EXPLAIN UNION using same routing which used for execution which allow return...

EXPLAIN UNION using same routing which used for execution which allow return correct bug messages (Bug #3639)
EXPLAIN of hidden SELECT of UNION


mysql-test/r/derived.result:
  explain of hidden select
mysql-test/r/subselect.result:
  explain of hidden select
mysql-test/r/union.result:
  explain of hidden select
  correct error messages on explain
mysql-test/t/subselect.test:
  show eliminated costants in WHERE clause
mysql-test/t/union.test:
  correct error messages on EXPLAIN with union
sql/item.cc:
  fixed name constructing for global ORDER BY items
sql/sql_class.h:
  select ID can be negative (for hidden SELECTs)
  removed unused field
sql/sql_lex.cc:
  new flag of UNION EXPLAIN
sql/sql_lex.h:
  new flag of UNION EXPLAIN
  select ID can be negative (for hidden SELECTs)
sql/sql_select.cc:
  EXPLAIN UNION using same routing which used for execution
  explain for hidden SELECT of UNION
sql/sql_union.cc:
  EXPLAIN UNION using same routing which used for execution

mysql-test/r/derived.result

@@ -94,11 +94,13 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	3	
 2	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	4	
 3	UNION	t1	ALL	NULL	NULL	NULL	NULL	4	
+-2	UNION RESULT	<union2>	ALL	NULL	NULL	NULL	NULL	NULL	
 explain select * from (select * from t1 union all select * from t1) a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	8	
 2	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	4	
 3	UNION	t1	ALL	NULL	NULL	NULL	NULL	4	
+-2	UNION RESULT	<union2>	ALL	NULL	NULL	NULL	NULL	NULL	
 CREATE TABLE t2 (a int not null);
 insert into t2 values(1);
 select * from (select * from t1 where t1.a=(select a from t2 where t2.a=t1.a)) a;
@@ -246,8 +248,10 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	<derived4>	ALL	NULL	NULL	NULL	NULL	2	
 4	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	2	
 5	UNION	t1	ALL	NULL	NULL	NULL	NULL	2	
+-4	UNION RESULT	<union4>	ALL	NULL	NULL	NULL	NULL	NULL	
 2	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	2	
 3	UNION	t1	ALL	NULL	NULL	NULL	NULL	2	
+-2	UNION RESULT	<union2>	ALL	NULL	NULL	NULL	NULL	NULL	
 drop table t1;
 CREATE TABLE `t1` (
 `N` int(11) unsigned NOT NULL default '0',

mysql-test/r/subselect.result

@@ -16,6 +16,7 @@ explain extended SELECT (SELECT 1) UNION SELECT (SELECT 2);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
 3	UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+-1	UNION RESULT	<union1>	ALL	NULL	NULL	NULL	NULL	NULL	
 Warnings:
 Note	1249	Select 2 was reduced during optimisation
 Note	1249	Select 4 was reduced during optimisation
@@ -28,6 +29,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
 3	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
 4	UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+-3	UNION RESULT	<union3>	ALL	NULL	NULL	NULL	NULL	NULL	
 Warnings:
 Note	1249	Select 2 was reduced during optimisation
 Note	1003	select high_priority (select 0 AS `0` union select 0 AS `0`) AS `(SELECT (SELECT 0 UNION SELECT 0))`
@@ -182,6 +184,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 2	SUBQUERY	t3	ALL	NULL	NULL	NULL	NULL	3	Using filesort
 3	UNION	t4	ALL	NULL	NULL	NULL	NULL	3	Using where; Using filesort
 4	SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	
+-1	UNION RESULT	<union1>	ALL	NULL	NULL	NULL	NULL	NULL	
 Warnings:
 Note	1003	(select high_priority test.t2.a AS `a`,test.t2.b AS `b` from test.t2 where (test.t2.b = (select test.t3.a AS `a` from test.t3 order by test.t3.a desc limit 1))) union (select test.t4.a AS `a`,test.t4.b AS `b` from test.t4 where (test.t4.b = (select (max(test.t2.a) * 4) AS `max(t2.a)*4` from test.t2)) order by test.t4.a)
 select (select a from t3 where a<t2.a*4 order by 1 desc limit 1), a from t2;
@@ -306,6 +309,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	2	
 2	DEPENDENT SUBQUERY	t1	system	NULL	NULL	NULL	NULL	1	
 3	DEPENDENT UNION	t5	ALL	NULL	NULL	NULL	NULL	2	Using where
+-2	UNION RESULT	<union2>	ALL	NULL	NULL	NULL	NULL	NULL	
 Warnings:
 Note	1276	Field or reference 't2.a' of SELECT #2 was resolved in SELECT #1
 Note	1276	Field or reference 't2.a' of SELECT #3 was resolved in SELECT #1
@@ -413,6 +417,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	index	NULL	topic	3	NULL	2	Using index
 2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
 3	UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+-2	UNION RESULT	<union2>	ALL	NULL	NULL	NULL	NULL	NULL	
 Warnings:
 Note	1003	select high_priority 1 AS `1` from test.t1
 drop table t1;
@@ -727,6 +732,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t2	index	NULL	id	5	NULL	2	Using where; Using index
 2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
 3	DEPENDENT UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+-2	UNION RESULT	<union2>	ALL	NULL	NULL	NULL	NULL	NULL	
 Warnings:
 Note	1003	select high_priority test.t2.id AS `id` from test.t2 where <in_optimizer>(test.t2.id,<exists>(select 1 AS `Not_used` having (<cache>(test.t2.id) = <null_helper>(1)) union select 1 AS `Not_used` having (<cache>(test.t2.id) = <null_helper>(3))))
 SELECT * FROM t2 WHERE id IN (SELECT 5 UNION SELECT 3);
@@ -1536,11 +1542,14 @@ e
 select * from t1 where 'f' > any (select s1 from t1 union select s1 from t1);
 s1
 e
-explain select * from t1 where 'f' > any (select s1 from t1 union select s1 from t1);
+explain extended select * from t1 where 'f' > any (select s1 from t1 union select s1 from t1);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	1	
 2	SUBQUERY	t1	system	NULL	NULL	NULL	NULL	1	
 3	UNION	t1	system	NULL	NULL	NULL	NULL	1	
+-2	UNION RESULT	<union2>	ALL	NULL	NULL	NULL	NULL	NULL	
+Warnings:
+Note	1003	select high_priority test.t1.s1 AS `s1` from test.t1
 drop table t1;
 CREATE TABLE t1 (number char(11) NOT NULL default '') ENGINE=MyISAM CHARSET=latin1;
 INSERT INTO t1 VALUES ('69294728265'),('18621828126'),('89356874041'),('95895001874');

mysql-test/r/union.result

@@ -86,6 +86,7 @@ explain extended (select a,b from t1 limit 2)  union all (select a,b from t2 ord
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	4	
 2	UNION	t2	ALL	NULL	NULL	NULL	NULL	4	Using filesort
+-1	UNION RESULT	<union1>	ALL	NULL	NULL	NULL	NULL	NULL	Using filesort
 Warnings:
 Note	1003	(select high_priority test.t1.a AS `a`,test.t1.b AS `b` from test.t1 limit 2) union all (select test.t2.a AS `a`,test.t2.b AS `b` from test.t2 order by test.t2.a limit 1) order by b desc
 (select sql_calc_found_rows  a,b from t1 limit 2)  union all (select a,b from t2 order by a) limit 2;
@@ -106,21 +107,15 @@ explain select a,b from t1 union all select a,b from t2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	4	
 2	UNION	t2	ALL	NULL	NULL	NULL	NULL	4	
+-1	UNION RESULT	<union1>	ALL	NULL	NULL	NULL	NULL	NULL	
 explain select xx from t1 union select 1;
 ERROR 42S22: Unknown column 'xx' in 'field list'
 explain select a,b from t1 union select 1;
-id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	4	
-2	UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+ERROR 21000: The used SELECT statements have a different number of columns
 explain select 1 union select a,b from t1 union select 1;
-id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
-2	UNION	t1	ALL	NULL	NULL	NULL	NULL	4	
-3	UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+ERROR 21000: The used SELECT statements have a different number of columns
 explain select a,b from t1 union select 1 limit 0;
-id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
-2	UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+ERROR 21000: The used SELECT statements have a different number of columns
 select a,b from t1 into outfile 'skr' union select a,b from t2;
 ERROR HY000: Wrong usage of UNION and INTO
 select a,b from t1 order by a union select a,b from t2;
@@ -475,6 +470,7 @@ explain extended (select * from t1 where a=1) union (select * from t2 where a=1)
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	const	PRIMARY	PRIMARY	4	const	1	
 2	UNION	t2	const	PRIMARY	PRIMARY	4	const	1	
+-1	UNION RESULT	<union1>	ALL	NULL	NULL	NULL	NULL	NULL	
 Warnings:
 Note	1003	(select high_priority test.t1.a AS `a`,test.t1.b AS `b` from test.t1 where (test.t1.a = 1)) union (select test.t2.a AS `a`,test.t2.b AS `b` from test.t2 where (test.t2.a = 1))
 (select * from t1 where a=5) union (select * from t2 where a=1);
@@ -497,10 +493,12 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 2	UNION	t1	index	PRIMARY	PRIMARY	4	NULL	4	Using index
 2	UNION	t2	index	PRIMARY	PRIMARY	4	NULL	3	Using where; Using index
+-1	UNION RESULT	<union1>	ALL	NULL	NULL	NULL	NULL	NULL	
 explain (select * from t1 where a=1) union (select * from t1 where b=1);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	const	PRIMARY	PRIMARY	4	const	1	
 2	UNION	t1	ref	b	b	5	const	1	Using where
+-1	UNION RESULT	<union1>	ALL	NULL	NULL	NULL	NULL	NULL	
 drop table t1,t2;
 create table t1 (   id int not null auto_increment, primary key (id)   ,user_name text );
 create table t2 (    id int not null auto_increment, primary key (id)   ,group_name text );
@@ -950,3 +948,8 @@ a
 1
 drop table t1, t2;
 set sql_select_limit=default;
+CREATE TABLE t1 (i int(11) default NULL,c char(1) default NULL,KEY i (i));
+CREATE TABLE t2 (i int(11) default NULL,c char(1) default NULL,KEY i (i));
+explain (select * from t1) union (select * from t2) order by not_existing_column;
+ERROR 42S22: Unknown column 'not_existing_column' in 'order clause'
+drop table t1, t2;

mysql-test/t/subselect.test

@@ -974,7 +974,7 @@ create table t1 (s1 char);
 insert into t1 values ('e');
 select * from t1 where 'f' > any (select s1 from t1);
 select * from t1 where 'f' > any (select s1 from t1 union select s1 from t1);
-explain select * from t1 where 'f' > any (select s1 from t1 union select s1 from t1);
+explain extended select * from t1 where 'f' > any (select s1 from t1 union select s1 from t1);
 drop table t1;
 
 #

mysql-test/t/union.test

@@ -39,8 +39,11 @@ explain select a,b from t1 union all select a,b from t2;
 
 --error  1054
 explain select xx from t1 union select 1;
+--error 1222
 explain select a,b from t1 union select 1;
+--error 1222
 explain select 1 union select a,b from t1 union select 1;
+--error 1222
 explain select a,b from t1 union select 1 limit 0;
 
 --error 1221
@@ -515,3 +518,12 @@ SET SQL_SELECT_LIMIT=1;
 select a from t1 union select a from t2 order by a;
 drop table t1, t2;
 set sql_select_limit=default;
+
+#
+# nonexisting column in global ORDER BY
+#
+CREATE TABLE t1 (i int(11) default NULL,c char(1) default NULL,KEY i (i));
+CREATE TABLE t2 (i int(11) default NULL,c char(1) default NULL,KEY i (i));
+--error 1054
+explain (select * from t1) union (select * from t2) order by not_existing_column;
+drop table t1, t2;

sql/item.cc

@@ -369,9 +369,14 @@ const char *Item_ident::full_name() const
   }
   else
   {
-    tmp=(char*) sql_alloc((uint) strlen(table_name)+
-			  (uint) strlen(field_name)+2);
-    strxmov(tmp,table_name,".",field_name,NullS);
+    if (table_name[0])
+    {
+      tmp= (char*) sql_alloc((uint) strlen(table_name) +
+			     (uint) strlen(field_name) + 2);
+      strxmov(tmp, table_name, ".", field_name, NullS);
+    }
+    else
+      tmp= (char*) field_name;
   }
   return tmp;
 }

sql/sql_class.h

@@ -768,7 +768,7 @@ public:
   uint	     current_tablenr,tmp_table;
   uint	     server_status,open_options,system_thread;
   uint32     db_length;
-  uint       select_number;             //number of select (used for EXPLAIN)
+  int        select_number;             //number of select (used for EXPLAIN)
   /* variables.transaction_isolation is reset to this after each commit */
   enum_tx_isolation session_tx_isolation;
   enum_check_fields count_cuted_fields;
@@ -1147,7 +1147,6 @@ class select_union :public select_result {
   TABLE *table;
   COPY_INFO info;
   TMP_TABLE_PARAM tmp_table_param;
-  bool not_describe;
 
   select_union(TABLE *table_par);
   ~select_union();

sql/sql_lex.cc

@@ -1007,6 +1007,7 @@ void st_select_lex_unit::init_query()
   fake_select_lex= 0;
   cleaned= 0;
   item_list.empty();
+  describe= 0;
 }
 
 void st_select_lex::init_query()

sql/sql_lex.h

@@ -336,6 +336,7 @@ public:
   st_select_lex *fake_select_lex;
 
   st_select_lex *union_distinct; /* pointer to the last UNION DISTINCT */
+  bool describe; /* union exec() called for EXPLAIN */
 
   void init_query();
   bool create_total_list(THD *thd, st_lex *lex, TABLE_LIST **result);
@@ -413,7 +414,7 @@ public:
 
   ulong table_join_options;
   uint in_sum_expr;
-  uint select_number; /* number of select (used for EXPLAIN) */
+  int select_number; /* number of select (used for EXPLAIN) */
   uint with_wild; /* item list contain '*' */
   bool  braces;   	/* SELECT ... UNION (SELECT ... ) <- this braces */
   /* TRUE when having fix field called in processing of this SELECT */

sql/sql_select.cc

@@ -991,7 +991,7 @@ JOIN::optimize()
       }
     }
     
-    if (select_lex->master_unit()->uncacheable)
+    if (select_lex->master_unit()->uncacheable || thd->lex->describe)
     {
       if (!(tmp_join= (JOIN*)thd->alloc(sizeof(JOIN))))
 	DBUG_RETURN(-1);
@@ -9132,7 +9132,8 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
 
   if (message)
   {
-    item_list.push_back(new Item_int((int32) join->select_lex->select_number));
+    item_list.push_back(new Item_int((longlong)
+				     join->select_lex->select_number));
     item_list.push_back(new Item_string(join->select_lex->type,
 					strlen(join->select_lex->type), cs));
     for (uint i=0 ; i < 7; i++)
@@ -9141,6 +9142,49 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
     if (result->send_data(item_list))
       join->error= 1;
   }
+  else if (join->select_lex == join->unit->fake_select_lex)
+  {
+    /* 
+       Here is guessing about fake_select_lex to avoid union SELECT
+       execution to get accurate information
+    */
+    char table_name_buffer[64];
+    item_list.empty();
+    /* id */
+    item_list.push_back(new Item_int((int32)
+				     join->select_lex->select_number));
+    /* select_type */
+    item_list.push_back(new Item_string(join->select_lex->type,
+					strlen(join->select_lex->type),
+					cs));
+    /* table */
+    int len= my_snprintf(table_name_buffer, sizeof(table_name_buffer)-1,
+			 "<union%d>",
+			 -join->select_lex->select_number);
+    item_list.push_back(new Item_string(table_name_buffer, len, cs));
+    item_list.push_back(new Item_string(join_type_str[JT_ALL],
+					  strlen(join_type_str[JT_ALL]),
+					  cs));
+    /* possible_keys */
+    item_list.push_back(item_null);
+    /* key*/
+    item_list.push_back(item_null);
+    /* key_len */
+    item_list.push_back(item_null);
+    /* ref */
+    item_list.push_back(item_null);
+    /* rows */
+    item_list.push_back(item_null);
+    /* extra */
+    if (join->unit->global_parameters->order_list.first)
+      item_list.push_back(new Item_string("Using filesort",
+					  14, cs));
+    else
+      item_list.push_back(new Item_string("", 0, cs));
+
+    if (result->send_data(item_list))
+      join->error= 1;
+  }
   else
   {
     table_map used_tables=0;
@@ -9150,36 +9194,41 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
       TABLE *table=tab->table;
       char buff[512],*buff_ptr=buff;
       char buff1[512], buff2[512];
-      char derived_name[64];
+      char table_name_buffer[64];
       String tmp1(buff1,sizeof(buff1),cs);
       String tmp2(buff2,sizeof(buff2),cs);
       tmp1.length(0);
       tmp2.length(0);
 
       item_list.empty();
+      /* id */
       item_list.push_back(new Item_int((int32)
 				       join->select_lex->select_number));
+      /* select_type */
       item_list.push_back(new Item_string(join->select_lex->type,
 					  strlen(join->select_lex->type),
 					  cs));
       if (tab->type == JT_ALL && tab->select && tab->select->quick)
 	tab->type= JT_RANGE;
+      /* table */
       if (table->derived_select_number)
       {
 	/* Derived table name generation */
-	int len= my_snprintf(derived_name, sizeof(derived_name)-1,
+	int len= my_snprintf(table_name_buffer, sizeof(table_name_buffer)-1,
 			     "<derived%u>",
 			     table->derived_select_number);
-	item_list.push_back(new Item_string(derived_name, len, cs));
+	item_list.push_back(new Item_string(table_name_buffer, len, cs));
       }
       else
 	item_list.push_back(new Item_string(table->table_name,
 					    strlen(table->table_name),
 					    cs));
+      /* type */
       item_list.push_back(new Item_string(join_type_str[tab->type],
 					  strlen(join_type_str[tab->type]),
 					  cs));
       uint j;
+      /* possible_keys */
       if (!tab->keys.is_clear_all())
       {
         for (j=0 ; j < table->keys ; j++)
@@ -9196,6 +9245,7 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
 	item_list.push_back(new Item_string(tmp1.ptr(),tmp1.length(),cs));
       else
 	item_list.push_back(item_null);
+      /* key key_len ref */
       if (tab->ref.key_parts)
       {
 	KEY *key_info=table->key_info+ tab->ref.key;
@@ -9234,9 +9284,11 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
 	item_list.push_back(item_null);
 	item_list.push_back(item_null);
       }
+      /* rows */
       item_list.push_back(new Item_int((longlong) (ulonglong)
 				       join->best_positions[i]. records_read,
 				       21));
+      /* extra */
       my_bool key_read=table->key_read;
       if ((tab->type == JT_NEXT || tab->type == JT_CONST) &&
           table->used_keys.is_set(tab->index))
@@ -9301,32 +9353,53 @@ int mysql_explain_union(THD *thd, SELECT_LEX_UNIT *unit, select_result *result)
   DBUG_ENTER("mysql_explain_union");
   int res= 0;
   SELECT_LEX *first= unit->first_select();
+
   for (SELECT_LEX *sl= first;
        sl;
        sl= sl->next_select())
   {
     // drop UNCACHEABLE_EXPLAIN, because it is for internal usage only
     uint8 uncacheable= (sl->uncacheable & ~UNCACHEABLE_EXPLAIN);
-
-    res= mysql_explain_select(thd, sl,
-			      (((&thd->lex->select_lex)==sl)?
-			       ((thd->lex->all_selects_list != sl) ? 
-				primary_key_name : "SIMPLE"):
-			       ((sl == first)?
-				((sl->linkage == DERIVED_TABLE_TYPE) ?
-				 "DERIVED":
-				((uncacheable & UNCACHEABLE_DEPENDENT) ?
-				 "DEPENDENT SUBQUERY":
-				 (uncacheable?"UNCACHEABLE SUBQUERY":
-				   "SUBQUERY"))):
-				((uncacheable & UNCACHEABLE_DEPENDENT) ?
-				 "DEPENDENT UNION":
-				 uncacheable?"UNCACHEABLE UNION":
-				  "UNION"))),
-			      result);
-    if (res)
-      break;
-
+    sl->type= (((&thd->lex->select_lex)==sl)?
+	       ((thd->lex->all_selects_list != sl) ? 
+		primary_key_name : "SIMPLE"):
+	       ((sl == first)?
+		((sl->linkage == DERIVED_TABLE_TYPE) ?
+		 "DERIVED":
+		 ((uncacheable & UNCACHEABLE_DEPENDENT) ?
+		  "DEPENDENT SUBQUERY":
+		  (uncacheable?"UNCACHEABLE SUBQUERY":
+		   "SUBQUERY"))):
+		((uncacheable & UNCACHEABLE_DEPENDENT) ?
+		 "DEPENDENT UNION":
+		 uncacheable?"UNCACHEABLE UNION":
+		 "UNION")));
+    sl->options|= SELECT_DESCRIBE;
+  }
+  if (first->next_select())
+  {
+    unit->fake_select_lex->select_number= -first->select_number;
+    unit->fake_select_lex->type= "UNION RESULT";
+    unit->fake_select_lex->options|= SELECT_DESCRIBE;
+    if (!(res= unit->prepare(thd, result, SELECT_NO_UNLOCK | SELECT_DESCRIBE)))
+      res= unit->exec();
+    res|= unit->cleanup();
+  }
+  else
+  {
+    thd->lex->current_select= first;
+    res= mysql_select(thd, &first->ref_pointer_array,
+			(TABLE_LIST*) first->table_list.first,
+			first->with_wild, first->item_list,
+			first->where,
+			first->order_list.elements +
+			first->group_list.elements,
+			(ORDER*) first->order_list.first,
+			(ORDER*) first->group_list.first,
+			first->having,
+			(ORDER*) thd->lex->proc_list.first,
+			first->options | thd->options | SELECT_DESCRIBE,
+			result, unit, first);
   }
   if (res > 0 || thd->net.report_error)
     res= -1; // mysql_explain_select do not report error
@@ -9334,30 +9407,6 @@ int mysql_explain_union(THD *thd, SELECT_LEX_UNIT *unit, select_result *result)
 }
 
 
-int mysql_explain_select(THD *thd, SELECT_LEX *select_lex, char const *type, 
-			 select_result *result)
-{
-  DBUG_ENTER("mysql_explain_select");
-  DBUG_PRINT("info", ("Select 0x%lx, type %s", (ulong)select_lex, type))
-  select_lex->type= type;
-  thd->lex->current_select= select_lex;
-  SELECT_LEX_UNIT *unit=  select_lex->master_unit();
-  int res= mysql_select(thd, &select_lex->ref_pointer_array,
-			(TABLE_LIST*) select_lex->table_list.first,
-			select_lex->with_wild, select_lex->item_list,
-			select_lex->where,
-			select_lex->order_list.elements +
-			select_lex->group_list.elements,
-			(ORDER*) select_lex->order_list.first,
-			(ORDER*) select_lex->group_list.first,
-			select_lex->having,
-			(ORDER*) thd->lex->proc_list.first,
-			select_lex->options | thd->options | SELECT_DESCRIBE,
-			result, unit, select_lex);
-  DBUG_RETURN(res);
-}
-
-
 void st_select_lex::print(THD *thd, String *str)
 {
   if (!thd)

sql/sql_union.cc

@@ -41,7 +41,7 @@ int mysql_union(THD *thd, LEX *lex, select_result *result,
 ***************************************************************************/
 
 select_union::select_union(TABLE *table_par)
-  :table(table_par), not_describe(0)
+  :table(table_par)
 {
   bzero((char*) &info,sizeof(info));
   /*
@@ -114,6 +114,8 @@ int st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
   select_result *tmp_result;
   DBUG_ENTER("st_select_lex_unit::prepare");
 
+  describe= test(additional_options & SELECT_DESCRIBE);
+
   /*
     result object should be reassigned even if preparing already done for
     max/min subquery (ALL/ANY optimization)
@@ -121,7 +123,26 @@ int st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
   result= sel_result;
 
   if (prepared)
+  {
+    if (describe)
+    {
+      /* fast reinit for EXPLAIN */
+      for (sl= first_select_in_union(); sl; sl= sl->next_select())
+      {
+	sl->join->result= result;
+	select_limit_cnt= HA_POS_ERROR;
+	offset_limit_cnt= 0;
+	if (!sl->join->procedure &&
+	    result->prepare(sl->join->fields_list, this))
+	{
+	  DBUG_RETURN(1);
+	}
+	sl->join->select_options|= SELECT_DESCRIBE;
+	sl->join->reinit();
+      }
+    }
     DBUG_RETURN(0);
+  }
   prepared= 1;
   res= 0;
   
@@ -134,8 +155,9 @@ int st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
   {
     if (!(tmp_result= union_result= new select_union(0)))
       goto err;
-    union_result->not_describe= 1;
     union_result->tmp_table_param.init();
+    if (describe)
+      tmp_result= sel_result;
   }
   else
   {
@@ -269,11 +291,11 @@ int st_select_lex_unit::exec()
   ulonglong add_rows=0;
   DBUG_ENTER("st_select_lex_unit::exec");
 
-  if (executed && !uncacheable)
+  if (executed && !uncacheable && !describe)
     DBUG_RETURN(0);
   executed= 1;
   
-  if (uncacheable || !item || !item->assigned())
+  if (uncacheable || !item || !item->assigned() || describe)
   {
     if (optimized && item && item->assigned())
     {
@@ -292,7 +314,7 @@ int st_select_lex_unit::exec()
 	res= sl->join->reinit();
       else
       {
-	if (sl != global_parameters)
+	if (sl != global_parameters && !describe)
 	{
 	  offset_limit_cnt= sl->offset_limit;
 	  select_limit_cnt= sl->select_limit+sl->offset_limit;
@@ -304,7 +326,7 @@ int st_select_lex_unit::exec()
 	    We can't use LIMIT at this stage if we are using ORDER BY for the
 	    whole query
 	  */
-	  if (sl->order_list.first)
+	  if (sl->order_list.first || describe)
 	    select_limit_cnt= HA_POS_ERROR;
 	  else
 	    select_limit_cnt= sl->select_limit+sl->offset_limit;
@@ -373,7 +395,7 @@ int st_select_lex_unit::exec()
 
     if (!thd->is_fatal_error)				// Check if EOM
     {
-      ulong options_tmp= thd->options;
+      ulong options_tmp= thd->options | fake_select_lex->options;
       thd->lex->current_select= fake_select_lex;
       offset_limit_cnt= global_parameters->offset_limit;
       select_limit_cnt= global_parameters->select_limit +