Answered: Why do we store a random bigint for cookie id instead the sessionid?

I'd say in all cases they should be interchangeable. In a really weird case in which a user migrates to a different server, session IDs will differ, but cookie IDs won't.

Aside from that, cookie IDs are generated by Q2A and they take 8 bytes (a MySQL big integer). However, PHP session IDs are more unpredictable, server-dependent and most likely would take more space.

For example, 5sd9ev06f3d2j35vgu1i5bri0i is a possible session ID. Those are 26 characters. Storing that would take 26 bytes, which will make searches less efficient.