I wouldn't say BRIN indexes are "the other way around"; index structure is still one where data values are looked up to find the area where occurrences exist.

I wouldn't say BRIN indexes are "the other way around"; index structure is still one where data values are looked up to find the area where occurrences exist. "Coarse" indexes like BRIN and ClickHouse's data-skipping indexes are still indexes in a broad sense of serving to narrow down a search.