t.idfamd5 != null then '1' else '2' end from (select NULL as idfamd5) as t;

Insert into bucketed table

Hive 2.x之前,向分桶表中插入数据时,需要(二者之一),

  • 方法1:set hive.enforce.bucketing = true

  • 方法2:

    1. 设置reducer数目,set mapred.reduce.tasks,使其等于bucket的数目
    2. 在select中使用,cluster by