Hi, Looking at the explanation abour ctid, it is "The physical location of the row version within its table. " From that line, I think ctid is unique in the table.
Unique but not stable - if you give your actual record an ID value the associated ctid for it may very well change over time and a given ctid can be associated with any number of IDs
And I also think ctid might be unique across the database since it is the physical location.
The concept doesn't even apply - the value itself only makes sense within a given physical table. i.e., the table is implied. It's like saying "I live at 123 Main St." to someone. Sure in any given place there can only be a single 123 Main St. but that really isn't useful by itself. And to extend back to the previous point, you may live there now but you will likely have a different address in the future and someone else will have 123 Main St.
