Tape. SQL databases emerged when data was stored on tape.
join table1, table2 where table1.id = table2.customer_id
type operations would have a tape for table1 in one drive, and a tape for table2 in the other drive. Things like fixed length records emerged to make it possible to fast forward the tape a specific number of inches to the point where the next record would begin, facilitating non-linear access.
Once that model was completely baked into the tooling, it didn't go away when the data moved to HDs then SSDs. The paradigms have outlived the hardware.
It's a bit like the save icon still being a floppy disk.
That's wrong. Since its inception, on of the main points of the relational mode is physical data independence.
No assumptions on how or where the data is physically stored is made by the model. Besides, it seems more plausible to me that Codd would be working on machines with disks, not tapes, by the time he proposed the model. See this, for example: https://www.theregister.com/2013/11/20/ibm_system_r_making_r...
So my understanding is relational databases were born on disks, not on tapes, but that distinction doesn't really matter to it. Additionally, it is the physical independence of the model that let all implementations based on it adapt when SSDs (and newer) storage systems arrived.
Finally, saying "the paradigms have outlived the hardware" doesn't make sense in this context. To repeat: the relational model was proposed precisely to isolate logical database design from hardware details. As a paradigm, it has been independent of hardware from conception, so of course it would outlive any specific hardware incarnation.
I’m no historian, but indexed disk-based structures - upon which relational databases were built – have been around a very long time. For example, IBM brought out ISAM in the mid 1960’s, roughly contemporary with the publication of the relational model. I could be wrong, but it doesn’t seem likely to me somehow that that those guys had tape foremost in mind.
Data locality still mattered in mechanical disks as it avoided seeking, and data locality still matters in SSD as a read brings in a whole block instead regardless of how much you need.
Tape was before ISAM iirc - things like merge-sort were big then, and the spinning tapes you'd see in those old black and white movies from the 60's. ISAM needed disks, and so did RDB's
join table1, table2 where table1.id = table2.customer_id
type operations would have a tape for table1 in one drive, and a tape for table2 in the other drive. Things like fixed length records emerged to make it possible to fast forward the tape a specific number of inches to the point where the next record would begin, facilitating non-linear access.
Once that model was completely baked into the tooling, it didn't go away when the data moved to HDs then SSDs. The paradigms have outlived the hardware.
It's a bit like the save icon still being a floppy disk.