[daoudc]

We actually just take the union and then re-rank. Because the lists are all small, this is cheap.

[marginalia_nu]

Point is, with a skip list (or similar), the lists don't need to be small. You can intersect data sets that are enormous very quickly using this algo[1] where a single linear read of both lists is the worst case scenario.

[1] https://nlp.stanford.edu/IR-book/html/htmledition/faster-pos...