avoiding of allocating JOIN structure on every UNION executing

(SCRUM)

sql/sql_select.cc

@@ -1381,12 +1381,24 @@ mysql_select(THD *thd, Item ***rref_pointer_array,
   JOIN *join;
   if (select_lex->join != 0)
   {
-    //here is EXPLAIN of subselect or derived table
     join= select_lex->join;
-    join->result= result;
-    if (!join->procedure && result->prepare(join->fields_list, unit))
+    if (select_lex->linkage != GLOBAL_OPTIONS_TYPE)
     {
-      DBUG_RETURN(-1);
+      //here is EXPLAIN of subselect or derived table
+      join->result= result;
+      if (!join->procedure && result->prepare(join->fields_list, unit))
+      {
+	DBUG_RETURN(-1);
+      }
+    }
+    else
+    {
+      if (join->prepare(rref_pointer_array, tables, wild_num,
+			conds, og_num, order, group, having, proc_param,
+			select_lex, unit, tables_and_fields_initied))
+      {
+	DBUG_RETURN(-1);
+      }
     }
     join->select_options= select_options;
     free_join= 0;
@@ -1396,7 +1408,6 @@ mysql_select(THD *thd, Item ***rref_pointer_array,
     join= new JOIN(thd, fields, select_options, result);
     thd->proc_info="init";
     thd->used_tables=0;                         // Updated by setup_fields
-
     if (join->prepare(rref_pointer_array, tables, wild_num,
 		      conds, og_num, order, group, having, proc_param,
 		      select_lex, unit, tables_and_fields_initied))
@@ -4300,6 +4311,7 @@ Field *create_tmp_field(THD *thd, TABLE *table,Item *item, Item::Type type,
     default: 
       // This case should never be choosen
       DBUG_ASSERT(0);
+      new_field= 0; // to satisfy compiler (uninitialized variable)
       break;
     }
     if (copy_func && item->is_result_field())

sql/sql_select.h

@@ -190,45 +190,62 @@ class JOIN :public Sql_alloc
   bool optimized; // flag to avoid double optimization in EXPLAIN
 
   JOIN(THD *thd_arg, List<Item> &fields, ulong select_options_arg,
-       select_result *result_arg):
-    join_tab(0),
-    table(0),
-    tables(0), const_tables(0),
-    sort_and_group(0), first_record(0),
-    do_send_rows(1),
-    send_records(0), found_records(0), examined_rows(0),
-    exec_tmp_table1(0), exec_tmp_table2(0),
-    thd(thd_arg),
-    sum_funcs(0),
-    procedure(0),
-    having(0), tmp_having(0),
-    select_options(select_options_arg),
-    result(result_arg),
-    lock(thd_arg->lock),
-    select_lex(0), //for safety
-    tmp_join(0),
-    select_distinct(test(select_options & SELECT_DISTINCT)),
-    no_order(0), simple_order(0), simple_group(0), skip_sort_order(0),
-    need_tmp(0),
-    hidden_group_fields (0), /*safety*/
-    buffer_result(test(select_options & OPTION_BUFFER_RESULT) &&
-		  !test(select_options & OPTION_FOUND_ROWS)),
-    all_fields(fields),
-    fields_list(fields),
-    error(0),
-    select(0),
-    ref_pointer_array(0), items0(0), items1(0), items2(0), items3(0),
-    ref_pointer_array_size(0),
-    zero_result_cause(0),
-    optimized(0)
+       select_result *result_arg)
+    :fields_list(fields)
   {
+    init(thd_arg, fields, select_options_arg, result_arg);
+  }
+  
+  void init(THD *thd_arg, List<Item> &fields, ulong select_options_arg,
+       select_result *result_arg)
+  {
+    join_tab= 0;
+    table= 0;
+    tables= 0;
+    const_tables= 0;
+    sort_and_group= 0;
+    first_record= 0;
+    do_send_rows= 1;
+    send_records= 0;
+    found_records= 0;
+    examined_rows= 0;
+    exec_tmp_table1= 0;
+    exec_tmp_table2= 0;
+    thd= thd_arg;
+    sum_funcs= 0;
+    procedure= 0;
+    having= 0;
+    tmp_having= 0;
+    select_options= select_options_arg;
+    result= result_arg;
+    lock= thd_arg->lock;
+    select_lex= 0; //for safety
+    tmp_join= 0;
+    select_distinct= test(select_options & SELECT_DISTINCT);
+    no_order= 0;
+    simple_order= 0;
+    simple_group= 0;
+    skip_sort_order= 0;
+    need_tmp= 0;
+    hidden_group_fields= 0; /*safety*/
+    buffer_result= test(select_options & OPTION_BUFFER_RESULT) &&
+      !test(select_options & OPTION_FOUND_ROWS);
+    all_fields= fields;
+    fields_list= fields;
+    error= 0;
+    select= 0;
+    ref_pointer_array= items0= items1= items2= items3= 0;
+    ref_pointer_array_size= 0;
+    zero_result_cause= 0;
+    optimized= 0;
+
     fields_list = fields;
     bzero((char*) &keyuse,sizeof(keyuse));
     tmp_table_param.copy_field=0;
     tmp_table_param.end_write_records= HA_POS_ERROR;
     rollup.state= ROLLUP::STATE_NONE;
   }
-  
+
   int prepare(Item ***rref_pointer_array, TABLE_LIST *tables, uint wind_num,
 	      COND *conds, uint og_num, ORDER *order, ORDER *group,
 	      Item *having, ORDER *proc_param, SELECT_LEX *select,

sql/sql_union.cc

@@ -340,6 +340,25 @@ int st_select_lex_unit::exec()
       fake_select_lex->table_list.link_in_list((byte *)&result_table_list,
 					       (byte **)
 					       &result_table_list.next);
+      JOIN *join= fake_select_lex->join;
+      if (!join)
+      {
+	/*
+	  allocate JOIN for fake select only once (privent
+	  mysql_select automatic allocation)
+	*/
+	fake_select_lex->join= new JOIN(thd, item_list, thd->options, result);
+      }
+      else
+      {
+	JOIN_TAB *tab,*end;
+	for (tab=join->join_tab,end=tab+join->tables ; tab != end ; tab++)
+	{
+	  delete tab->select;
+	  delete tab->quick;
+	}
+	join->init(thd, item_list, thd->options, result);
+      }
       res= mysql_select(thd, &fake_select_lex->ref_pointer_array,
 			&result_table_list,
 			0, item_list, NULL,
@@ -378,14 +397,21 @@ int st_select_lex_unit::cleanup()
       free_tmp_table(thd, table);
     table= 0; // Safety
   }
+  JOIN *join;
   for (SELECT_LEX *sl= first_select_in_union(); sl; sl= sl->next_select())
   {
-    JOIN *join;
     if ((join= sl->join))
     {
       error|= sl->join->cleanup();
       delete join;
     }
   }
+  if (fake_select_lex && (join= fake_select_lex->join))
+  {
+    join->tables_list= 0;
+    join->tables= 0;
+    error|= join->cleanup();
+    delete join;
+  }
   DBUG_RETURN(error);
 }