we refer to the main transaction as
   the top-level transaction.
  </p><p>
   If a subtransaction is assigned a non-virtual transaction ID,
   its transaction ID is referred to as a <span class="quote">“<span class="quote">subxid</span>”</span>.
   Read-only subtransactions are not assigned subxids, but once they
   attempt to write, they will be assigned one. This also causes all of
   a subxid's parents, up to and including the top-level transaction,
   to be assigned non-virtual transaction ids.  We