Of course MySQL has obviously the deep support for custom storage engines, then it has loadable functions, auth, component service and these are currently internal extensibility but going forward we are working on a way to manage this extensibility externally through a new proposal which I will be going over in the upcoming MySQL contributor summit in November. Postgres though highly externally extensible doesn't cut into the core like MySQL or MariaDB does thus making it hard to for example make TidesDB integration as TidesDB has to handle the MVCC, for example. OrioleDB is another example and it has to utilize a custom Postgres fork for this reason, the core not being very extensible, though I hear this is being worked on heavily.