created_at2022-10-01 12:09:27.771114
updated_at2024-05-07 23:23:50.410454
descriptionEasily create user defined functions (UDFs) for MariaDB and MySQL.
Trevor Gross (tgross35)



UDF: MariaDB/MySQL User Defined Functions in Rust

This crate aims to make it extremely simple to implement UDFs for SQL, in a minimally error-prone fashion.

Looking for prewritten useful UDFs? Check out the UDF suite, which provides downloadable binaries for some useful functions: https://github.com/pluots/udf-suite.

View the docs here: https://docs.rs/udf/latest

UDF Theory

Basic SQL UDFs consist of three exposed functions:

  • An initialization function where arguments are checked and memory is allocated
  • A processing function where a result is returned
  • A deinitialization function where anything on the heap is cleaned up (performed automatically in this library)

Aggregate UDFs (those that work on more than one row at a time) simply need to register two to three additional functions.

This library handles everything that used to be difficult about writing UDFs (dynamic registration, allocation/deallocation, error handling, nullable values, logging) and makes it trivial to add any function to your SQL server instance. It also inclues a mock interface, for testing your function implementation without needing a server.


The steps to create a working UDF using this library are:

  • Create a new rust project (cargo new --lib my-udf), add udf as a dependency (cd my-udf; cargo add udf) and change the crate type to a cdylib by adding the following to Cargo.toml:

    crate-type = ["cdylib"]
  • Make a struct or enum that will share data between initializing and processing steps (it may be empty). The default name of your UDF will be your struct's name converted to snake case.

  • Implement the BasicUdf trait on this struct

  • Implement the AggregateUdf trait if you want it to be an aggregate function

  • Add #[udf::register] to each of these impl blocks (optionally with a (name = "my_name") argument)

  • Compile the project with cargo build --release (output will be target/release/libmy_udf.so)

  • Load the struct into MariaDB/MySql using CREATE FUNCTION ...

  • Use the function in SQL!

For an example of some UDFs written using this library, see either the udf-examples/ directory or the udf-suite repository.

Detailed overview

This section goes into the details of implementing a UDF with this library, but it is non-exhaustive. For that, see the documentation, or the udf-examples directory for well-annotated examples.

Struct creation

The first step is to create a struct (or enum) that will be used to share data between all relevant SQL functions. These include:

  • init Called once per result set. Here, you can store const data to your struct (if applicable)
  • process Called once per row (or per group for aggregate functions). This function uses data in the struct and in the current row's arguments
  • clear Aggregate only, called once per group at the beginning. Reset the struct as needed.
  • add Aggregate only, called once per row within a group. Perform needed calculations and save the data in the struct.
  • remove Window functions only, called to remove a value from a group

It is quite possible, especially for simple functions, that there is no data that needs sharing. In this case, just make an empty struct and no allocation will take place.

/// Function `sum_int` just adds all arguments as integers and needs no shared data
struct SumInt;

/// Function `avg` on the other hand may want to save data to perform aggregation
struct Avg {
    running_total: f64

There is a bit of a caveat for functions returning buffers (string & decimal functions): if there is a possibility that string length exceeds MYSQL_RESULT_BUFFER_SIZE (255), then the string to be returned must be contained within the struct (the process function will then return a reference).

/// Generate random lipsum that may be longer than 255 bytes
struct Lipsum {
    res: String

Trait Implementation

The next step is to implement the BasicUdf and optionally AggregateUdf traits. See the docs for more information.

If you use rust-analyzer with your IDE, it can help you out. Just type impl BasicUdf for MyStruct {} and place your cursor between the brackets - it will offer to autofill the function skeletons (ctrl+. or cmd+. brings up this menu if it doesn't show up by default).

use udf::prelude::*;

struct SumInt;

impl BasicUdf for SumInt {
    type Returns<'a> = Option<i64>;

    fn init<'a>(
      cfg: &UdfCfg<Init>,
      args: &'a ArgList<'a, Init>
    ) -> Result<Self, String> {
      // ...

    fn process<'a>(
        &'a mut self,
        cfg: &UdfCfg<Process>,
        args: &ArgList<Process>,
        error: Option<NonZeroU8>,
    ) -> Result<Self::Returns<'a>, ProcessError> {
      // ...


Assuming the above has been followed, all that is needed is to produce a C dynamic library for the project. This can be done by specifying crate-type = ["cdylib"] in your Cargo.toml. After this, compiling with cargo build --release will produce a loadable .so file (located in target/release).

Important version note: this crate relies on a feature called generic associated types (GATs) which are only available on rust >= 1.65. This version only just became stable (2022-11-03), so be sure to run rustup update if you run into compiler issues.

CI runs tests on both Linux and Windows, and this crate should work for either. MacOS is untested, but will likely work as well.

Symbol Inspection

If you would like to verify that the correct C-callable functions are present, you can inspect the dynamic library with nm.

# Output of example .so
$ nm -gC --defined-only target/release/libudf_examples.so
00000000000081b0 T avg_cost
0000000000008200 T avg_cost_add
00000000000081e0 T avg_cost_clear
0000000000008190 T avg_cost_deinit
0000000000008100 T avg_cost_init
0000000000009730 T is_const
0000000000009710 T is_const_deinit
0000000000009680 T is_const_init
0000000000009320 T sql_sequence


Once compiled, the produced object file needs to be copied to the location of the plugin_dir SQL variable - usually, this is /usr/lib/mysql/plugin/.

Once that has been done, CREATE FUNCTION can be used in MariaDB/MySql to load it.

Docker Use

Testing in Docker is highly recommended, so as to avoid disturbing a host SQL installation. See the udf-examples readme for instructions on how to do this.


The udf-examples crate contains examples of various UDFs, as well as instructions on how to compile them. See the readme there.

Logging & Debugging Note

If you need to log things like warnings during normal use of the function, anything printed to stderr will appear in the server logs (which can be viewed with e.g. docker logs mariadb_udf_test if testing in Docker). The udf_log! macro will print a message that matches the formatting of other SQL log information. You can also enable the crate features logging-debug for function entry/exitpoint debugging, or logging-debug-calls for information on the exact call parameters from the MariaDB/MySQL server.

The best way to debug is to use the udf::mock module to create s.all unit tests. These can be run to validate correctness, or stepped through with a debugger if needed (this use case is likely somewhat rare). All types implement Debug so they can also be easily printed (the builtin dbg! macro prints to stderr, so this will also appear in logs):

let arg0 = dbg!(args.get(0).unwrap())
[udf_examples/src/avgcost.rs:58] &self = AvgCost {
    count: 0,
    total_qty: 0,
    total_price: 0.0,

[udf_examples/src/avgcost.rs:60] args.get(0).unwrap() = SqlArg {
    value: Int(
    attribute: "qty",
    maybe_null: true,
    arg_type: Cell {
        value: INT_RESULT,
    marker: PhantomData<udf::traits::Process>,


This work is dual-licensed under Apache 2.0 and GPL 2.0 (or any later version) as of version 0.5.1. You can choose either of them if you use this work.

SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later

Commit count: 168

cargo fmt