Function get_index_for_order

sql/sql_select.cc:33637–33719

*/

uint get_index_for_order(ORDER *order, TABLE *table, SQL_SELECT *select,
                         ha_rows limit, ha_rows *scanned_limit,
                         bool *need_sort, bool *reverse)
{
  if (!order)
  {
    *need_sort= FALSE;
    if (select && select->quick)
      return select->quick->index; // index or MAX_KEY, use quick select as is
    else
      return table->file->key_used_on_scan; // MAX_KEY or index for some engine
  }

  if (!is_simple_order(order)) // just to cut further expensive checks
  {
    *need_sort= TRUE;
    return MAX_KEY;
  }

  if (select && select->quick)
  {
    if (select->quick->index == MAX_KEY)
    {
      *need_sort= TRUE;
      return MAX_KEY;
    }

    uint used_key_parts;
    switch (test_if_order_by_key(NULL, order, table, select->quick->index,
                                 &used_key_parts)) {
    case 1: // desired order
      *need_sort= FALSE; 
      *scanned_limit= MY_MIN(limit, select->quick->records);
      return select->quick->index;
    case 0: // unacceptable order
      *need_sort= TRUE;
      return MAX_KEY;
    case -1: // desired order, but opposite direction
      {
        QUICK_SELECT_I *reverse_quick;
        if ((reverse_quick=
               select->quick->make_reverse(used_key_parts)))
        {
          select->set_quick(reverse_quick);
          *need_sort= FALSE;
          *scanned_limit= MY_MIN(limit, select->quick->records);
          return select->quick->index;
        }
        else
        {
          *need_sort= TRUE;
          return MAX_KEY;
        }
      }
    }
    DBUG_ASSERT(0);
  }
  else if (limit != HA_POS_ERROR)