fixed union rexecution bug

mysql-test/r/subselect.result:
  test for union rexecution bug
mysql-test/t/subselect.test:
  test for union rexecution bug

mysql-test/r/subselect.result

@@ -5,6 +5,9 @@ SELECT (SELECT 1) UNION SELECT (SELECT 2);
 (SELECT 1)
 1
 2
+SELECT (SELECT (SELECT 0 UNION SELECT 0));
+(SELECT (SELECT 0 UNION SELECT 0))
+0
 drop table if exists t1,t2,t3,t4,t5,attend,clinic,inscrit;
 create table t1 (a int);
 create table t2 (a int, b int);

mysql-test/t/subselect.test

-
 select (select 2);
 SELECT (SELECT 1) UNION SELECT (SELECT 2);
+SELECT (SELECT (SELECT 0 UNION SELECT 0));
 drop table if exists t1,t2,t3,t4,t5,attend,clinic,inscrit;
 create table t1 (a int);
 create table t2 (a int, b int);

sql/sql_lex.cc

@@ -945,7 +945,7 @@ void st_select_lex_unit::init_query()
   select_limit_cnt= HA_POS_ERROR;
   offset_limit_cnt= 0;
   union_option= 0;
-  prepared= optimized= 0;
+  prepared= optimized= executed= 0;
   item= 0;
 }
 

sql/sql_lex.h

@@ -227,8 +227,9 @@ 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)
+    prepared, // prepare phase already performed for UNION (unit)
+    optimized, // optimize phase already performed for UNION (unit)
+    executed; // already executed
 public:
   /*
     Pointer to 'last' select or pointer to unit where stored

sql/sql_union.cc

@@ -199,8 +199,12 @@ int st_select_lex_unit::exec()
 {
   DBUG_ENTER("st_select_lex_unit::exec");
   SELECT_LEX *lex_select_save= thd->lex.select;
-
-  if(depended || !item || !item->assigned())
+  
+  if (executed && !depended)
+    DBUG_RETURN(0);
+  executed= 1;
+  
+  if (depended || !item || !item->assigned())
   {
     if (optimized && item && item->assigned())
       item->assigned(0); // We will reinit & rexecute unit