Fixed bug mdev-4063 (bug #56927).

This bug could result in returning 0 for the expressions of the form 
<aggregate_function>(distinct field) when the system variable  
max_heap_table_size was set to a small enough number.
It happened because the method Unique::walk() did not support
the case when more than one pass was needed to merge the trees
of distinct values saved in an external file.

Backported a fix in grant_lowercase.test from mariadb 5.5.

mysql-test/r/sum_distinct-big.result

@@ -103,5 +103,20 @@ sm
 10323810
 10325070
 10326330
+# 
+# Bug mdev-4063: SUM(DISTINCT...) with small'max_heap_table_size 
+# (bug #56927)
+# 
+SET max_heap_table_size=default;
+INSERT INTO t1 SELECT id+16384 FROM t1;
+DELETE FROM t2;
+INSERT INTO t2 SELECT id FROM t1 ORDER BY id*rand();
+SELECT SUM(DISTINCT id) sm FROM t2;
+sm
+536887296
+SET max_heap_table_size=16384;
+SELECT SUM(DISTINCT id) sm FROM t2;
+sm
+536887296
 DROP TABLE t1;
 DROP TABLE t2;

mysql-test/t/grant_lowercase.test

 # test cases for strmov(tmp_db, db) -> strnmov replacement in sql_acl.cc
+--source include/not_embedded.inc
 
 #
 # http://seclists.org/fulldisclosure/2012/Dec/4

mysql-test/t/sum_distinct-big.test

@@ -63,5 +63,22 @@ SELECT SUM(DISTINCT id) sm FROM t1;
 SELECT SUM(DISTINCT id) sm FROM t2;
 SELECT SUM(DISTINCT id) sm FROM t1 GROUP BY id % 13;
 
+--echo # 
+--echo # Bug mdev-4063: SUM(DISTINCT...) with small'max_heap_table_size 
+--echo # (bug #56927)
+--echo # 
+
+SET max_heap_table_size=default;
+
+INSERT INTO t1 SELECT id+16384 FROM t1;
+DELETE FROM t2;
+INSERT INTO t2 SELECT id FROM t1 ORDER BY id*rand();
+
+SELECT SUM(DISTINCT id) sm FROM t2;
+
+SET max_heap_table_size=16384;
+
+SELECT SUM(DISTINCT id) sm FROM t2;
+
 DROP TABLE t1;
 DROP TABLE t2;

sql/item_sum.cc

@@ -1084,7 +1084,7 @@ void Item_sum_distinct::calculate_val_and_count()
     if (tree)
     {
       table->field[0]->set_notnull();
-      tree->walk(item_sum_distinct_walk, (void*) this);
+      tree->walk(table, item_sum_distinct_walk, (void*) this);
     }
     is_evaluated= TRUE;
   }
@@ -2583,7 +2583,7 @@ longlong Item_sum_count_distinct::val_int()
     if (tree->elements == 0)
       return (longlong) tree->elements_in_tree(); // everything fits in memory
     count= 0;
-    tree->walk(count_distinct_walk, (void*) &count);
+    tree->walk(table, count_distinct_walk, (void*) &count);
     is_evaluated= TRUE;
     return (longlong) count;
   }

sql/sql_class.h

@@ -3008,6 +3008,8 @@ class Unique :public Sql_alloc
   bool flush();
   uint size;
 
+  bool merge(TABLE *table, uchar *buff, bool without_last_merge);
+
 public:
   ulong elements;
   Unique(qsort_cmp2 comp_func, void *comp_func_fixed_arg,
@@ -3035,7 +3037,7 @@ public:
   }
 
   void reset();
-  bool walk(tree_walk_action action, void *walk_action_arg);
+  bool walk(TABLE *table, tree_walk_action action, void *walk_action_arg);
 
   uint get_size() const { return size; }
   ulonglong get_max_in_memory_size() const { return max_in_memory_size; }

sql/uniques.cc

@@ -538,6 +538,7 @@ end:
   SYNOPSIS
     Unique:walk()
   All params are 'IN':
+    table   parameter for the call of the merge method
     action  function-visitor, typed in include/my_tree.h
             function is called for each unique element
     arg     argument for visitor, which is passed to it on each call
@@ -546,66 +547,68 @@ end:
     <> 0 error
  */
 
-bool Unique::walk(tree_walk_action action, void *walk_action_arg)
+bool Unique::walk(TABLE *table, tree_walk_action action, void *walk_action_arg)
 {
-  int res;
+  int res= 0;
   uchar *merge_buffer;
 
   if (elements == 0)                       /* the whole tree is in memory */
     return tree_walk(&tree, action, walk_action_arg, left_root_right);
 
+  table->sort.found_records=elements+tree.elements_in_tree;
   /* flush current tree to the file to have some memory for merge buffer */
   if (flush())
     return 1;
   if (flush_io_cache(&file) || reinit_io_cache(&file, READ_CACHE, 0L, 0, 0))
     return 1;
-  if (!(merge_buffer= (uchar *) my_malloc((ulong) max_in_memory_size, MYF(0))))
+  ulong buff_sz= (max_in_memory_size / size + 1) * size;
+  if (!(merge_buffer= (uchar *) my_malloc((ulong) buff_sz, MYF(0))))
     return 1;
-  res= merge_walk(merge_buffer, (ulong) max_in_memory_size, size,
-                  (BUFFPEK *) file_ptrs.buffer,
-                  (BUFFPEK *) file_ptrs.buffer + file_ptrs.elements,
-                  action, walk_action_arg,
-                  tree.compare, tree.custom_arg, &file);
+  if (buff_sz < (ulong) (size * (file_ptrs.elements + 1)))
+    res= merge(table, merge_buffer, buff_sz >= size * MERGEBUFF2) ;
+  
+  if (!res)
+  {  
+    res= merge_walk(merge_buffer, (ulong) max_in_memory_size, size,
+                    (BUFFPEK *) file_ptrs.buffer,
+                    (BUFFPEK *) file_ptrs.buffer + file_ptrs.elements,
+                    action, walk_action_arg,
+                    tree.compare, tree.custom_arg, &file);
+  }
   my_free((char*) merge_buffer, MYF(0));
   return res;
 }
 
+
 /*
-  Modify the TABLE element so that when one calls init_records()
-  the rows will be read in priority order.
-*/
+  DESCRIPTION
+    Perform multi-pass sort merge of the elements accessed through table->sort,
+    using the buffer buff as the merge buffer. The last pass is not performed
+    if without_last_merge is TRUE.
+  SYNOPSIS
+    Unique:merge()
+  All params are 'IN':
+    table               the parameter to access sort context
+    buff                merge buffer
+    without_last_merge  TRUE <=> do not perform the last merge
+  RETURN VALUE
+    0    OK
+    <> 0 error
+ */
 
-bool Unique::get(TABLE *table)
+bool Unique::merge(TABLE *table, uchar *buff, bool without_last_merge)
 {
   SORTPARAM sort_param;
-  table->sort.found_records=elements+tree.elements_in_tree;
-
-  if (my_b_tell(&file) == 0)
-  {
-    /* Whole tree is in memory;  Don't use disk if you don't need to */
-    if ((record_pointers=table->sort.record_pointers= (uchar*)
-	 my_malloc(size * tree.elements_in_tree, MYF(0))))
-    {
-      (void) tree_walk(&tree, (tree_walk_action) unique_write_to_ptrs,
-		       this, left_root_right);
-      return 0;
-    }
-  }
-  /* Not enough memory; Save the result to file && free memory used by tree */
-  if (flush())
-    return 1;
-
-  IO_CACHE *outfile=table->sort.io_cache;
+  IO_CACHE *outfile= table->sort.io_cache;
   BUFFPEK *file_ptr= (BUFFPEK*) file_ptrs.buffer;
   uint maxbuffer= file_ptrs.elements - 1;
-  uchar *sort_buffer;
   my_off_t save_pos;
-  bool error=1;
-
-      /* Open cached file if it isn't open */
-  outfile=table->sort.io_cache=(IO_CACHE*) my_malloc(sizeof(IO_CACHE),
-                                MYF(MY_ZEROFILL));
+  bool error= 1;
 
+  /* Open cached file if it isn't open */
+  if (!outfile)
+    outfile= table->sort.io_cache= (IO_CACHE*) my_malloc(sizeof(IO_CACHE),
+                                                         MYF(MY_ZEROFILL));
   if (!outfile ||
       (! my_b_inited(outfile) &&
        open_cached_file(outfile,mysql_tmpdir,TEMP_PREFIX,READ_RECORD_BUFFER,
@@ -615,42 +618,81 @@ bool Unique::get(TABLE *table)
 
   bzero((char*) &sort_param,sizeof(sort_param));
   sort_param.max_rows= elements;
-  sort_param.sort_form=table;
+  sort_param.sort_form= table;
   sort_param.rec_length= sort_param.sort_length= sort_param.ref_length=
     size;
   sort_param.keys= (uint) (max_in_memory_size / sort_param.sort_length);
-  sort_param.not_killable=1;
+  sort_param.not_killable= 1;
 
-  if (!(sort_buffer=(uchar*) my_malloc((sort_param.keys+1) *
-				       sort_param.sort_length,
-				       MYF(0))))
-    return 1;
-  sort_param.unique_buff= sort_buffer+(sort_param.keys*
-				       sort_param.sort_length);
+  sort_param.unique_buff= buff + (sort_param.keys * sort_param.sort_length);
 
   sort_param.compare= (qsort2_cmp) buffpek_compare;
   sort_param.cmp_context.key_compare= tree.compare;
   sort_param.cmp_context.key_compare_arg= tree.custom_arg;
 
   /* Merge the buffers to one file, removing duplicates */
-  if (merge_many_buff(&sort_param,sort_buffer,file_ptr,&maxbuffer,&file))
+  if (merge_many_buff(&sort_param,buff,file_ptr,&maxbuffer,&file))
     goto err;
   if (flush_io_cache(&file) ||
       reinit_io_cache(&file,READ_CACHE,0L,0,0))
     goto err;
-  if (merge_buffers(&sort_param, &file, outfile, sort_buffer, file_ptr,
+  if (without_last_merge)
+  {
+    file_ptrs.elements= maxbuffer+1;
+    return 0;
+  }
+  if (merge_buffers(&sort_param, &file, outfile, buff, file_ptr,
 		    file_ptr, file_ptr+maxbuffer,0))
     goto err;
-  error=0;
+  error= 0;
 err:
-  x_free(sort_buffer);
   if (flush_io_cache(outfile))
-    error=1;
+    error= 1;
 
   /* Setup io_cache for reading */
-  save_pos=outfile->pos_in_file;
+  save_pos= outfile->pos_in_file;
   if (reinit_io_cache(outfile,READ_CACHE,0L,0,0))
-    error=1;
+    error= 1;
   outfile->end_of_file=save_pos;
   return error;
 }
+
+
+/*
+  Modify the TABLE element so that when one calls init_records()
+  the rows will be read in priority order.
+*/
+
+bool Unique::get(TABLE *table)
+{
+  bool rc= 1;
+  uchar *sort_buffer= NULL;
+  table->sort.found_records= elements+tree.elements_in_tree;
+
+  if (my_b_tell(&file) == 0)
+  {
+    /* Whole tree is in memory;  Don't use disk if you don't need to */
+    if ((record_pointers=table->sort.record_pointers= (uchar*)
+	 my_malloc(size * tree.elements_in_tree, MYF(0))))
+    {
+      (void) tree_walk(&tree, (tree_walk_action) unique_write_to_ptrs,
+		       this, left_root_right);
+      return 0;
+    }
+  }
+  /* Not enough memory; Save the result to file && free memory used by tree */
+  if (flush())
+    return 1;
+  
+  ulong buff_sz= (max_in_memory_size / size + 1) * size;
+  if (!(sort_buffer= (uchar*) my_malloc(buff_sz, MYF(0))))
+    return 1;
+
+  if (merge(table, sort_buffer, FALSE))
+    goto err;  
+  rc= 0;  
+
+err:  
+  x_free(sort_buffer);  
+  return rc;
+}