MDEV-29612 ReadViewBase::snapshot() misses an optimization

ReadViewBase::snapshot(): In case m_low_limit_no==m_low_limit_id
and m_ids would include everything between that and m_up_limit_id,
set all fields to m_up_limit_id and clear m_ids, to speed up
changes_visible() and append().

rw_trx_hash_t::debug_iterator(): Add an assertion.

storage/innobase/include/trx0sys.h

@@ -523,6 +523,7 @@ class rw_trx_hash_t
     if (element->trx)
       validate_element(element->trx);
     element->mutex.wr_unlock();
+    ut_ad(element->id < element->no);
     return arg->action(element, arg->argument);
   }
 #endif

storage/innobase/read/read0read.cc

@@ -173,9 +173,22 @@ For details see: row_vers_old_has_index_entry() and row_purge_poss_sec()
 inline void ReadViewBase::snapshot(trx_t *trx)
 {
   trx_sys.snapshot_ids(trx, &m_ids, &m_low_limit_id, &m_low_limit_no);
+  if (m_ids.empty())
+  {
+    m_up_limit_id= m_low_limit_id;
+    return;
+  }
+
   std::sort(m_ids.begin(), m_ids.end());
-  m_up_limit_id= m_ids.empty() ? m_low_limit_id : m_ids.front();
+  m_up_limit_id= m_ids.front();
   ut_ad(m_up_limit_id <= m_low_limit_id);
+
+  if (m_low_limit_no == m_low_limit_id &&
+      m_low_limit_id == m_up_limit_id + m_ids.size())
+  {
+    m_ids.clear();
+    m_low_limit_id= m_low_limit_no= m_up_limit_id;
+  }
 }