Method optimize

sql/sql_union.cc:2230–2334

Run optimization phase. @return false unit successfully passed optimization phase. @return TRUE an error occur. */

  @return TRUE an error occur.
*/
bool st_select_lex_unit::optimize()
{
  SELECT_LEX *lex_select_save= thd->lex->current_select;
  SELECT_LEX *select_cursor=first_select();
  DBUG_ENTER("st_select_lex_unit::optimize");

  if (optimized && !uncacheable && !describe)
    DBUG_RETURN(false);

  if (with_element && with_element->is_recursive && optimize_started)
    DBUG_RETURN(false);
  optimize_started= true;

  if (uncacheable || !item || !item->assigned() || describe)
  {
    if (item)
      item->reset_value_registration();
    if (optimized && item)
    {
      if (item->assigned())
      {
        item->assigned(0); // We will reinit & reexecute unit
        item->reset();
      }
      if (table->is_created())
      {
        table->file->ha_delete_all_rows();
        table->file->info(HA_STATUS_VARIABLE);
      }
      /* re-enabling indexes for next subselect iteration */
      if ((union_result->force_enable_index_if_needed() || union_distinct))
      {
        if(table->file->ha_enable_indexes(key_map(table->s->keys), false))
          DBUG_ASSERT(0);
        else
          table->no_keyread= 0;
      }
    }
    for (SELECT_LEX *sl= select_cursor; sl; sl= sl->next_select())
    {
      if (sl->tvc)
      {
	sl->tvc->select_options=
          (lim.is_unlimited() || sl->braces) ?
          sl->options & ~OPTION_FOUND_ROWS : sl->options | found_rows_for_union;
	if (sl->tvc->optimize(thd))
        {
          thd->lex->current_select= lex_select_save;
          DBUG_RETURN(TRUE);
        }
        if (derived)
	  sl->increase_derived_records(sl->tvc->get_records());
	continue;
      }
      thd->lex->current_select= sl;

      if (optimized)
	saved_error= sl->join->reinit();