(I'm the author of the post - just woke up and saw this discussion. Hi!)

Will put together a follow-up with some more information, but in summary: * yeah, no built-ins for graph operations in Spanner other than relational SQL for standard joins * the key thing that make this work are the ability to easily construct global indexes that aren't sharded by the primary key and reasonably fast joins between them * it's also helpful that Spanner does a reasonable job of parallelizing queries (e.g. a lot of times we'll get a 15x increase in speed vs. a sequential plan) * we then do the fan-out across the graph in our Java Spanner client - each distributed SQL index read takes ~10 ms so we can do multiple round trips of graph traversal in the client