Creating user-defined function (UDF)

User-Defined Functions (UDFs) can be used to manipulate stored data with a function of the user's choice.

Allows users to define functions that can be applied to data stored in a table as part of a query result. The function must be created prior to its use in a SELECT statement. The function will be performed on each row of the table. To use user-defined functions with Java or Javascript, enable_user_defined_functions must be set true in the cassandra.yaml file setting to enable the functions. User-defined functions are defined within a keyspace; if no keyspace is defined, the current keyspace is used. User-defined functions are executed in a sandbox

By default, Cassandra supports defining functions in java and javascript. Other scripting languages, such as Python, Ruby, and Scala can be added by adding a JAR to the classpath. Install the JAR file into $CASSANDRA_HOME/lib/jsr223/[language]/[jar-name].jar where language is 'jruby', 'jython', or 'scala'

Procedure

Create a function, specifying the data type of the returned value, the language, and the actual code of the function to be performed. The following function, fLog(), computes the logarithmic value of each input. It is a built-in java function and used to generate linear plots of non-linear data. For this example, it presents a simple math function to show the capabilities of user-defined functions.
```
cqlsh> CREATE OR REPLACE FUNCTION fLog (input double) CALLED ON NULL INPUT RETURNS double LANGUAGE java AS 'return Double.valueOf(Math.log(input.doubleValue()));';
```
Note:
- CALLED ON NULL INPUT ensures the function will always be executed.
- RETURNS NULL ON NULL INPUT ensures the function will always return NULL if any of the input arguments is NULL.
- RETURNS defines the data type of the value returned by the function.
A function can be replaced with a different function if OR REPLACE is used as shown in the example above. Optionally, the IF NOT EXISTS keywords can be used to create the function only if another function with the same signature does not exist in the keyspace. OR REPLACE and IF NOT EXISTS cannot be used in the same command.
```
CREATE FUNCTION IF NOT EXISTS fLog (input double) 
   CALLED ON NULL INPUT 
   RETURNS double 
   LANGUAGE java AS '
     return Double.valueOf(Math.log(input.doubleValue()));
     ';
```