Fix for BUG#8532.

The problem was in that the code that analyses the applicability of the
QUICK_GROUP_MIN_MAX access method for DISTINC queries assumed that there
are no duplicate column references in the DISTINCT clause, and it added
non-exiting key parts for the duplicate column references. 

The solution adds a test to check whether the select list already contained
a field with the same name. If such field was already present, then it was
already decided to use its key part for index access. In this such case we
must skip the duplicate field instead of counting it as a new field.


mysql-test/r/group_min_max.result:
  Added test results for BUG#8532.
mysql-test/t/group_min_max.test:
  Added tests for BUG#8532.
sql/opt_range.cc:
  The problem was in that the code that analyses the applicability of the
  QUICK_GROUP_MIN_MAX access method for DISTINC queries assumed that there
  are no duplicate column references in the DISTINCT clause, and it added
  non-exiting key parts for the duplicate column references. 
  
  The solution adds a test to check whether the select list already contained
  a field with the same name. If such field was already present, then it was
  already decided to use its key part for index access. In this such case we
  must skip the duplicate field instead of counting it as a new field.

1 files changed, 10 insertions, 1 deletions

diff --git a/sql/opt_range.cc b/sql/opt_range.cc
index 12e5c60312b..812d5a41cbc 100644
--- a/sql/opt_range.cc
+++ b/sql/opt_range.cc
@@ -6878,6 +6878,7 @@ get_best_group_min_max(PARAM *param, SEL_TREE *tree)
   SEL_ARG *cur_index_tree= NULL;
   ha_rows cur_quick_prefix_records= 0;
   uint cur_param_idx;
+  key_map cur_used_key_parts;
 
   for (uint cur_index= 0 ; cur_index_info != cur_index_info_end ;
        cur_index_info++, cur_index++)
@@ -6925,17 +6926,25 @@ get_best_group_min_max(PARAM *param, SEL_TREE *tree)
     else if (join->select_distinct)
     {
       select_items_it.rewind();
+      cur_used_key_parts.clear_all();
       while ((item= select_items_it++))
       {
         item_field= (Item_field*) item; /* (SA5) already checked above. */
         /* Find the order of the key part in the index. */
         key_part_nr= get_field_keypart(cur_index_info, item_field->field);
+        /*
+          Check if this attribute was already present in the select list.
+          If it was present, then its corresponding key part was alredy used.
+        */
+        if (cur_used_key_parts.is_set(key_part_nr))
+          continue;
         if (key_part_nr < 1 || key_part_nr > join->fields_list.elements)
           goto next_index;
         cur_part= cur_index_info->key_part + key_part_nr - 1;
         cur_group_prefix_len+= cur_part->store_length;
+        cur_used_key_parts.set_bit(key_part_nr);
+        ++cur_group_key_parts;
       }
-      cur_group_key_parts= join->fields_list.elements;
     }
     else
       DBUG_ASSERT(FALSE);