Fixed bug #25602. A query with DISTINCT in the select list to which

the loose scan optimization for grouping queries was applied returned 
a wrong result set when the query was used with the SQL_BIG_RESULT
option.

The SQL_BIG_RESULT option forces to use sorting algorithm for grouping
queries instead of employing a suitable index. The current loose scan
optimization is applied only for one table queries when the suitable
index is covering. It does not make sense to use sort algorithm in this
case. However the create_sort_index function does not take into account
the possible choice of the loose scan to implement the DISTINCT operator
which makes sorting unnecessary. Moreover the current implementation of
the loose scan for queries with distinct assumes that sorting will
never happen. Thus in this case create_sort_index should not call
the function filesort.

mysql-test/r/group_min_max.result

@@ -2286,3 +2286,24 @@ Variable_name	Value
 Handler_read_key	8
 Handler_read_next	1
 DROP TABLE t1,t2,t3;
+CREATE TABLE t1 (a int, INDEX idx(a));
+INSERT INTO t1 VALUES
+(4), (2), (1), (2), (4), (2), (1), (4),
+(4), (2), (1), (2), (2), (4), (1), (4);
+EXPLAIN SELECT DISTINCT(a) FROM t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	NULL	idx	5	NULL	9	Using index for group-by
+SELECT DISTINCT(a) FROM t1;
+a
+1
+2
+4
+EXPLAIN SELECT SQL_BIG_RESULT DISTINCT(a) FROM t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	NULL	idx	5	NULL	9	Using index for group-by
+SELECT SQL_BIG_RESULT DISTINCT(a) FROM t1;
+a
+1
+2
+4
+DROP TABLE t1;

mysql-test/t/group_min_max.test

@@ -870,3 +870,25 @@ DELETE FROM t3 WHERE (SELECT (SELECT MAX(b) FROM t1 GROUP BY a HAVING a < 2) x
 SHOW STATUS LIKE 'handler_read__e%';
 
 DROP TABLE t1,t2,t3;
+
+#
+# Bug#25602: queries with DISTINCT and SQL_BIG_RESULT hint 
+#            for which loose scan optimization is applied
+#
+
+CREATE TABLE t1 (a int, INDEX idx(a));
+INSERT INTO t1 VALUES
+  (4), (2), (1), (2), (4), (2), (1), (4),
+  (4), (2), (1), (2), (2), (4), (1), (4);
+
+EXPLAIN SELECT DISTINCT(a) FROM t1;
+SELECT DISTINCT(a) FROM t1;
+EXPLAIN SELECT SQL_BIG_RESULT DISTINCT(a) FROM t1;
+SELECT SQL_BIG_RESULT DISTINCT(a) FROM t1;
+
+DROP TABLE t1;
+
+
+
+
+

sql/sql_select.cc

@@ -12552,10 +12552,14 @@ create_sort_index(THD *thd, JOIN *join, ORDER *order,
 
   /*
     When there is SQL_BIG_RESULT do not sort using index for GROUP BY,
-    and thus force sorting on disk.
+    and thus force sorting on disk unless a group min-max optimization
+    is going to be used as it is applied now only for one table queries
+    with covering indexes.
   */
   if ((order != join->group_list || 
-       !(join->select_options & SELECT_BIG_RESULT)) &&
+       !(join->select_options & SELECT_BIG_RESULT) ||
+       select && select->quick &&
+       select->quick->get_type() == QUICK_SELECT_I::QS_TYPE_GROUP_MIN_MAX) &&
       test_if_skip_sort_order(tab,order,select_limit,0))
     DBUG_RETURN(0);
   for (ORDER *ord= join->order; ord; ord= ord->next)