MDEV-27442 Wrong result upon query with DISTINCT and EXISTS subquery

The problem was that get_best_group_min_max() did not check if fields used
by the "group_min_max optimization" where used in sub queries.
Because of this, it did not detect that a key (b,a) was used in the WHERE
clause for the statement:
SELECT DISTINCT b FROM t1 WHERE EXISTS ( SELECT 1 FROM DUAL WHERE a > 1 ).

Fixed by also traversing the sub queries when checking if a field is used.
This disables group_min_max_optimization for the above query.

Reviewer: Sergei Petrunia

mysql-test/main/group_min_max.result

@@ -4027,3 +4027,19 @@ drop table t1;
 #
 # End of 10.1 tests
 #
+#
+# MDEV-27442 Wrong result upon query with DISTINCT and EXISTS subquery
+#
+CREATE TABLE t1 (a int, b int, KEY b (b,a)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (0,100),(2,100),(2,101),(3,102);
+# Must not use Using index for group-by
+explain SELECT DISTINCT b FROM t1 WHERE EXISTS ( SELECT 1 FROM DUAL WHERE a > 1 );
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	index	NULL	b	10	NULL	4	Using where; Using index
+2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+SELECT DISTINCT b FROM t1 WHERE EXISTS ( SELECT 1 FROM DUAL WHERE a > 1 );
+b
+100
+101
+102
+DROP TABLE t1;

mysql-test/main/group_min_max.test

@@ -1689,3 +1689,14 @@ drop table t1;
 --echo #
 --echo # End of 10.1 tests
 --echo #
+
+--echo #
+--echo # MDEV-27442 Wrong result upon query with DISTINCT and EXISTS subquery
+--echo #
+
+CREATE TABLE t1 (a int, b int, KEY b (b,a)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (0,100),(2,100),(2,101),(3,102);
+--echo # Must not use Using index for group-by
+explain SELECT DISTINCT b FROM t1 WHERE EXISTS ( SELECT 1 FROM DUAL WHERE a > 1 );
+SELECT DISTINCT b FROM t1 WHERE EXISTS ( SELECT 1 FROM DUAL WHERE a > 1 );
+DROP TABLE t1;

sql/opt_range.cc

@@ -14000,7 +14000,7 @@ get_best_group_min_max(PARAM *param, SEL_TREE *tree, double read_time)
         key_part_range[1]= last_part;
 
         /* Check if cur_part is referenced in the WHERE clause. */
-        if (join->conds->walk(&Item::find_item_in_field_list_processor, 0,
+        if (join->conds->walk(&Item::find_item_in_field_list_processor, true,
                               key_part_range))
         {
           cause= "keypart reference from where clause";