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User Defined Functions (UDF) in ClickHouse allow users to extend the native capabilities of the database by creating custom functions for specific use cases. UDFs in ClickHouse are usually written in C++ for performance reasons.
Steps to Build UDF in ClickHouse
1. Set Up the Development Environment
- Clone the ClickHouse repository.
git clone https://github.com/ClickHouse/ClickHouse.git
Install the necessary build tools and libraries.
2. Write the UDF
UDFs in ClickHouse are typically written in C++. For this example, let’s create a simple UDF that calculates the factorial of a number.
class FunctionFactorial : public IFunction
static constexpr auto name = “factorial”;
static FunctionPtr create(const Context &)
String getName() const override
size_t getNumberOfArguments() const override
DataTypePtr getReturnTypeImpl(const DataTypes & arguments) const override
void executeImpl(Block & block, const ColumnNumbers & arguments, size_t result, size_t input_rows_count) override
auto * result_column = block.getByPosition(result).column.get();
const ColumnWithTypeAndName & argument_column = block.getByPosition(arguments);
for (size_t row = 0; row < input_rows_count; ++row)
UInt64 value = argument_column.column->getUInt(row);
UInt64 factorial = 1;
for (UInt64 i = 1; i <= value; ++i)
factorial *= i;
void registerFunctionFactorial(FunctionFactory & factory)
3. Register the UDF
- Add the registration code at the end of the UDF file.
void registerFunctionFactorial(FunctionFactory &);
Add the registration function to the build system.
4. Compile and Install ClickHouse with the UDF
- Build ClickHouse with the custom function.
- Install and restart ClickHouse.
5. Use the UDF
SELECT factorial(5); — Expected output: 120
Benefits of UDFs in ClickHouse:
- Customization: Tailor ClickHouse to your specific needs.
- Performance: Being written in C++, UDFs are compiled into native code, ensuring high performance.
- Flexibility: Integrate third-party libraries or proprietary algorithms directly into ClickHouse.
ClickHouse, with its capabilities to support User Defined Functions, is not just a database, but a powerful platform for high-performance real-time analytics. For businesses operating at webscale, the ability to customize and enhance their analytics infrastructure is invaluable. By developing advanced real-time analytics using ClickHouse, companies can derive actionable insights from vast amounts of data in real-time, allowing them to make informed decisions rapidly and stay ahead in the competitive global marketplace.
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