The relation is the table, normalization, foreign keys, candidate keys etc are all extensions to that base model for Codd.
Some of the impedance mismatch is due to that, and not just the declarative nature or extensions
Specifically I think the Alice book covers how with a candidate key + the remaining tuples in the row form the row but only the candidate key has an identity, the rest of the row is a substring.
Some quotes from the link, but searching for 'user' will hit what I think justifies my interpretation.
> Future users of large data banks must be protected from having to know how the data is organized in the machine (the internal representation).
> To sum up, it is proposed that most users should interact with a relational model of the data consisting of a collection of time-varying relationships (rather than relations). Each user need not know more about any relationship than its name together with the names of its domains (role qualified whenever necessary): Even this information might be offered in menu style by the system (subject to security and privacy constraints) upon request by the user.
