Use UDFs in JavaScript

JavaScript code is inlined in CREATE FUNCTION statement and then run on the embedded QuickJS virtual machine in RisingWave. It does not support access to external networks and is limited to computational tasks only. Compared to other languages, JavaScript UDFs offer the easiest way to define UDFs in RisingWave.

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Define your functions

You can use the CREATE FUNCTION command to create JavaScript UDFs. See the syntax as follows:

CREATE FUNCTION function_name ( arg_name arg_type [, ...] )
    [ RETURNS return_type | RETURNS TABLE ( column_name column_type [, ...] ) ]
    LANGUAGE javascript
    AS [ $$ function_body $$ | 'function_body' ];

The argument names you defined can be used in the function body. For example:

CREATE FUNCTION gcd(a int, b int) RETURNS int LANGUAGE javascript AS $$
    if(a == null || b == null) {
        return null;
    }
    while (b != 0) {
        let t = b;
        b = a % b;
        a = t;
    }
    return a;
$$;

See the correspondence between SQL types and JavaScript types in the Data type mapping. You need to ensure that the type of the return value is either null or consistent with the type in the RETURNS clause.

If the function you defined returns a table, you need to use the yield statement to return the data of each row. For example:

CREATE FUNCTION series(n int) RETURNS TABLE (x int) LANGUAGE javascript AS $$
    for(let i = 0; i < n; i++) {
        yield i;
    }
$$;

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Use your functions

Once the UDFs are created in RisingWave, you can use them in SQL queries just like any built-in functions. For example:

SELECT gcd(25, 15);
SELECT * from series(5);

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Define your aggregate functions

You can create aggregate functions using the CREATE AGGREGATE command. Refer to the syntax below:

CREATE AGGREGATE function_name ( argument_type [, ...] )
    RETURNS return_type
    LANGUAGE javascript
    AS $$ function_body $$

In the function_body, the code should define several exported functions to implement the aggregate function.

Required functions:

• create_state() -> state: Create a new state.
• accumulate(state, *args) -> state: Accumulate a new value into the state, returning the updated state.

Optional functions:

• finish(state) -> value: Get the result of the aggregate function. If not defined, the state is returned as the result.
• retract(state, *args) -> state: Retract a value from the state, returning the updated state. If not defined, the state can not be updated incrementally in materialized views and performance may be affected.

The following command creates an aggregate function named weighted_avg to calculate the weighted average.

create aggregate weighted_avg(value int, weight int) returns float language javascript as $$
    export function create_state() {
        return { sum: 0, weight: 0 };
    }
    export function accumulate(state, value, weight) {
        if (value == null || weight == null) {
            return state;
        }
        state.sum += value * weight;
        state.weight += weight;
        return state;
    }
    export function retract(state, value, weight) {
        if (value == null || weight == null) {
            return state;
        }
        state.sum -= value * weight;
        state.weight -= weight;
        return state;
    }
    export function finish(state) {
        if (state.weight == 0) {
            return null;
        }
        return state.sum / state.weight;
    }
$$;

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Data type mapping

The following table shows the data type mapping between SQL and JavaScript: