What is Bucket map side join?

Introduction to Bucket Map Join For suppose if one table has 2 buckets then the other table must have either 2 buckets or a multiple of 2 buckets (2, 4, 6, and so on). Further, since the preceding condition is satisfied then the joining can be done on the mapper side only. Else a normal inner join is performed.

What is MAP join and SMB join in Hive?

In SMB join in Hive, each mapper reads a bucket from the first table and the corresponding bucket from the second table and then a merge sort join is performed. Sort Merge Bucket (SMB) join in hive is mainly used as there is no limit on file or partition or table join.

What are the joins in Hive?

Moreover, there are several types of Hive join – HiveQL Select Joins: Hive inner join, hive left outer join, hive right outer join, and hive full outer join. We will also learn Hive Join tables in depth.

How does map join work?

Map join is a feature used in Hive queries to increase its efficiency in terms of speed. Join is a condition used to combine the data from 2 tables. So, when we perform a normal join, the job is sent to a Map-Reduce task which splits the main task into 2 stages – “Map stage” and “Reduce stage”.

What is Bucket join?

It means that only the matching buckets of small tables are replicated onto each mapper while joining. By doing this, the efficiency of the query is improved drastically. In a bucket map join, data is not sorted.

How does Hive join work?

First, let’s discuss how join works in Hive. A common join operation will be compiled to a MapReduce task, as shown in figure 1. A common join task involves a map stage and a reduce stage. A mapper reads from join tables and emits the join key and join value pair into an intermediate file.

What is the difference between left join and left outer join?

There really is no difference between a LEFT JOIN and a LEFT OUTER JOIN. Both versions of the syntax will produce the exact same result in PL/SQL. Some people do recommend including outer in a LEFT JOIN clause so it’s clear that you’re creating an outer join, but that’s entirely optional.

What is the difference between left join and left outer join in Hive?

There is actually no difference between a left join and a left outer join – they both refer to the exact same operation in SQL. An example will help clear this up.

When should we use Map side join?

The Map side join and the reduce side join. Map side join is usually used when one data set is large and the other data set is small. Whereas the Reduce side join can join both the large data sets. The Map side join is faster as it does not have to wait for all mappers to complete as in case of reducer.