I don't know how things are tracked right now. My gut feeling (sometimes wrong) is that there is a per-user-per-category timestamp which is used to generate these magic dots. If you want it per-post then that jumps from users x categories to users x posts - potentially a huge table.

@manassehkatz You wouldn't need to store timestamps, just read/unread status, so it would just be the table of users and the list of posts they've read. Updating the lists might be a pain though.

@Moshi Whether it is UserId + PostId + Boolean or UserId + PostId + Timestamp makes very little difference. Timestamp would make for slightly larger records. Boolean would make the update process more complicated (because you would have to update all the little records each time). There are some other variants I can think of. But end result is a ton of little records for relatively little benefit. MySQL (and PostgreSQL too) doesn't handle that all that well when you

scale from thousands up to millions. Been there, done that. Most of the actual disk spaces gets taken up by the indexes. No matter how you slice it, its a mess. Not worth doing IMHO. Personally, the way I read (here or SE) is I go to the relevant site/community/category and then start at the top and work my way down until I see stuff that I know I already read (based on displayed timestamps).

@manassehkatz I didn't mean UserID + PostID, I meant having one record per userID with a list of postIDs that correspond to which posts the user has read.

Typical databases don't have records like that - i.e., some fixed fields plus a variable number of additional fields (in this case, the PostIds could go into the thousands very quickly). What they instead have is two types of master records (User and Post) and a ManyToMany Table connecting them (UserId + PostId + whatever other necessary information, if any).

@manassehkatz Typical databases can't store arrays?

No. They fake it. Either for a small fixed size array by using a bunch of fields, or for a large variable size array by using another table.

@manassehkatz ah, I see. your comments make a lot more sense to me now