> Serializing access to shared data items fundamentally requires some serial execution.
Sure, but typical MVCC systems that provide SI will serialize transactions in some cases anyway. For example, if two transactions try to update the same row in Postgres, the second will block until the first one commits/aborts. This is true under RC, RR, or SERIALIZABLE.
> This fundamental overhead is a key reason why almost every commodity RDBMS defaults to one of these weaker isolation levels and why databases like Oracle don't support serializable isolation at all.
I don't think that's true: if you already have an MVCC implementation that provides SI, the additional overhead of providing SSI is relatively modest. I'd say the main reason Oracle doesn't provide SERIALIZABLE is historical: their MVCC implementation predated Cahill's work by many years, and people literally didn't know how to use MVCC to provide SSI at the time.
Sure, but typical MVCC systems that provide SI will serialize transactions in some cases anyway. For example, if two transactions try to update the same row in Postgres, the second will block until the first one commits/aborts. This is true under RC, RR, or SERIALIZABLE.
> This fundamental overhead is a key reason why almost every commodity RDBMS defaults to one of these weaker isolation levels and why databases like Oracle don't support serializable isolation at all.
I don't think that's true: if you already have an MVCC implementation that provides SI, the additional overhead of providing SSI is relatively modest. I'd say the main reason Oracle doesn't provide SERIALIZABLE is historical: their MVCC implementation predated Cahill's work by many years, and people literally didn't know how to use MVCC to provide SSI at the time.