Страницата на картата на Hadoop се присъединява към условна задача - сауоп, кошер, hiveql, mapper

Уча се да се присъединява към карта на кошера и попаднах на условна задача. Имам следните въпроси относно условната задача

1. Каква е условната задача при присъединяването към картата на hadoop?
2. Как условната задача помага при идентифицирането на малката таблица в картата?

3. каква е разликата между следните свойства на кошера и тяхното значение

   задайте hive.auto.convert.join.noconditionaltask.size;

   задайте hive.mapjoin.smalltable.filesize;

Може ли някой да отговори на горните въпроси и да ми помогне да разбера условната задача в присъединяването към картата.

Отговори:

    1. What is a conditional task in hadoop map join ?

During compilation time, the query processor generates a
conditional task containing a list of tasks and among this one of the tasks gets resolved to run during execution time.



2. How does conditional task help in identifying the small table in map join?

During the execution stage, the conditional task knows the exact file size of each input table, even if the table is an intermediate one. If all the tables are too large to be converted into map join, then just run the common join task as previously. If one of the tables is large and others are small enough to run map join, then the conditional task will pick the corresponding map join local task to run. By this mechanism, it can convert the common join into a map join automatically and dynamically.

3. What is difference between the following hive properties and their significance


hive. Auto. Convert. Join

default value: false

This is used for auto join conversion. Once auto join is enabled, you need not specify the map-join in the query.

hive. Auto. Convert. Join. Noconditionaltask

default value: true



This controls whether hive should enable the optimization of converting common join into map-join based on the input file size or not.If the total size of small tables is larger than 25MB, then the conditional task will choose the original common join to run.

hive. Auto. Convert. Join. Noconditionaltask. Size

default value: 10000000

If the sum of size for n-1 of the tables/partitions for an n-way join is smaller than this size, the join is directly converted to a map join (there is no conditional task). The default is 10mb. But this is dependent on hive. Auto. Convert. Join. Noconditionaltask and works only when it is true.

За повече информация: https://cwiki.apache.org/confluence/download/attachments/27362054/Hive+Summit+2011-join.pdf