Get tooled up before grappling with Google's Spanner database

Cost?

How is that cost likely to compare with, say, an Oracle setup that attempts to do the same thing?

The advantage of relational databases…

… is that they're based on relational algebra and thus have a solid mathematical foundation. Removing redundancy via normalisation gives the reliability and flexibility of predicate logic. JOINs themselves are an artefact of SQL, which is itself is not relational – there is no room in relational algebra for NULL – and gets a lot of things wrong as a result. Modern relational databases can provide incredible performance (things like Swarm64 are aiming for 30 m rows/s @ 40 byte writing) which should be sufficient for most transactional workloads but this requires local clients. Worldwide replication will inherently introduce some latency. Chances are, however, that if you're hitting those kind of limits you already know these things. Unless you're doing NoSQL because "webscale" and wondering why your data keeps getting corrupt or deleted.

The use of a dedicated network with synchronisation via atomic clocks is useful in its own right and possibly the most important aspect of the implementation.

"Hence NoSQL, the collection of database-like alternatives that didn't use SQL"

I think you'll find they actually are databases.

Throw a spanner in

Spanner is an interesting data store. It works great where you have billions of things and each of those things has some small varying number of other things related to it - this is the nested tables trick. Parent and nested child tables are stored together so there's no I/O seek penalty for getting them all together. For other cases it's not relational even though a flimsy SQL interpreter makes it look like it is. Updates are transactional with a compare & set type of operation, which can be done in MySQL too.

The real question is if you trust Google. My experience is that recoverable glitches happen several times a day, momentary failures happen almost every day, and there's some significant screw-up outage at least once a month. Google believes they're perfect so Spanner comes with absolutely no monitoring dashboard.

In my test between Google's hosted Spanner and Google's hosted MySQL 2, Spanner won by an enormous margin. Their MySQL 2 instances always seems to be abnormally slow so it's a biased test. I'd never host data on Google unless there was a real need for it.