The HiveQL language can be extended by means of plugins and third-party functions. In Hive, there are three types of functions characterized by the number of rows they take as input and produce as output:
User Defined Functions (UDFs): are simpler functions that act on one row at a time.
User Defined Aggregate Functions (UDAFs): take multiple rows as input and generate multiple rows as output. These are aggregate functions to be used in conjunction with a
GROUP BYstatement (similar toCOUNT(),AVG(),MIN(),MAX(), and so on).User Defined Table Functions (UDTFs): take multiple rows as input and generate a logical table comprised of multiple rows that can be used in join expressions.
Tip
These APIs are provided only in Java. For other languages, it is possible to stream data through a user-defined script using the TRANSFORM, MAP, and REDUCE clauses that act as a frontend to Hadoop's streaming capabilities.
Two APIs are available to write UDFs. A simple API org.apache.hadoop...