How to write and use custom functions in Hive.

6 months ago

2 minutes

In Hive, you can write and use custom functions by creating UDFs (User Defined Functions). UDFs can be unary functions, binary functions, or aggregate functions, and can be used to process, transform, or calculate data.

Here are the basic steps for writing and using custom functions in Hive.

To create a Java class for UDF: Begin by writing a Java class to implement the custom function’s logic. In the Java class, you need to inherit from Hive’s UDF class and implement the evaluate method to define the function’s logic. For instance, you can write a simple UDF to reverse a string.

import org.apache.hadoop.hive.ql.exec.UDF;

public class ReverseStringUDF extends UDF {
    public String evaluate(String input) {
        StringBuilder reversed = new StringBuilder(input).reverse();
        return reversed.toString();
    }
}

Compile Java classes: Compile the written Java classes into a jar file for loading and using in Hive. Maven or other build tools can be used for compiling Java classes.
Registering UDF in Hive: Add the compiled jar file to Hive’s classpath and register the UDF. You can use the ADD JAR command to load the jar file and the CREATE FUNCTION command to register the UDF. For example, to register the ReverseStringUDF function written above.

ADD JAR /path/to/ReverseStringUDF.jar;
CREATE FUNCTION reverse_string AS 'com.example.ReverseStringUDF' USING JAR 'ReverseStringUDF.jar';

By using custom functions, after successful registration, you can use custom functions in Hive. For example, you can use the ReverseStringUDF function mentioned above to reverse strings.

SELECT reverse_string('hello world');

These are the basic steps for writing and using custom functions in Hive. By creating UDFs, you can expand the functionality of Hive and meet more flexible and personalized data processing needs.