UDF stands for User-Defined Function in Hive. A UDF is a function that can be defined and used by a user to perform operations on data stored in Hive tables. Hive supports a wide variety of built-in functions for data processing, but sometimes the user needs to perform custom operations that are not supported by the built-in functions. In such cases, UDFs can be written in various programming languages such as Java, Python, and others to extend the functionality of Hive.

A UDF can be used in Hive SELECT statements just like any other built-in function. For example, if you have written a UDF that takes a string as input and returns its length, you can use it in a SELECT statement like this:

SELECT my_udf(column_name)
FROM my_table;

Here, “my_udf” is the name of the UDF, and “column_name” is the name of the column on which you want to apply the UDF.

To create a UDF, you need to write the function in a programming language like Java, package it into a JAR file, and then register it in Hive. Once the UDF is registered, you can use it in your Hive queries.

Overall, UDFs provide a powerful way to extend the functionality of Hive and perform custom data processing operations on your data.

Here is an example of how to write a UDF in Java and create a JAR file for it:

1] Write the UDF in Java:

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

public class ReverseStringUDF extends UDF {
  public Text evaluate(final Text s) {
    if (s == null) { return null; }
    return new Text(new StringBuilder(s.toString()).reverse().toString());
  }
}

In this example, we are defining a UDF called “ReverseStringUDF” that takes a string as input and returns its reverse. The UDF extends the UDF class from the org.apache.hadoop.hive.ql.exec package.

2 Compile the Java code

javac -classpath /path/to/hive/lib/*:/path/to/hadoop/lib/* ReverseStringUDF.java

This will compile the Java code and generate a class file.

3 Package the UDF into a JAR file

jar -cvf reverse-string-udf.jar ReverseStringUDF*.class

This will create a JAR file called “reverse-string-udf.jar” that contains the compiled UDF class.

4 Register the UDF in Hive:

add jar /path/to/reverse-string-udf.jar;
create temporary function reverse_string as 'ReverseStringUDF';

This will register the UDF in Hive. The “create temporary function” statement is used to create a temporary function in Hive that can be used in the current session. The name of the function is “reverse_string”, and the fully-qualified class name of the UDF is “ReverseStringUDF”.

Once the UDF is registered, you can use it in Hive queries like this

SELECT reverse_string(column_name)
FROM my_table;

This will return the reverse of the string in the “column_name” column.

Note: In the example, you need to replace “/path/to/hive/lib” and “/path/to/hadoop/lib” with the actual paths to the Hive and Hadoop libraries on your system