Introduction

ClickHouse® is well known for its extensive collection of built-in SQL functions. From string manipulation and mathematical operations to JSON processing, geospatial analytics, machine learning helpers, and date-time functions, the database provides everything needed for most analytical workloads.

However, every organization eventually encounters business logic that cannot be expressed using built-in SQL functions alone. Financial institutions may require proprietary risk calculations, retailers may have custom loyalty score algorithms, and manufacturing companies may need domain-specific validation rules.

Prior to ClickHouse® 26.3, implementing these custom operations often meant exporting data from ClickHouse®, processing it in an external application written in Python, Go, Rust, or Java, and then loading the results back into the database. Although functional, this approach introduces unnecessary data movement, increases latency, complicates data pipelines, and creates additional operational overhead.

ClickHouse® 26.3 introduces an exciting new capability: WebAssembly (WASM) User-Defined Functions (UDFs).

Instead of moving data outside the database, developers can now execute custom logic directly inside ClickHouse® through a secure WebAssembly runtime. Developers can write functions in languages such as Rust, C, C++, TinyGo, or AssemblyScript, compile them into a .wasm module, register them with ClickHouse®, and invoke them just like built-in SQL functions.

In this article, we'll explore what WASM UDFs are, how they work, where they are useful, their benefits and limitations, and best practices for using them effectively.

What Is a User-Defined Function (UDF)?

A User-Defined Function (UDF) is a custom function created by users to perform operations that are not available as built-in database functions.

For example, suppose your organization calculates customer loyalty using a proprietary algorithm based on purchase frequency, lifetime value, product categories, and customer tenure.

Without a UDF, this logic might be repeated across dozens of SQL queries or implemented in an external application.

With a UDF, the entire calculation becomes reusable.

Built-in functions:

• SUM()
• AVG()
• COUNT()
• LENGTH()

Custom function:

loyalty_score()

Once defined, it behaves like any other SQL function.

What Is WebAssembly (WASM)?

WebAssembly (WASM) is a portable binary instruction format designed for secure, high-performance execution.

Unlike native plugins, which execute directly on the operating system, WebAssembly modules run inside an isolated sandbox.

The typical workflow looks like this:

Rust / C / C++ / TinyGo / AssemblyScript
                │
                ▼
      Compile to .wasm
                │
                ▼
     ClickHouse® WASM Runtime
                │
                ▼
        SQL Query Execution

Because WebAssembly is language-independent, developers are free to choose whichever supported language best fits their project.

Why WASM UDFs Matter

Before ClickHouse® 26.3, implementing custom business logic usually required:

1. Exporting data from ClickHouse®.
2. Processing it in another application.
3. Writing the processed results back into ClickHouse®.

This introduces several disadvantages:

• Additional infrastructure
• More network traffic
• Higher latency
• Extra storage
• More operational complexity
• Increased maintenance effort

With WASM UDFs:

• Processing stays inside ClickHouse®.
• Data never leaves the database.
• SQL becomes more expressive.
• Business logic becomes reusable.
• Pipelines become simpler.

How WASM UDFs Work

The development workflow is straightforward.

Write Function
      │
      ▼
Compile to WebAssembly (.wasm)
      │
      ▼
Register Module
      │
      ▼
Create SQL Function
      │
      ▼
Execute Inside SQL Queries

Once registered, the custom function behaves just like any built-in ClickHouse® function.

Supported Programming Languages

Any language capable of compiling to WebAssembly can be used.

Common choices include:

• Rust
• C
• C++
• TinyGo
• AssemblyScript

Among these, Rust is generally considered the preferred option because of its memory safety, excellent tooling, and mature WebAssembly ecosystem.

Enabling WASM UDF Support

In ClickHouse® 26.3, WASM UDF support is still experimental.

It must be enabled explicitly.

<enable_wasm_udf>1</enable_wasm_udf>

After enabling the setting, restart ClickHouse®.

The server can then load WebAssembly modules.

Note: Because the feature is experimental, it should be thoroughly tested before enabling it in production environments.

Registering a WASM Module

After compiling your code into a .wasm file, register the module with ClickHouse®.

Conceptually:

custom_logic.wasm
        │
        ▼
ClickHouse®
        │
        ▼
Registered WASM Module

Each module can expose one or more exported functions.

Creating a WASM Function

After the module has been registered, create a SQL function that references one of the exported methods.

Calling the function is no different from calling any built-in SQL function.

SELECT
    customer_id,
    wasm_loyalty_score(total_orders, total_spent)
FROM customers;

The custom logic executes internally while ClickHouse® processes each row.

Example Use Case

Imagine an insurance company that calculates premiums using hundreds of proprietary business rules written in Rust.

Traditional workflow

ClickHouse®

     │

Export Data

     │

Rust Application

     │

Import Results

Using WASM UDFs

ClickHouse®

      │

WASM Premium Calculator

      │

Query Results

Everything happens inside ClickHouse®, eliminating unnecessary data movement.

Real-World Use Cases

1. Financial Calculations

Banks, insurance providers, and fintech companies frequently implement proprietary algorithms.

Examples include:

• Credit scoring
• Loan eligibility
• Risk calculations
• Interest computations

2. Data Validation

Organizations often need custom validation rules.

Examples:

• Customer ID validation
• Tax number verification
• Product code validation
• Checksum calculations

3. Text Processing

Many companies require organization-specific text operations.

Examples:

• Product normalization
• Keyword extraction
• Custom tokenization
• Domain-specific parsing

4. Scientific Computing

Research and engineering teams can implement mathematical algorithms unavailable in standard SQL.

5. Machine Learning Preprocessing

Prepare features before passing data into downstream ML pipelines.

Examples include:

• Feature scaling
• Normalization
• Feature encoding
• Custom transformations

Benefits of WASM UDFs

WASM UDFs vs External Processing

Runtime Security

One of WebAssembly's biggest advantages is isolation.

The runtime:

• Executes inside a secure sandbox.
• Prevents direct operating system access.
• Restricts unsafe operations.
• Protects the ClickHouse® server from untrusted native code.

Compared with traditional native plugins, this provides a much safer extension mechanism.

Best Practices

When developing WASM UDFs:

• Keep functions deterministic.
• Avoid unnecessary memory allocations.
• Focus each function on a single responsibility.
• Benchmark performance before deployment.
• Reuse compiled modules whenever possible.
• Prefer built-in ClickHouse® functions when they already solve the problem efficiently.
• Thoroughly test modules before production deployment.

Current Limitations

Although powerful, WASM UDFs are still evolving.

Current limitations include:

• Experimental in ClickHouse® 26.3
• Small runtime overhead compared to built-in functions
• Not intended to replace native SQL functionality
• Requires additional testing before production use

As the feature matures, many of these limitations are expected to improve.

When Should You Use WASM UDFs?

WASM UDFs are an excellent choice when implementing:

• Proprietary business rules
• Financial calculations
• Data validation
• Feature engineering
• Specialized mathematical algorithms
• Domain-specific text processing

However, if an equivalent built-in ClickHouse® function already exists, it will generally remain the preferred option for simplicity and performance.

Architecture Overview

              Application
                    │
                    ▼
               SQL Query
                    │
                    ▼
          ClickHouse® 26.3
                    │
      ┌─────────────┴─────────────┐
      │                           │
      ▼                           ▼
Built-in SQL Functions      WASM Runtime
      │                           │
      └─────────────┬─────────────┘
                    ▼
              Query Results

Best Practices for Production Adoption

If you're planning to experiment with WASM UDFs, consider the following recommendations:

• Start with non-critical workloads.
• Validate correctness against existing implementations.
• Benchmark execution times with realistic datasets.
• Monitor CPU and memory usage during execution.
• Version your WebAssembly modules for easier deployment and rollback.
• Keep your business logic modular so individual functions remain easy to maintain.

Following these practices will help you safely evaluate the feature while it continues to mature.

Final Thoughts

WebAssembly User-Defined Functions represent one of the most exciting extensibility features introduced in ClickHouse® 26.3.

By allowing developers to execute custom business logic directly inside the database, WASM UDFs reduce data movement, simplify analytics pipelines, and enable reusable domain-specific functionality without modifying the ClickHouse® source code.

Although the feature is currently experimental, it opens the door to a wide range of possibilities—from financial calculations and data validation to machine learning preprocessing and scientific computing—all while benefiting from WebAssembly's secure sandboxed execution model.

As ClickHouse® continues to evolve, WASM UDFs have the potential to become a powerful extension mechanism for organizations that need flexible, high-performance analytics with custom business logic executed exactly where the data lives.