MDEV-20057 Distinct SUM on CROSS JOIN and grouped returns wrong result

SELECT DISTINCT did not work with expressions with sum functions.
Distinct was only done on the values stored in the intermediate temporary
tables, which only stored the value of each sum function.

In other words:
SELECT DISTINCT sum(a),sum(b),avg(c) ... worked.
SELECT DISTINCT sum(a),sum(b) > 2,sum(c)+sum(d) would not work.

The later query would do ONLY apply distinct on the sum(a) part.

Reviewer: Sergei Petrunia <sergey@mariadb.com>


This was fixed by extending remove_dup_with_hash_index() and
remove_dup_with_compare() to take into account the columns in the result
list that where not stored in the temporary table.

Note that in many cases the above dup removal functions are not used as
the optimizer may be able to either remove duplicates early or it will
discover that duplicate remove is not needed. The later happens for
example if the group by fields is part of the result.

Other things:
- Backported from 11.0 the change of Sort_param.tmp_buffer from char* to
  String.
- Changed Type_handler::make_sort_key() to take String as a parameter
  instead of Sort_param. This was done to allow make_sort_key() functions
  to be reused by distinct elimination functions.
  This makes Type_handler_string_result::make_sort_key() similar to code
  in 11.0
- Simplied error handling in remove_dup_with_compare() to remove code
  duplication.

sql/sql_sort.h

@@ -19,6 +19,7 @@
 #include "my_base.h"                            /* ha_rows */
 #include <my_sys.h>                             /* qsort2_cmp */
 #include "queues.h"
+#include "sql_string.h"
 
 typedef struct st_buffpek BUFFPEK;
 
@@ -82,14 +83,20 @@ public:
 
   uchar *unique_buff;
   bool not_killable;
-  char* tmp_buffer;
+  String tmp_buffer;
   // The fields below are used only by Unique class.
   qsort2_cmp compare;
   BUFFPEK_COMPARE_CONTEXT cmp_context;
 
   Sort_param()
   {
-    memset(this, 0, sizeof(*this));
+    memset(reinterpret_cast<void*>(this), 0, sizeof(*this));
+    tmp_buffer.set_thread_specific();
+    /*
+      Fix memset() clearing the charset.
+      TODO: The constructor should be eventually rewritten not to use memset().
+    */
+    tmp_buffer.set_charset(&my_charset_bin);
   }
   void init_for_filesort(uint sortlen, TABLE *table,
                          ha_rows maxrows, bool sort_positions);