We can't use transaction ids for that however, because they're necessarily assigned by the time a transaction starts to write, whereas visibility is determined

We can't use transaction ids for that however, because they're necessarily assigned by the time a transaction starts to write, whereas visibility is determined by the time transactions commit. Snapshots, the datastructure that determines which transaction's writes ought to be visible and which not, use transaction ids as cutoff values however, to make visibility determinations cheaper. A second large reasons why we'd not want to go for randomness is that we need to store data for each transaction id, namely whether it committed or not. If we'd assign them randomly we'd need to keep around a lot more of that data, and accesses would be a lot more expensive because there'd basically not be any locality.