[strken]

GraphQL came from Facebook, where it's used to wrap other services and aggregate the results into a tree of data. I've also used it this way.

As I understand, most of the constraints (writes are done in isolation, you don't have transactions, etc.) are there because they're consistent with an architecture where you can't actually implement those things. Transactions are something that happens further down the chain of services, and a single mutation may or may not be implemented using transactions that may or may not be distributed across multiple services. A mutation could cause information to be sent to message queues, stored in various data stores with different technologies, cached in multiple places, persisted to a data warehouse, or other side effects, and transactions are an implementation detail.

You could use foreign data wrappers in postgres, or some other equivalent, but most of the good bits of SQL would take a lot of work or wouldn't even be possible.