As far as I'm understanding your situtation it seems like you're using range partitioning and then hoping to get benefits of hash partitioning from a range part

As far as I'm understanding your situtation it seems like you're using range partitioning and then hoping to get benefits of hash partitioning from a range partition. I think you would need to use hash partitions to avoid look ups on every every table. You could do something weird like have each range partition have an exclusion filter and make sure that each of your randomly generated IDs accommodate that exclusion filter, and generate IDs until you find one.. But it sounds like you would benefit from a hash partition here instead of a range one.