Hive : Understanding the MAPJOIN Operator in Hive with an Example

When dealing with large datasets, optimizing join operations is crucial to improving query performance. One of the techniques to achieve this is by using the MAPJOIN operator. In this article, we will discuss the MAPJOIN operator in detail and demonstrate its practical usage through an example.

What is the MAPJOIN Operator?

The MAPJOIN operator is a query optimization feature in Hive that allows users to perform a Map-side join instead of a Reduce-side join, which is the default join mechanism. The MAPJOIN operator is used when one of the tables being joined is small enough to fit in memory. By loading the smaller table into memory and joining it with the larger table in the map phase, the MAPJOIN operator can significantly improve the performance of a join operation.

Benefits of using the MAPJOIN Operator

1. It eliminates the need for a separate reduce phase, resulting in faster join operations.
2. It reduces the amount of data shuffled between the map and reduce phases, thus decreasing network overhead.
3. It can improve the overall performance of the query, especially when dealing with a small table and a large table.

Example: Using the MAPJOIN Operator

In this example, we will use the MAPJOIN operator to analyze a dataset containing the details of fresh graduates and another dataset containing the details of colleges. We will demonstrate how this operator can be used to efficiently join the two tables and improve query performance.

Dataset Schema (freshers_in_details)

Dataset Schema (freshers_in_colleges)

Now, let’s say we want to find the average CGPA of students in each college. We can use the MAPJOIN operator to achieve this as follows:

SET hive.auto.convert.join=true;
SET hive.mapjoin.smalltable.filesize=5000000; -- This value should be adjusted based on the size of the smaller table

CREATE TABLE freshers_in_avg_cgpa AS
SELECT f.college_id, c.college_name, AVG(f.cgpa) as avg_cgpa
FROM freshers_in_details f
JOIN freshers_in_colleges c ON f.college_id = c.college_id
GROUP BY f.college_id, c.college_name;