Chapter 13. Functions
User-Defined Functions (UDFs) are a powerful feature that allow users to extend HiveQL. As we’ll see, you implement them in Java and once you add them to your session (interactive or driven by a script), they work just like built-in functions, even the online help. Hive has several types of user-defined functions, each of which performs a particular “class” of transformations on input data.
In an ETL workload, a process might have several processing steps. The Hive language has multiple ways to pipeline the output from one step to the next and produce multiple outputs during a single query. Users also have the ability to create their own functions for custom processing. Without this feature a process might have to include a custom MapReduce step or move the data into another system to apply the changes. Interconnecting systems add complexity and increase the chance of misconfigurations or other errors. Moving data between systems is time consuming when dealing with gigabyte- or terabyte-sized data sets. In contrast, UDFs run in the same processes as the tasks for your Hive queries, so they work efficiently and eliminate the complexity of integration with other systems. This chapter covers best practices associated with creating and using UDFs.
Discovering and Describing Functions
Before writing custom UDFs, let’s familiarize ourselves with the ones that are already part of Hive. Note that it’s common in the Hive community to use “UDF” to refer to any function, ...