It's worth noting that in current conditions the assumptions may be unwarranted.
First, while storage used to be orders of magnitude slower than memory, not SSD storage is just a single order of magnitude slower;
Second, in many domains now it's often practical to ensure that your data set can fit in memory. For example, if your system is for storing financial transactions (which is a prime market for Oracle), then your enterprise has to be quite large to get a terabyte of transactions and you can put a terabyte (or much more) of RAM in a database system if you choose to.
First, while storage used to be orders of magnitude slower than memory, not SSD storage is just a single order of magnitude slower;
Second, in many domains now it's often practical to ensure that your data set can fit in memory. For example, if your system is for storing financial transactions (which is a prime market for Oracle), then your enterprise has to be quite large to get a terabyte of transactions and you can put a terabyte (or much more) of RAM in a database system if you choose to.