Patch for Bug#13805127: Stored program cache produces wrong result in same THD.

Background:

  - as described in MySQL Internals Prepared Stored
    (http://forge.mysql.com/wiki/MySQL_Internals_Prepared_Stored),
    the Optimizer sometimes does destructive changes to the parsed
    LEX-object (Item-tree), which makes it impossible to re-use
    that tree for PS/SP re-execution.

  - in order to be able to re-use the Item-tree, the destructive
    changes are remembered and rolled back after the statement execution.

The problem, discovered by this bug, was that the objects representing
GROUP-BY clause did not restored after query execution. So, the GROUP-BY
part of the statement could not be properly re-initialized for re-execution
after destructive changes.

Those objects do not take part in the Item-tree, so they can not be saved
using the approach for Item-tree.

The fix is as follows:

  - introduce a new array in st_select_lex to store the original
    ORDER pointers, representing the GROUP-BY clause;

  - Initialize this array in fix_prepare_information().

  - restore the list of GROUP-BY items in reinit_stmt_before_use().

1 files changed, 14 insertions, 2 deletions

diff --git a/sql/sql_lex.h b/sql/sql_lex.h
index c89057a4f75..6d004d0fa24 100644
--- a/sql/sql_lex.h
+++ b/sql/sql_lex.h
@@ -24,6 +24,7 @@
 #include "sql_trigger.h"
 #include "item.h"               /* From item_subselect.h: subselect_union_engine */
 #include "thr_lock.h"                  /* thr_lock_type, TL_UNLOCK */
+#include "mem_root_array.h"
 
 /* YACC and LEX Definitions */
 
@@ -258,6 +259,7 @@ enum enum_drop_mode
 #define TL_OPTION_ALIAS         8
 
 typedef List<Item> List_item;
+typedef Mem_root_array<ORDER*, true> Group_list_ptrs;
 
 /* SERVERS CACHE CHANGES */
 typedef struct st_lex_server_options
@@ -693,7 +695,16 @@ public:
   enum olap_type olap;
   /* FROM clause - points to the beginning of the TABLE_LIST::next_local list. */
   SQL_I_List<TABLE_LIST>  table_list;
-  SQL_I_List<ORDER>       group_list; /* GROUP BY clause. */
+
+  /*
+    GROUP BY clause.
+    This list may be mutated during optimization (by remove_const()),
+    so for prepared statements, we keep a copy of the ORDER.next pointers in
+    group_list_ptrs, and re-establish the original list before each execution.
+  */
+  SQL_I_List<ORDER>       group_list;
+  Group_list_ptrs        *group_list_ptrs;
+
   List<Item>          item_list;  /* list of fields & expressions */
   List<String>        interval_list;
   bool	              is_item_list_lookup;
@@ -870,7 +881,8 @@ public:
   bool test_limit();
 
   friend void lex_start(THD *thd);
-  st_select_lex() : n_sum_items(0), n_child_sum_items(0) {}
+  st_select_lex() : group_list_ptrs(NULL), n_sum_items(0), n_child_sum_items(0)
+  {}
   void make_empty_select()
   {
     init_query();