MartenDB. :)
I am thinking more along the lines of an embedded db like SQLite but does native serialization, indexing and query optimization against .Net objects and types.
Then to make a "database server" is really just an app server running your c#. If you really wanted to make it like pg then you would have an app server that accepts code snippets, compiles runs and returns results. Of course you would need proper sandboxing like .net fiddle. You could also do something in between like serializing Linq expression trees to send them to server for processing.
In an ideal world where your code runs (client, app server, db server) is a choice based on locality needs not forced by runtimes (javascript in the browser, .Net on the app server, pg/Sql on the db server).
You can pretty much get close to this now with javascript/node/plV8, but again I prefer C#/.Net.
It’s stores it as jsonb. So you don’t need to worry about weird relationships you only worry about your root aggregate. You can index it. Apply full text search. And persisted columns in the map if for convince or speed.
So Yeah it maps the linq to the equiv sql for you. But allows you to work with your object and worry less about persistence. Cos you can add and remove properties. It’s the closest thing I’ve come to, to being able to forget about the database and focus on code.
I Guess unless you write your own database from the ground up so it supports the language as a feature without dropping back to something else. It won’t happen.
However the mismatch is always there, the overhead of mapping types, inefficient serialization (key names repeated over and over in json vs a real schema), certain statements cannot be transpiled to sql so you have pathologic cases at unexpected times, in ability to run any statement you want in the db tier to take advantage of locality and so on.
Most everyone wants a object database since most people are working with objects which is why ORM are so popular.
Closet thing I ever saw was db4o.
This really adds up on a large database. Mongo used to recommend short key names and I even think some client would translate them based on some definition to try and save disk/memory/io
https://github.com/postgrespro/zson tries to do key compression.
Also its not that Linq can't do what you want, its that if you do certain things that can't be translated to SQL the whole intermediate result will be retrieved from the db, deserialized then the linq statement will be run in memory on the client. Its like putting ToList in the middle of your linq statement where a maybe a few million rows might need to be processed to give the very small result.
If the linq statement was being run in process on the db server at least it would happen local to the data and would be more akin to a table scan vs index, still not great but much better then sending all the results to the client for processing.
The only downside is that the only interface to your data is through the application, but there's certainly a use-case for something like that.
More than 20 years ago I actually used it as a true application server, where the database application layer would even generate all the HTML code required to displayed your app ui.
And with database schemas you have proper sandboxing.