The Cloud Spanner SQL frontend/analyzer has been open sourced by google as ZetaSQL. https://github.com/google/zetasql

It is amazingly good.

You give it textual SQL (+ schema + all your function definitions) and it returns a really clean logical query plan. It is also happy to do this via a protobufs so you can use it from languages other than C++. It is also tested and documented up the wazoo. It has been such a pleasure to work with.

Anyway, the big problem with ZetaSQL is that it is not a common SQL dialect.

It seems that the only reasonable way to do this PostgreSQL interface for Cloud Spanner is to add a second parser (and other extensions) to ZetaSQL. If I am correct, I really really hope they open source that part of ZetaSQL as well - it would be a massive step forward for open source SQL tooling.