How to get return value from a Diesel sql function

Question

I have a SQL function (in Postgres) that I'd like to call using Diesel. I've declared it using the sql_function! macro, and I can call the function OK. But I'm not able to get the return value — it always returns 1, which I'm assuming is a row count. How can I get the actual return value?

sql_function! {
    fn my_add(a: Int4, b: Int4) -> Int4;
}

let my_add_fn = my_add(2, 4);
let result = diesel::select(my_add_fn).execute(&conn).unwrap();
dbg!(&result); // This prints 1. Should be 6

When I call my_add from the Postgres CLI, it works correctly:

select my_add(2, 4);
 my_add 
--------
      6
(1 row)

And here is the my_add function

CREATE OR REPLACE FUNCTION my_add(
    a integer,
    b integer
)
    RETURNS integer
AS $$
BEGIN
    return a + b;
END;
$$ LANGUAGE plpgsql
    IMMUTABLE;

Answer 1

This is an old question but figured I'd post a response in case anyone else lands here.

When you call execute as you did, what you're returned is only the number of affected rows. See: getting started guide, and search for "get_result". In a select, load is probably the best choice, as a comment already noted - (docs).

To get back the result(s) from an insert or update query, replace execute with get_result or get_results for multiple rows (which will give you back a Vec<T> instead of just T) ... more docs.