Index: sql/uniques.cc
===================================================================
--- sql/uniques.cc	(revision 1)
+++ sql/uniques.cc	(working copy)
@@ -417,7 +417,7 @@
     <> 0  error
 */
 
-static bool merge_walk(uchar *merge_buffer, ulong merge_buffer_size,
+static bool merge_walk(uchar *in_merge_buffer, ulong in_merge_buffer_size,
                        uint key_length, BUFFPEK *begin, BUFFPEK *end,
                        tree_walk_action walk_action, void *walk_action_arg,
                        qsort_cmp2 compare, const void *compare_arg,
@@ -425,11 +425,30 @@
 {
   BUFFPEK_COMPARE_CONTEXT compare_context = { compare, compare_arg };
   QUEUE queue;
+
+  /* return 1 here make the result to be 0, looks buggy*/
   if (end <= begin ||
-      merge_buffer_size < (ulong) (key_length * (end - begin + 1)) ||
+  //    merge_buffer_size < (ulong) (key_length * (end - begin + 1)) ||
       init_queue(&queue, (uint) (end - begin), offsetof(BUFFPEK, key), 0,
                  buffpek_compare, &compare_context))
     return 1;
+
+  bool exceed_tmp_table_size= false;
+  uchar *merge_buffer= NULL;
+  ulong merge_buffer_size= (ulong) key_length * (end - begin + 1);
+  
+  if (in_merge_buffer_size >= merge_buffer_size)
+  {
+    merge_buffer= in_merge_buffer;
+    merge_buffer_size= in_merge_buffer_size;
+  }
+  else
+  {
+    exceed_tmp_table_size= true;
+
+    merge_buffer= (uchar*)my_malloc(merge_buffer_size, MYF(0));
+  }
+
   /* we need space for one key when a piece of merge buffer is re-read */
   merge_buffer_size-= key_length;
   uchar *save_key_buff= merge_buffer + merge_buffer_size;
@@ -524,6 +543,11 @@
   res= 0;
 end:
   delete_queue(&queue);
+
+  /* delete temp merge_buffer if necessary */
+  if (exceed_tmp_table_size)
+    my_free(merge_buffer);
+
   return res;
 }