I‘m sorry, my question was incomplete and badly written.
The extension geoip2 does lookup a requests ip in a local geoip database so that the result can be used later on to eg. filter requests.
I‘d like to query a ‘regular’ database using parts of a request (not necessarily only the ip) and then use the result further on in the response.
Edit to add: The implementation of geoip seems to be very straight forward, I may just try to use it as a starting point for an own implementation.