Creating user-defined function (UDF)

Write custom functions using Java and other programming languages for use in SELECT, INSERT, and UPDATE statements.

Write custom functions using Java and other programming languages for use in SELECT, INSERT, and UPDATE statements. Function are only available within the keyspace where it is defined.

You can define functions that are executed against 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 is performed on each row of the table.

cassandra.yaml

The cassandra.yaml file is located in the installation_location/conf directory.

Adding additional languages

DataStax Distribution of Apache Cassandra™ supports functions written in Java and JavaScript by default. Add support for other scripting languages, Python, Ruby, and Scala by adding a JAR to the classpath. Install the JAR file into: installation_location/lib/jsr223/[language]/[jar-name].jar where language is 'jruby', 'jython', or 'scala'.

Prerequisites

By default the ability to add user-defined functions is disabled. To enable, change the following settings in the file:

Java: Set enable_user_defined_functions to true
Javascript and other custom languages: Set enable_scripted_user_defined_functions to true.

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.
```
CREATE FUNCTION cycling.flog(target_name double)
    CALLED ON NULL INPUT
    RETURNS double
    LANGUAGE java
    AS $$return Double.valueOf(Math.log(input.doubleValue()));$$;
```
Note:
- target_name
- Actions when the input from the target column is null:
  - CALLED ON NULL INPUTensures 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 CQL 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 OR REPLACE FUNCTION cycling.flog(input double)
    CALLED ON NULL INPUT
    RETURNS double
    LANGUAGE java
    AS $$return Double.valueOf(Math.log(input.doubleValue()));$$;
```