[yason]

I think the parent meant the value vs. reference separation.

Consider that you create a record, give it a primary key N, then start referring to that value by the primary key and at some point make an update to the record, the same primary key now refers to another value. The primary key is just a reference pointer to a placeholder (=record) and depending on whatnot the value in the placeholder can change to anything. So, you have to be careful of what you mean by the primary key because it's just a reference, not a value. In the value paradigm your primary key would be a hash, like in git, that would forever be that one value instead of referring to some value.

[talaketu]

Exactly. Primary key is a subset of attributes - thus necessarily populated by values.

It's worth considering the correspondence between the concept of functional dependency in the relational model, and the concept of a pure function in functional programming. The issue under discussion, then, is whether referential transparency is afforded in the database.

While referential transparency in a database is achieved momentarily at the right isolation level, it is not achieved in the eternal sense of a pure function.

This is because the functional dependency in FP encodes an intensional definition, whereas the functional dependency captured in a relation is extensional, usually modelling the state of the knowledge of the relevant world, and therefore being subject to change.