A fix and test case for Bug#6088 "FOUND_ROWS returns wrong values for

prepared statements when LIMIT is used" and post-review comments.
The fix changes the approach we calculate the need for ORDER BY 
in UNION: the previous was not PS friendly, as it damaged SELECT_LEX 
options in case of single select.

mysql-test/r/ps.result

@@ -410,3 +410,27 @@ a	a
 1.1	1.2
 2.1	2.2
 deallocate prepare stmt;
+create table t1 (a int);
+insert into t1 (a) values (1), (2), (3), (4), (5), (6), (7), (8), (9), (10);
+prepare stmt from "select sql_calc_found_rows * from t1 limit 2";
+execute stmt;
+a
+1
+2
+select found_rows();
+found_rows()
+10
+execute stmt;
+a
+1
+2
+select found_rows();
+found_rows()
+10
+execute stmt;
+a
+1
+2
+select found_rows();
+found_rows()
+10

mysql-test/r/subselect.result

@@ -176,17 +176,17 @@ a	b
 a	b
 1	7
 2	7
-3	8
 4	8
+3	8
 explain extended (select * from t2 where t2.b=(select a from t3 order by 1 desc limit 1)) union (select * from t4 where t4.b=(select max(t2.a)*4 from t2) order by a);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where
 2	SUBQUERY	t3	ALL	NULL	NULL	NULL	NULL	3	Using filesort
-3	UNION	t4	ALL	NULL	NULL	NULL	NULL	3	Using where; Using filesort
+3	UNION	t4	ALL	NULL	NULL	NULL	NULL	3	Using where
 4	SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	
 NULL	UNION RESULT	<union1,3>	ALL	NULL	NULL	NULL	NULL	NULL	
 Warnings:
-Note	1003	(select 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)
+Note	1003	(select 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 a)
 select (select a from t3 where a<t2.a*4 order by 1 desc limit 1), a from t2;
 (select a from t3 where a<t2.a*4 order by 1 desc limit 1)	a
 3	1

mysql-test/t/ps.test

@@ -415,3 +415,17 @@ execute stmt;
 execute stmt;
 execute stmt;
 deallocate prepare stmt;
+
+#
+# Bug#6088 "FOUND_ROWS returns wrong values for prepared statements when
+# LIMIT is used"
+# 
+create table t1 (a int);
+insert into t1 (a) values (1), (2), (3), (4), (5), (6), (7), (8), (9), (10);
+prepare stmt from "select sql_calc_found_rows * from t1 limit 2";
+execute stmt;
+select found_rows();
+execute stmt;
+select found_rows();
+execute stmt;
+select found_rows();

sql/sql_union.cc

@@ -147,6 +147,7 @@ int st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
   SELECT_LEX *lex_select_save= thd_arg->lex->current_select;
   SELECT_LEX *sl, *first_select;
   select_result *tmp_result;
+  bool is_union;
   DBUG_ENTER("st_select_lex_unit::prepare");
 
   describe= test(additional_options & SELECT_DESCRIBE);
@@ -183,10 +184,11 @@ int st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
   
   thd_arg->lex->current_select= sl= first_select= first_select_in_union();
   found_rows_for_union= first_select->options & OPTION_FOUND_ROWS;
+  is_union= test(first_select->next_select());
 
   /* Global option */
 
-  if (first_select->next_select())
+  if (is_union)
   {
     if (!(tmp_result= union_result= new select_union(0)))
       goto err;
@@ -195,14 +197,11 @@ int st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
       tmp_result= sel_result;
   }
   else
-  {
     tmp_result= sel_result;
-    // single select should be processed like select in p[arantses
-    first_select->braces= 1;
-  }
 
   for (;sl; sl= sl->next_select())
   {
+    bool can_skip_order_by;
     sl->options|=  SELECT_NO_UNLOCK;
     JOIN *join= new JOIN(thd_arg, sl->item_list, 
 			 sl->options | thd_arg->options | additional_options,
@@ -217,14 +216,17 @@ int st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
       select_limit_cnt= HA_POS_ERROR;		// no limit
     if (select_limit_cnt == HA_POS_ERROR || sl->braces)
       sl->options&= ~OPTION_FOUND_ROWS;
-    
+
+    can_skip_order_by= is_union &&
+                       (!sl->braces || select_limit_cnt == HA_POS_ERROR);
+
     res= join->prepare(&sl->ref_pointer_array,
 		       (TABLE_LIST*) sl->table_list.first, sl->with_wild,
 		       sl->where,
-		       ((sl->braces) ? sl->order_list.elements : 0) +
-		       sl->group_list.elements,
-		       (sl->braces) ? 
-		       (ORDER *)sl->order_list.first : (ORDER *) 0,
+                       (can_skip_order_by ? 0 : sl->order_list.elements) +
+                       sl->group_list.elements,
+                       can_skip_order_by ?
+                       (ORDER*) 0 : (ORDER *)sl->order_list.first,
 		       (ORDER*) sl->group_list.first,
 		       sl->having,
 		       (ORDER*) NULL,
@@ -264,10 +266,8 @@ int st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
     }
   }
 
-  if (first_select->next_select())
+  if (is_union)
   {
-    /* This is not a single select */
-
     /*
       Check that it was possible to aggregate
       all collations together for UNION.
@@ -364,8 +364,6 @@ int st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
       }
     }
   }
-  else
-    first_select->braces= 0; // remove our changes
 
   thd_arg->lex->current_select= lex_select_save;