MDEV-20109: Optimizer ignores distinct key created for materialized...

(Backported to 10.2-compatiblity, build 22257)

Adjust the testcase according to the review input

mysql-test/r/subselect_sj.result

@@ -3184,7 +3184,7 @@ insert into t4 select floor(rand()*1000) from t2 limit 500;
 # |    1 | PRIMARY      | t3          | ALL  | NULL          | ... | 10000 | Using where; Using join buffer (flat, BNL join) |
 #
 # Instead, it should use eq_ref on the materialized table.
-explain select * from t3 where a in (select a from t4 group by a);
+explain select * from t3 where a in (select a from t4);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t3	ALL	NULL	NULL	NULL	NULL	10000	
 1	PRIMARY	<subquery2>	eq_ref	distinct_key	distinct_key	4	func	1	

mysql-test/r/subselect_sj_jcl6.result

@@ -3197,7 +3197,7 @@ insert into t4 select floor(rand()*1000) from t2 limit 500;
 # |    1 | PRIMARY      | t3          | ALL  | NULL          | ... | 10000 | Using where; Using join buffer (flat, BNL join) |
 #
 # Instead, it should use eq_ref on the materialized table.
-explain select * from t3 where a in (select a from t4 group by a);
+explain select * from t3 where a in (select a from t4);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t3	ALL	NULL	NULL	NULL	NULL	10000	
 1	PRIMARY	<subquery2>	eq_ref	distinct_key	distinct_key	4	func	1	

mysql-test/t/subselect_sj.test

@@ -2867,7 +2867,7 @@ insert into t4 select floor(rand()*1000) from t2 limit 500;
 --echo #
 --echo # Instead, it should use eq_ref on the materialized table.
 
-explain select * from t3 where a in (select a from t4 group by a);
+explain select * from t3 where a in (select a from t4);
 
 drop table t1, t2, t3, t4;