Create a user-defined function (UDF)

Write custom functions using Java or JavaScript to use in SELECT, INSERT, and UPDATE statements or as a building block for a user-defined aggregate (UDA). Functions are only available within the keyspace where it is defined. To preserve fast performance, use user-defined functions for short computations, and create Java UDFs instead of Javascript UDFs.

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.

Enable and create a UDF

Create a keyspace or use an existing one.
In cassandra.yaml, enable UDFs, which are disabled by default:
- Java: Set enable_user_defined_functions to true.
- JavaScript (in addition to Java): Set enable_scripted_user_defined_functions to true.
Use CREATE FUNCTION to create a UDF, 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 OR REPLACE FUNCTION cycling.flog ( input double ) 
  CALLED ON NULL INPUT 
  RETURNS double 
  LANGUAGE java AS 
    $$ return Double.valueOf(Math.log(input.doubleValue())); $$ 
;
```
Results

Options

Actions when the input from the target column is null:
- CALLED ON NULL INPUT ensures the function always executes when called.
- RETURNS NULL ON NULL INPUT ensures the function always returns NULL if any of the input arguments are NULL.
RETURNS <type> defines the CQL data type of the value returned by the function.
OR REPLACE replaces an existing function with a new function definition.
IF NOT EXISTS keywords can be used to create the function only if another function with the same signature doesn’t already exist in the keyspace. If a function with the same signature already exists, the command silently does nothing. If IF NOT EXISTS is omitted, then an error is returned if a function with the same name already exists.

OR REPLACE and IF NOT EXISTS are mutually exclusive.

Create a user-defined function (UDF)

Enable and create a UDF

Options

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