Pagination only makes sense in the context of an ordered collection; if there is no stable sort order then you can’t paginate. So you identify the last record seen with whatever fields you are ordering by, and if the last record has been deleted, then it doesn’t matter because you are only fetching the items greater than those values according to the sort order.

Anyway, there is plenty of documentation out there for cursor-based pagination; Hacker News comments isn’t the right place to explain the implementation details.

1. This kind of pagination can be done for any key as long as its data type admits total ordering.

2. The WHERE condition typically uses `>` or `<`, so it doesn't fail even when that record is deleted before the next client request referring to it.

If the IDs are monotonous, it doesn't matter.

(for posterity, this appears to have been a typo for monotonic)