Function Plugin¶

Creating a Shared Library for UDFs¶

User defined functions (UDFs) allow users to create custom functions without the need to rebuild the executable. There are many benefits to UDFs, such as:

Simplify SQL queries by creating UDFs for repetitive logic.
Implement custom logic pertaining to the specific business use cases of the users.
Once defined, easily reusable and called multiple times just like built in functions.
Shorter compile times.

To create the UDF, create a new C++ file in the same format as the below example file named ExampleFunction.cpp:
```
#include "presto_cpp/main/dynamic_registry/DynamicFunctionRegistrar.h"

template <typename T>
struct NameOfStruct {
   VELOX_DEFINE_FUNCTION_TYPES(T);
   FOLLY_ALWAYS_INLINE bool call(out_type<int64_t>& result, const arg_type<Varchar> in) {
      ...
   }
};

extern "C" {
   void registerExtensions() {
      facebook::presto::registerPrestoFunction<
         NameOfStruct,
         int64_t, facebook::velox::Varchar>("function_name", "test.namespace");
      }
}
```
Note: The int64_t return type and the Varchar input type can be changed as needed. Additional or no arguments may be specified as well. For more examples, see the examples. The functions should follow the Velox scalar function API. Return types and argument types include:
- simple types: BOOLEAN, TINYINT, SMALLINT, INTEGER, BIGINT, DOUBLE, TIMESTAMP, DATE, VARCHAR, VARBINARY.
- complex types: ARRAY, MAP, ROW. They can be mixed with other simple and complex types. For example: MAP(INTEGER, ARRAY(BIGINT)).

Create a shared library using CMakeLists.txt like the following:

add_library(name_of_dynamic_fn SHARED ExampleFunction.cpp)
target_link_libraries(name_of_dynamic_fn PRIVATE presto_dynamic_function_registrar fmt::fmt gflags::gflags xsimd)

Place your shared libraries in the plugin directory. The path to this directory defaults to the plugin directory relative to the directory in which the process is being run but it is configurable in the plugin.dir property set in Presto C++ Plugins.