It's more than that - virtually large all datasets I've worked with (> 20B documents) had a keyspace of under 128 keys/columns per nested level of a JSON document. Meaning an optimizing DB could express most keys as single byte (or less with packing). I don't thing column reordering would be necessary unless you exhausted the first 128 keys and you wanted to optimize the base-128 keyspace assignment. Such an operation could be done during compaction or any other operation that rewrites a table.