User-defined functions

Hive defines the following three types of UDF:

• UDFs: These are regular user-defined functions that operate row-wise and output one result for one row, such as most built-in mathematic and string functions.

• UDAFs: These are user-defined aggregating functions that operate row-wise or group-wise and output one row or one row for each group as a result, such as the MAX and COUNT built-in functions.

• UDTFs: These are user-defined table-generating functions that also operate row-wise, but they produce multiple rows/tables as a result, such as the EXPLODE function. UDTF can be used either after SELECT or after the LATERAL VIEW statement.

  Note

  Since Hive is implemented in Java, UDFs should be written in Java as well. Since Java supports running code in other languages through the javax.script API (see http://docs.oracle.com/javase/6/docs/api/javax/script/package-summary.html), UDFs can be written in languages other than Java. In this book, we only focus on Java UDFs.

We'll start...