Wow, thanks for that info, very useful. When you say “I really hope they open source that part”, do you mean the mechanism to plug other dialects in, or the PostgreSQL interface?
This is the lexical structure and syntax docs for the new postgres inteface to cloud spanner:
https://cloud.google.com/spanner/docs/postgresql/lexical
And this is the zetasql lexical structure and syntax docs:
https://github.com/google/zetasql/blob/master/docs/lexical.m...
Notice that the new PG docs are an edit of the Zeta ones - evidence that my hypothesis is correct.