 ensure that a parent
   xid is always lower than any of its child subxids.
  </p><p>
   The immediate parent xid of each subxid is recorded in the
   <code class="filename">pg_subtrans</code> directory. No entry is made for
   top-level xids since they do not have a parent, nor is an entry made
   for read-only subtransactions.
  </p><p>
   When a subtransaction commits, all of its committed child
   subtransactions with subxids will also be considered subcommitted
   in that transaction.  When a subtransa