When we are reading a Lance file the scheduler will usually submit requests in the order that they are needed. The exception to this is any field that needs indirect I/O (for example, a list field). When the scheduler reaches that field it will immediately schedule the I/O requests to fetch the data offsets. However, we cannot schedule the data at this time because we do not know what I/O to issue. Instead, the data is requested as soon as the offsets are fetched. By this time the scheduler may have scheduled many more requests.

We want these indirect IOPS to take priority over IOPS scheduled for later pages. We can easily assign a priority for all IOPS based on the top-level row number of the first row the request is fetching (e.g. if we have List<List> and we ask for rows 10..20 then three different requests will be generated and they will all have priority 10.

One potential wrinkle is that these priorities only apply with the context of a single file. If two files are being read simultaneously then we the normal first-come-first-serve priority should apply. So the priority order is roughly...

if lhs.file == rhs.file:
  return lhs.priority < rhs.priority
else:
  return lhs.submit_timestamp < rhs.submit_timestamp

However, the above function is not transitive and so it wouldn't work as a sort function.