Function test_if_subkey

sql/sql_select.cc:27369–27399

*/

static uint
test_if_subkey(ORDER *order, TABLE *table, uint ref, uint ref_key_parts,
	       const key_map *usable_keys)
{
  uint nr;
  uint min_length= (uint) ~0;
  uint best= MAX_KEY;
  KEY_PART_INFO *ref_key_part= table->key_info[ref].key_part;
  KEY_PART_INFO *ref_key_part_end= ref_key_part + ref_key_parts;
  
  /*
    Find the shortest key that
    - produces the required ordering
    - has key #ref (up to ref_key_parts) as its subkey.
  */
  for (nr= 0 ; nr < table->s->keys ; nr++)
  {
    uint not_used;
    if (usable_keys->is_set(nr) &&
	table->key_info[nr].key_length < min_length &&
	table->key_info[nr].user_defined_key_parts >= ref_key_parts &&
	is_subkey(table->key_info[nr].key_part, ref_key_part,
		  ref_key_part_end) &&
	test_if_order_by_key(NULL, order, table, nr, &not_used))
    {
      min_length= table->key_info[nr].key_length;
      best= nr;
    }
  }
  return best;
}


/**