[morgo]

I can't speak to PostgreSQL, but I work on the TiDB team having previously worked on the MySQL team.

Ensuring compatibility from a clean slate is hard, but to make MySQL distributed requires more than the current storage engine API provides. Thus, you would be likely be creating a fork and no longer getting the benefits of having an upstream[1].

The surgery you need to perform on the code would also reduce you from full compatibility (switching from pessimistic to optimistic locking means some statements no longer work). There are some great side benefits to a clean slate too: In TiDB we're able to use modern languages such as Go for the TiDB Server, and Rust for TiKV.

([1] Phrasing this question to upstream: I think they'd like to offer such functionality, but introducing this level of backward incompatibility is hard. They are also very concerned about performance regression bugs, which would be very hard to prevent with all the refactoring required.)