The more I deal with scalable relational DBMSes (Spanner in particular), the more I doubt their usefulness even at large scale. Relational + fully-consistent are always at odds with scalability. It seems like either you have a classic NoSQL use case and use that, or you shard at the application level rather than in the database.

Could someone share a use case where you truly benefited migrating from Postgres/MySQL to Spanner/Citus/Cockroach, and there was no better solution? I'd like my hunch to be wrong.

that really should be the first heuristic for almost any systems design problem - if you can afford to buy a big enough pair of machines to fit your data in hot-swap postgres then just do that.

don't bother with mongo or mysql or dynamo or cassandra or bigtable or spanner or ... until your lack of profitability or size means you can't afford to just use postgres.

I wonder what the costs are compared to Aurora. I haven't been on AWS for a while but loved the service when I was.