Presto is the open-source SQL query engine for data lakes. It supports many native functions which are usually sufficient for most use cases. However, there is maybe a corner case where you need to implement your own function. To simplify this, Presto allows users to define expressions as SQL functions. These are dynamic functions separated from the Presto source code, managed by a functions namespace manager that you can set up with a MySQL database. In fact, this is one of the most widely used features of Presto at Facebook, with over 1000s of functions defined.
Function Namespace Manager
A function namespace is a special catalog.schema that stores functions in the format like mysql.test. Each catalog.schema can be a function namespace. A function namespace manager is a plugin that manages a set of these function catalog schemas. The catalog can be mapped to connectors in Presto (a connector for functions, no tables or view) and allows the Presto engine to perform actions such as creating, altering, and deleting functions.