External database server functions are provided for SQL Server, PostgreSQL, and Db2 as part of MFDBFH's File View Generation support. These external functions can optionally be installed as alternatives to the T-SQL functions provided for SQL Server and PostgreSQL, but are mandatory for Db2. You would typically only need to install the external functions for SQL Server or PostgreSQL if the supplied T-SQL functions were not providing the desired performance.
Both the T-SQL and the external function types are supplied with your Micro Focus product.