Fixed bug mdev-4369.

The function was adjusted to be able to aggregate
the counters of the merged elements. 
Before this change it was not possible to guarantee the correctness
of the counters passed to the the call-back parameter walk_action.
As a result, when some elements of a Unique object were flushed into
disk the function passed to merge_walk() as the call-back parameter
could return wrong counters of elements. This could lead to building
wrong histograms. 

mysql-test/r/statistics.result

@@ -1511,4 +1511,43 @@ test	t1	a	1	5	0.0000	1.0000	10	DOUBLE_PREC_HB	0000FF3FFF7FFFBFFFFF
 set histogram_size=default;
 set histogram_type=default;
 drop table t1;
+#
+# Bug mdev-4369: histogram for a column with many distinct values 
+#
+CREATE TABLE t1 (id int);
+CREATE TABLE t2 (id int);
+INSERT INTO t1 (id) VALUES (1), (1), (1),(1);
+INSERT INTO t1 (id) SELECT id FROM t1;
+INSERT INTO t1 SELECT id+1 FROM t1;
+INSERT INTO t1 SELECT id+2 FROM t1;
+INSERT INTO t1 SELECT id+4 FROM t1;
+INSERT INTO t1 SELECT id+8 FROM t1;
+INSERT INTO t1 SELECT id+16 FROM t1;
+INSERT INTO t1 SELECT id+32 FROM t1;
+INSERT INTO t1 SELECT id+64 FROM t1;
+INSERT INTO t1 SELECT id+128 FROM t1;
+INSERT INTO t1 SELECT id+256 FROM t1;
+INSERT INTO t1 SELECT id+512 FROM t1;
+INSERT INTO t2 SELECT id FROM t1 ORDER BY id*rand();
+SELECT COUNT(*) FROM t2;
+COUNT(*)
+8192
+SELECT COUNT(DISTINCT id) FROM t2;
+COUNT(DISTINCT id)
+1024
+set @@tmp_table_size=1024*16;
+set @@max_heap_table_size=1024*16;
+set histogram_size=63;
+analyze table t2 persistent for all;
+Table	Op	Msg_type	Msg_text
+test.t2	analyze	status	OK
+select db_name, table_name, column_name,
+min_value, max_value,
+nulls_ratio, avg_frequency,
+hist_size, hist_type, HEX(histogram)
+FROM mysql.column_stats;
+db_name	table_name	column_name	min_value	max_value	nulls_ratio	avg_frequency	hist_size	hist_type	HEX(histogram)
+test	t2	id	1	1024	0.0000	8.0000	63	SINGLE_PREC_HB	03070B0F13171B1F23272B2F33373B3F43474B4F53575B5F63676B6F73777B7F83878B8F93979B9FA3A7ABAFB3B7BBBFC3C7CBCFD3D7DBDFE3E7EBEFF3F7FB
+set histogram_size=default;
+drop table t1, t2;
 set use_stat_tables=@save_use_stat_tables;

mysql-test/t/statistics.test

@@ -638,5 +638,48 @@ set histogram_type=default;
 
 drop table t1;
 
+--echo #
+--echo # Bug mdev-4369: histogram for a column with many distinct values 
+--echo #
+
+
+CREATE TABLE t1 (id int);
+CREATE TABLE t2 (id int);
+
+INSERT INTO t1 (id) VALUES (1), (1), (1),(1);
+INSERT INTO t1 (id) SELECT id FROM t1;
+INSERT INTO t1 SELECT id+1 FROM t1;
+INSERT INTO t1 SELECT id+2 FROM t1;
+INSERT INTO t1 SELECT id+4 FROM t1;
+INSERT INTO t1 SELECT id+8 FROM t1;
+INSERT INTO t1 SELECT id+16 FROM t1;
+INSERT INTO t1 SELECT id+32 FROM t1;
+INSERT INTO t1 SELECT id+64 FROM t1;
+INSERT INTO t1 SELECT id+128 FROM t1;
+INSERT INTO t1 SELECT id+256 FROM t1;
+INSERT INTO t1 SELECT id+512 FROM t1;
+
+INSERT INTO t2 SELECT id FROM t1 ORDER BY id*rand();
+
+SELECT COUNT(*) FROM t2;
+SELECT COUNT(DISTINCT id) FROM t2;
+
+set @@tmp_table_size=1024*16;
+set @@max_heap_table_size=1024*16;
+
+set histogram_size=63;
+
+analyze table t2 persistent for all;
+
+select db_name, table_name, column_name,
+       min_value, max_value,
+       nulls_ratio, avg_frequency,
+       hist_size, hist_type, HEX(histogram)
+  FROM mysql.column_stats;
+
+set histogram_size=default;
+
+drop table t1, t2;
+
 set use_stat_tables=@save_use_stat_tables;
 

sql/sql_class.h

@@ -3997,6 +3997,7 @@ class Unique :public Sql_alloc
   uint size;
   uint full_size;
   uint min_dupl_count;   /* always 0 for unions, > 0 for intersections */
+  bool with_counters;
 
   bool merge(TABLE *table, uchar *buff, bool without_last_merge);
 

sql/sql_statistics.cc

@@ -1348,7 +1348,7 @@ class Count_distinct_field: public Sql_alloc
     tree_key_length= field->pack_length();
 
     tree= new Unique((qsort_cmp2) simple_str_key_cmp, (void*) field,
-                     tree_key_length, max_heap_table_size);
+                     tree_key_length, max_heap_table_size, 1);
   }
 
   virtual ~Count_distinct_field()
@@ -1435,7 +1435,7 @@ class Count_distinct_field_bit: public Count_distinct_field
 
     tree= new Unique((qsort_cmp2) simple_ulonglong_key_cmp,
                      (void*) &tree_key_length,
-                     tree_key_length, max_heap_table_size);
+                     tree_key_length, max_heap_table_size, 1);
   }
 
   bool add()

sql/uniques.cc

@@ -86,6 +86,7 @@ Unique::Unique(qsort_cmp2 comp_func, void * comp_func_fixed_arg,
   full_size= size;
   if (min_dupl_count_arg)
     full_size+= sizeof(element_count);
+  with_counters= test(min_dupl_count_arg);
   my_b_clear(&file);
   init_tree(&tree, (ulong) (max_in_memory_size / 16), 0, size, comp_func,
             NULL, comp_func_fixed_arg, MYF(MY_THREAD_SPECIFIC));
@@ -428,6 +429,22 @@ static int buffpek_compare(void *arg, uchar *key_ptr1, uchar *key_ptr2)
 C_MODE_END
 
 
+inline
+element_count get_counter_from_merged_element(void *ptr, uint ofs)
+{
+  element_count cnt;
+  memcpy((uchar *) &cnt, (uchar *) ptr + ofs, sizeof(element_count));
+  return cnt;
+}
+
+
+inline
+void put_counter_into_merged_element(void *ptr, uint ofs, element_count cnt)
+{
+  memcpy((uchar *) ptr + ofs, (uchar *) &cnt, sizeof(element_count));
+}
+
+
 /*
   DESCRIPTION
 
@@ -457,6 +474,8 @@ C_MODE_END
     file               file with all trees dumped. Trees in the file
                        must contain sorted unique values. Cache must be
                        initialized in read mode.
+    with counters      take into account counters for equal merged
+                       elements
   RETURN VALUE
     0     ok
     <> 0  error
@@ -466,7 +485,7 @@ static bool merge_walk(uchar *merge_buffer, ulong merge_buffer_size,
                        uint key_length, BUFFPEK *begin, BUFFPEK *end,
                        tree_walk_action walk_action, void *walk_action_arg,
                        qsort_cmp2 compare, void *compare_arg,
-                       IO_CACHE *file)
+                       IO_CACHE *file, bool with_counters)
 {
   BUFFPEK_COMPARE_CONTEXT compare_context = { compare, compare_arg };
   QUEUE queue;
@@ -485,6 +504,8 @@ static bool merge_walk(uchar *merge_buffer, ulong merge_buffer_size,
   uint bytes_read;               /* to hold return value of read_to_buffer */
   BUFFPEK *top;
   int res= 1;
+  uint cnt_ofs= key_length - (with_counters ? sizeof(element_count) : 0);
+  element_count cnt;
   /*
     Invariant: queue must contain top element from each tree, until a tree
     is not completely walked through.
@@ -543,9 +564,17 @@ static bool merge_walk(uchar *merge_buffer, ulong merge_buffer_size,
     /* new top has been obtained; if old top is unique, apply the action */
     if (compare(compare_arg, old_key, top->key))
     {
-      if (walk_action(old_key, 1, walk_action_arg))
+      cnt= with_counters ?
+           get_counter_from_merged_element(old_key, cnt_ofs) : 1;
+      if (walk_action(old_key, cnt, walk_action_arg))
         goto end;
     }
+    else if (with_counters)
+    {
+      cnt= get_counter_from_merged_element(top->key, cnt_ofs);
+      cnt+= get_counter_from_merged_element(old_key, cnt_ofs);
+      put_counter_into_merged_element(top->key, cnt_ofs, cnt);
+    }
   }
   /*
     Applying walk_action to the tail of the last tree: this is safe because
@@ -556,7 +585,10 @@ static bool merge_walk(uchar *merge_buffer, ulong merge_buffer_size,
   {
     do
     {
-      if (walk_action(top->key, 1, walk_action_arg))
+      
+      cnt= with_counters ?
+           get_counter_from_merged_element(top->key, cnt_ofs) : 1;
+      if (walk_action(top->key, cnt, walk_action_arg))
         goto end;
       top->key+= key_length;
     }
@@ -620,7 +652,7 @@ bool Unique::walk(TABLE *table, tree_walk_action action, void *walk_action_arg)
                     (BUFFPEK *) file_ptrs.buffer,
                     (BUFFPEK *) file_ptrs.buffer + file_ptrs.elements,
                     action, walk_action_arg,
-                    tree.compare, tree.custom_arg, &file);
+                    tree.compare, tree.custom_arg, &file, with_counters);
   }
   my_free(merge_buffer);
   return res;