Yeah ZomboDB looks interesting, I guess the main difference is Groonga is an embedded database, more like sqlite; whereas Elastic Search you still gotta spin up an instance and be responsible for it.
I guess you're still responsible for the Groonga db on the postgres server too but theoretically it seems more secure than spinning up another service on a different server
Looks like it might be similar to https://zombodb.com/ which I haven't tried yet but which looks like a really interesting solution in this space.