When I needed to interface with SQLite from Ada, I discovered that SQLite library provided with the language lacked regular expressions, and I had to add that functionality myself. During this exercise, I learned that you can relatively easily add Ada functions to be executed by SQLite. From performance and ability to debug standpoint, I'd prefer this to a whole language runtime added to the database.

In general, I'd prefer to minimize non-SQL code that runs in the database because it's hard to reason about its implications on the already complicated planning and execution of SQL code. Especially if such code can observe or change the state of the variables involved in a transaction. I feel like to not have this feature backfire, I'd want to have a way to either disallow access to the variables, or create a framework for the foreign code where its made to comply with transaction rules and have to make promises about its own code to not violate them.

The other reason to not do this is that most functions someone might want to add are very simple, enough so that even if you don’t know C, you can probably muddle your way through it.

JS in an RDBMS… shudder