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

@@ -1657,6 +1657,44 @@ a
 b
 c
 d
+select distinct a1,a1 from t1;
+a1	a1
+a	a
+b	b
+c	c
+d	d
+select distinct a2,a1,a2,a1 from t1;
+a2	a1	a2	a1
+a	a	a	a
+b	a	b	a
+a	b	a	b
+b	b	b	b
+a	c	a	c
+b	c	b	c
+a	d	a	d
+b	d	b	d
+select distinct t1.a1,t2.a1 from t1,t2;
+a1	a1
+a	a
+b	a
+c	a
+d	a
+a	b
+b	b
+c	b
+d	b
+a	c
+b	c
+c	c
+d	c
+a	d
+b	d
+c	d
+d	d
+a	e
+b	e
+c	e
+d	e
 explain select distinct a1,a2,b from t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	range	NULL	idx_t1_1	147	NULL	17	Using index for group-by

mysql-test/t/group_min_max.test

@@ -475,11 +475,15 @@ select distinct a1,a2,b,c from t2 where (a2 >= 'b') and (b = 'a') and (c = 'i121
 select distinct a1,a2,b from t2 where (a1 > 'a') and (a2 > 'a') and (b = 'c');
 select distinct b from t2 where (a2 >= 'b') and (b = 'a');
 
--- BUG 6303
+-- BUG #6303
 select distinct t_00.a1
 from t1 t_00
 where exists ( select * from t2 where a1 = t_00.a1 );
 
+-- BUG #8532 - SELECT DISTINCT a, a causes server to crash
+select distinct a1,a1 from t1;
+select distinct a2,a1,a2,a1 from t1;
+select distinct t1.a1,t2.a1 from t1,t2;
 
 --
 -- DISTINCT queries with GROUP-BY

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);