Understanding Custom Functions in DuckDB
Tags: pandas, duckdb, udf, sql
Published: 2024-01-16 04:26:58

DuckDB's support for custom functions is a crucial feature that allows users to extend the database's capabilities by incorporating their logic and operations. Custom functions are user-defined functions (UDFs) that can be implemented in languages such as Python and then seamlessly integrated into DuckDB. This extensibility is invaluable when users encounter specific analytical challenges not addressed by the built-in functions. **For instance, SQL often struggles to infer datetime formats, leading to the need for complex case-when statements. The parse_dates custom function showcased here, leveraging Pandas capabilities, becomes a powerful solution to overcome this limitation.**

## The parse_dates Function
The parse_dates function, in the provided Python code, is a practical example of a custom function designed to handle date parsing within DuckDB. This function leverages the popular Pandas library to parse dates based on user-defined formats. The flexibility of the function allows users to specify date formats and handles different scenarios gracefully, using Pandas' pd.to_datetime method.



```
def parse_dates(col, fmt):
    """
    Method to parse the dates based on the format provided,
    this will be created as a UDF in DuckDB
    """
    try:
        if fmt[0].lower() == "y":
            return pd.to_datetime(col, yearfirst=True, errors="coerce")
        if fmt[0].lower() == "m":
            return pd.to_datetime(col, dayfirst=True, errors="coerce")
    except (IndexError, ValueError):
        pass
    return None
```


This function is particularly useful in scenarios where the date formats in the dataset might vary, providing a flexible solution for date parsing within DuckDB.

## Integrating parse_dates into DuckDB
The process of integrating the parse_dates function into DuckDB involves creating a corresponding function within the database. The create_function method checks whether the function already exists and, if not, registers it with DuckDB. The provided SQL query ensures that there are no duplicate entries before attempting to create the function.

```
def create_function(conn):
    """
    Create a function in DuckDB. Currently, it's hardcoded
    we can modify later based on the use case
    """
    function_check = """SELECT DISTINCT  function_name
                        FROM duckdb_functions()
                        WHERE lower(function_type) = 'scalar'
                        AND lower(function_name) in ('parse_dates')
                        ORDER BY function_name;"""

    function_check_output = conn.query(function_check)
    try:
        if not function_check_output:
            conn.create_function("parse_dates", parse_dates, [VARCHAR, VARCHAR], TIMESTAMP)
    except (duckdb.Error, ValueError) as error:
        raise ValueError(
            f"Failed to create function 'parse_dates': {str(error)}"
        ) from error

```

This step ensures that the custom function is available for use in DuckDB's SQL queries.

## Unregistering the Custom Function
The unregister_function method allows users to remove the custom function from DuckDB. If, for any reason, users want to unregister the parse_dates function, this method facilitates the removal of the function from DuckDB.

```
def unregister_function(conn):
    """
    Unregister a function in DuckDB.
    """
    conn.remove_function("parse_dates")

```
This feature emphasizes the dynamic nature of DuckDB, allowing users to manage and tailor the set of available functions according to their evolving needs.

## Conclusion
The integration of custom functions, such as the parse_dates example, exemplifies DuckDB's commitment to providing users with a customizable and extensible platform for data analysis. As users explore and create their custom functions, they gain the ability to enhance DuckDB's capabilities to address unique challenges in data analysis workflows. Custom functions not only open up new possibilities but also empower users to shape their analytical environment to suit their specific requirements, making DuckDB a versatile and user-friendly database for diverse analytical tasks.