A proper fix for bug #57688.

Introduced a new flag in the class Item. The flag is set
to 1 only for items that are used in GROUP BY lists of
queries with ROLLUP.

sql/item.cc

@@ -380,6 +380,7 @@ Item::Item():
 {
   marker= 0;
   maybe_null=null_value=with_sum_func=unsigned_flag=0;
+  in_rollup= 0;
   decimals= 0; max_length= 0;
   with_subselect= 0;
   cmp_context= IMPOSSIBLE_RESULT;
@@ -420,6 +421,7 @@ Item::Item(THD *thd, Item *item):
   marker(item->marker),
   decimals(item->decimals),
   maybe_null(item->maybe_null),
+  in_rollup(item->in_rollup),
   null_value(item->null_value),
   unsigned_flag(item->unsigned_flag),
   with_sum_func(item->with_sum_func),

sql/item.h

@@ -518,6 +518,8 @@ class Item {
   int8 marker;
   uint8 decimals;
   my_bool maybe_null;			/* If item may be null */
+  my_bool in_rollup;                    /* If used in GROUP BY list
+                                           of a query with ROLLUP */ 
   my_bool null_value;			/* if item is null */
   my_bool unsigned_flag;
   my_bool with_sum_func;

sql/sql_select.cc

@@ -8919,6 +8919,8 @@ simplify_joins(JOIN *join, List<TABLE_LIST> *join_list, COND *conds, bool top)
         For some of the inner tables there are conjunctive predicates
         that reject nulls => the outer join can be replaced by an inner join.
       */
+      if (table->outer_join && !table->embedding && table->table)
+        table->table->maybe_null= FALSE;
       table->outer_join= 0;
       if (table->on_expr)
       {
@@ -9005,6 +9007,8 @@ simplify_joins(JOIN *join, List<TABLE_LIST> *join_list, COND *conds, bool top)
       while ((tbl= it++))
       {
         tbl->embedding= table->embedding;
+        if (!tbl->embedding && !tbl->on_expr && tbl->table)
+          tbl->table->maybe_null= FALSE;
         tbl->join_list= table->join_list;
       }
       li.replace(nested_join->join_list);
@@ -9882,7 +9886,7 @@ Field *create_tmp_field(THD *thd, TABLE *table,Item *item, Item::Type type,
       If item have to be able to store NULLs but underlaid field can't do it,
       create_tmp_field_from_field() can't be used for tmp field creation.
     */
-    if (field->maybe_null && !field->field->maybe_null())
+    if (field->maybe_null && field->in_rollup && !field->field->maybe_null())
     {
       result= create_tmp_field_from_item(thd, item, table, NULL,
                                          modify_item, convert_blob_length);
@@ -13508,8 +13512,6 @@ static bool
 list_contains_unique_index(TABLE *table,
                           bool (*find_func) (Field *, void *), void *data)
 {
-  if (table->pos_in_table_list->outer_join)
-    return 0;
   for (uint keynr= 0; keynr < table->s->keys; keynr++)
   {
     if (keynr == table->s->primary_key ||
@@ -13523,7 +13525,7 @@ list_contains_unique_index(TABLE *table,
            key_part < key_part_end;
            key_part++)
       {
-        if (key_part->field->real_maybe_null() || 
+        if (key_part->field->maybe_null() ||
             !find_func(key_part->field, data))
           break;
       }
@@ -16337,6 +16339,7 @@ static bool change_group_ref(THD *thd, Item_func *expr, ORDER *group_list,
     if (arg_changed)
     {
       expr->maybe_null= 1;
+      expr->in_rollup= 1;
       *changed= TRUE;
     }
   }
@@ -16400,6 +16403,7 @@ bool JOIN::rollup_init()
       if (*group_tmp->item == item)
       {
         item->maybe_null= 1;
+        item->in_rollup= 1;
         found_in_group= 1;
         break;
       }