fixed bug of multi-level EXPLAIN

mysql-test/r/subselect.result:
  test of multi-level EXPLAIN
mysql-test/t/subselect.test:
  test of multi-level EXPLAIN

mysql-test/r/subselect.result

@@ -159,6 +159,14 @@ UNIQUE KEY `email` (`email`)
 INSERT INTO inscrit (pseudo,email) VALUES ('joce','test');
 INSERT INTO inscrit (pseudo,email) VALUES ('joce1','test1');
 INSERT INTO inscrit (pseudo,email) VALUES ('2joce1','2test1');
+EXPLAIN SELECT pseudo,(SELECT email FROM inscrit WHERE pseudo=(SELECT
+pseudo FROM inscrit WHERE pseudo='joce')) FROM inscrit WHERE pseudo=(SELECT
+pseudo FROM inscrit WHERE pseudo='joce');
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	inscrit	const	PRIMARY	PRIMARY	35	const	1	
+4	SUBSELECT	inscrit	const	PRIMARY	PRIMARY	35	const	1	
+2	SUBSELECT	inscrit	const	PRIMARY	PRIMARY	35	const	1	
+3	SUBSELECT	inscrit	const	PRIMARY	PRIMARY	35	const	1	
 SELECT pseudo FROM inscrit WHERE pseudo=(SELECT pseudo,email FROM
 inscrit WHERE pseudo='joce');
 Subselect returns more than 1 field

mysql-test/t/subselect.test

@@ -82,6 +82,9 @@ CREATE TABLE `inscrit` (
 INSERT INTO inscrit (pseudo,email) VALUES ('joce','test');
 INSERT INTO inscrit (pseudo,email) VALUES ('joce1','test1');
 INSERT INTO inscrit (pseudo,email) VALUES ('2joce1','2test1');
+EXPLAIN SELECT pseudo,(SELECT email FROM inscrit WHERE pseudo=(SELECT
+pseudo FROM inscrit WHERE pseudo='joce')) FROM inscrit WHERE pseudo=(SELECT
+pseudo FROM inscrit WHERE pseudo='joce');
 -- error 1239
 SELECT pseudo FROM inscrit WHERE pseudo=(SELECT pseudo,email FROM
 inscrit WHERE pseudo='joce');

sql/item_subselect.cc

@@ -212,7 +212,7 @@ subselect_single_select_engine::subselect_single_select_engine(THD *thd,
 							       select_subselect *result,
 							       Item_subselect *item):
   subselect_engine(thd, item, result),
-   executed(0), optimized(0)
+    prepared(0), optimized(0), executed(0)
 {
   select_lex= select;
   SELECT_LEX_UNIT *unit= select_lex->master_unit();
@@ -251,6 +251,9 @@ subselect_union_engine::subselect_union_engine(THD *thd,
 
 int subselect_single_select_engine::prepare()
 {
+  if (prepared)
+    return 0;
+  prepared= 1;
   SELECT_LEX *save_select= thd->lex.select;
   thd->lex.select= select_lex;
   if(join->prepare((TABLE_LIST*) select_lex->table_list.first,

sql/item_subselect.h

@@ -179,8 +179,9 @@ class subselect_engine
 
 class subselect_single_select_engine: public subselect_engine
 {
-  my_bool executed; /* simple subselect is executed */
+  my_bool prepared; /* simple subselect is prepared */
   my_bool optimized; /* simple subselect is optimized */
+  my_bool executed; /* simple subselect is executed */
   st_select_lex *select_lex; /* corresponding select_lex */
   JOIN * join; /* corresponding JOIN structure */
 public:

sql/sql_lex.cc

@@ -944,7 +944,7 @@ void st_select_lex_unit::init_query()
   global_parameters= this;
   select_limit_cnt= HA_POS_ERROR;
   offset_limit_cnt= 0;
-  optimized= 0;
+  prepared= optimized= 0;
   item= 0;
 }
 

sql/sql_lex.h

@@ -227,6 +227,7 @@ class st_select_lex_unit: public st_select_lex_node {
   select_result *result;
   int res;
   bool describe, found_rows_for_union,
+    prepared, //prepare phase already performed for UNION (unit)
     optimized; // optimize phase already performed for UNION (unit)
 public:
   /*

sql/sql_union.cc

@@ -104,11 +104,16 @@ bool select_union::flush()
 typedef JOIN * JOIN_P;
 int st_select_lex_unit::prepare(THD *thd, select_result *result)
 {
+  DBUG_ENTER("st_select_lex_unit::prepare");
+
+  if (prepared)
+    DBUG_RETURN(0);
+  prepared= 1;
+    
   describe=(first_select()->options & SELECT_DESCRIBE) ? 1 : 0;
   res= 0;
   found_rows_for_union= false;
   TMP_TABLE_PARAM tmp_table_param;
-  DBUG_ENTER("st_select_lex_unit::prepare");
   this->thd= thd;
   this->result= result;
   SELECT_LEX *lex_select_save= thd->lex.select;