1 files changed, 22 insertions, 6 deletions

diff --git a/sql/item_func.cc b/sql/item_func.cc
index 583371aed32..82912f8a0fa 100644
--- a/sql/item_func.cc
+++ b/sql/item_func.cc
@@ -163,14 +163,13 @@ bool Item_func::agg_arg_charsets(DTCollation &coll,
   }
 
   THD *thd= current_thd;
-  Item_arena *arena= thd->current_arena, backup;
+  Item_arena *arena, backup;
   bool res= FALSE;
   /*
     In case we're in statement prepare, create conversion item
     in its memory: it will be reused on each execute.
   */
-  if (arena->is_stmt_prepare())
-    thd->set_n_backup_item_arena(arena, &backup);
+  arena= thd->change_arena_if_needed(&backup);
 
   for (arg= args, last= args + nargs; arg < last; arg++)
   {
@@ -198,7 +197,7 @@ bool Item_func::agg_arg_charsets(DTCollation &coll,
     conv->fix_fields(thd, 0, &conv);
     *arg= conv;
   }
-  if (arena->is_stmt_prepare())
+  if (arena)
     thd->restore_backup_item_arena(arena, &backup);
   return res;
 }
@@ -2429,6 +2428,7 @@ static user_var_entry *get_variable(HASH *hash, LEX_STRING &name,
     entry->value=0;
     entry->length=0;
     entry->update_query_id=0;
+    entry->collation.set(NULL, DERIVATION_NONE);
     /*
       If we are here, we were called from a SET or a query which sets a
       variable. Imagine it is this:
@@ -2470,7 +2470,24 @@ bool Item_func_set_user_var::fix_fields(THD *thd, TABLE_LIST *tables,
      is different from query_id).
   */
   entry->update_query_id= thd->query_id;
-  entry->collation.set(args[0]->collation);
+  /*
+    As it is wrong and confusing to associate any 
+    character set with NULL, @a should be latin2
+    after this query sequence:
+
+      SET @a=_latin2'string';
+      SET @a=NULL;
+
+    I.e. the second query should not change the charset
+    to the current default value, but should keep the 
+    original value assigned during the first query.
+    In order to do it, we don't copy charset
+    from the argument if the argument is NULL
+    and the variable has previously been initialized.
+  */
+  if (!entry->collation.collation || !args[0]->null_value)
+    entry->collation.set(args[0]->collation);
+  collation.set(entry->collation);
   cached_result_type= args[0]->result_type();
   return 0;
 }
@@ -2498,7 +2515,6 @@ bool Item_func_set_user_var::update_hash(void *ptr, uint length,
       my_free(entry->value,MYF(0));
     entry->value=0;
     entry->length=0;
-    entry->collation.set(cs, dv);
   }
   else
   {