Arroyo SQL can be extended with user-defined functions. Currently, only Rust UDFs are supported. UDFs can be defined as part of the SQL API call, or via the Web UI.

UDFs are defined as Rust functions. The parameters and return type of the UDF are determined by the definition of the function. All types must be valid SQL data types.

Here’s an example of a simple UDF that squares an integer:

fn my_square(x: i64) -> i64 {
  x * x

Once registered, this can be used in SQL queries:

SELECT my_square( FROM nexmark


UDFs can depend on external crates. To add dependencies, you can define a special comment in the UDF definition like this:

regex = "1.10.2"

use regex::Regex;

fn my_regex_matches(s: String) -> bool {
    let re = Regex::new(r"test").unwrap();

Internally, the contents of the [dependencies] comment are used to generate a Cargo.toml file for the UDF. See the Cargo.toml reference for more details on the syntax.


Nullability handling is controlled via the type signature of the UDF parameters and return types. If a parameter is an Option type (for example Option<i64>), then it will be called with all inputs, even if they are NULL. If the parameter is not an Option type (for example i64), then it will only be called with non-NULL inputs.

Similarly, if the return type is an Option type, then the output type is nullable, otherwise it is non-nullable. If

In table form:

InputParameter typeReturn typeCalled onNullability
NullableTTNon-null valuesNullable
NullableOption<T>TAll valuesNon-null
NullableTOption<T>Non-null valuesNullable
NullableOption<T>Option<T>All valuesNullable
Non-nullTTAll valuesNon-null
Non-nullOption<T>TAll valuesNon-null
Non-nullTOption<T>All valuesNullable
Non-nullOption<T>Option<T>All valuesNullable

Debugging UDFs

If UDFs panic, they will produce log lines in the worker logs. These are not currently forwarded to the Web UI, so you will need to find the logs where the workers are running. For the default process scheduler, this will be in the controller logs. For kubernetes and nomad schedulers, this will be within the actual worker pods.