https://cloud.google.com/blog/topics/developers-practitioner...
EDIT: of note, Spanner has its own first-party JDBC driver now too (fairly recent, IIUC): https://cloud.google.com/spanner/docs/open-source-jdbc
"The PostgreSQL language in Cloud Spanner": https://cloud.google.com/spanner/docs/postgresql/overview
"PostgreSQL interface" under the Concepts section: https://cloud.google.com/spanner/docs/postgresql-interface