Function test_if_order_by_key

sql/sql_select.cc:27169–27284

*/

static int test_if_order_by_key(JOIN *join, ORDER *order, TABLE *table,
                                uint idx, uint *used_key_parts)
{
  KEY_PART_INFO *key_part,*key_part_end;
  key_part=table->key_info[idx].key_part;
  key_part_end=key_part + table->key_info[idx].ext_key_parts;
  key_part_map const_key_parts=table->const_key_parts[idx];
  uint user_defined_kp= table->key_info[idx].user_defined_key_parts;
  int reverse=0;
  uint key_parts;
  bool have_pk_suffix= false;
  uint pk= table->s->primary_key;
  ORDER::enum_order keypart_order;
  DBUG_ENTER("test_if_order_by_key");
 
  if ((table->file->ha_table_flags() & HA_PRIMARY_KEY_IN_READ_INDEX) && 
      idx < table->s->keys &&
      table->key_info[idx].ext_key_part_map && pk != MAX_KEY && pk != idx)
  {
    have_pk_suffix= true;
  }

  if ((*order->item)->real_item()->type() != Item::FIELD_ITEM)
  {
    if (order->next || order->direction != ORDER::ORDER_ASC)
      DBUG_RETURN(0);

    DBUG_RETURN((*order->item)->part_of_sortkey().is_set(idx));
  }

  for (; order ; order=order->next, const_key_parts>>=1)
  {
    Item_field *item_field= ((Item_field*) (*order->item)->real_item());
    int flag;

    /*
      Skip key parts that are constants in the WHERE clause.
      These are already skipped in the ORDER BY by const_expression_in_where()
      for top level queries.
    */
    for (; const_key_parts & 1 ; const_key_parts>>= 1)
    {
      if (item_field->contains(key_part->field))
      {
        /* Subquery with ORDER BY, continue with next field */
        goto next_order_field;
      }
      key_part++;
    }

    /*
      This check was in this function historically (although I think it's
      better to check it outside of this function):

      "Test if the primary key parts were all const (i.e. there's one row).
       The sorting doesn't matter"

       So, we're checking that