MDEV-25635 Assertion failure when pushing from HAVING into WHERE of view

This bug could manifest itself after pushing a where condition over a
mergeable derived table / view / CTE DT into a grouping view / derived
table / CTE V whose item list contained set functions with constant
arguments such as MIN(2), SUM(1) etc. In such cases the field references
used in the condition pushed into the view V that correspond set functions
are wrapped into Item_direct_view_ref wrappers. Due to a wrong implementation
of the virtual method const_item() for the class Item_direct_view_ref the
wrapped set functions with constant arguments could be erroneously taken
for constant items. This could lead to a wrong result set returned by the
main select query in 10.2. In 10.4 where a possibility of pushing condition
from HAVING into WHERE had been added this could cause a crash.

Approved by Sergey Petrunya <sergey.petrunya@mariadb.com>

mysql-test/r/derived_cond_pushdown.result

@@ -10634,4 +10634,43 @@ m
 7
 drop view v1;
 drop table t1;
+#
+# MDEV-25635: pushdown into grouping view using aggregate functions
+#             with constant arguments via a mergeable derived table
+#
+create table t1 (a int);
+insert into t1 values (3), (7), (1), (3), (7), (7), (3);
+create view v1 as select a, sum(1) as f, sum(1) as g from t1 group by a;
+select * from v1;
+a	f	g
+1	1	1
+3	3	3
+7	3	3
+select * from (select * from v1) as dt where a=f and a=g;
+a	f	g
+1	1	1
+3	3	3
+explain extended select * from (select * from v1) as dt where a=f and a=g;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	<derived3>	ALL	NULL	NULL	NULL	NULL	7	100.00	Using where
+3	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	7	100.00	Using temporary; Using filesort
+Warnings:
+Note	1003	select `v1`.`a` AS `a`,`v1`.`f` AS `f`,`v1`.`g` AS `g` from `test`.`v1` where `v1`.`a` = `v1`.`f` and `v1`.`a` = `v1`.`g`
+create view v2 as select a, min(1) as f, min(1) as g from t1 group by a;
+select * from v2;
+a	f	g
+1	1	1
+3	1	1
+7	1	1
+select * from (select * from v2) as dt where a=f and a=g;
+a	f	g
+1	1	1
+explain extended select * from (select * from v2) as dt where a=f and a=g;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	<derived3>	ALL	NULL	NULL	NULL	NULL	7	100.00	Using where
+3	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	7	100.00	Using temporary; Using filesort
+Warnings:
+Note	1003	select `v2`.`a` AS `a`,`v2`.`f` AS `f`,`v2`.`g` AS `g` from `test`.`v2` where `v2`.`f` = `v2`.`a` and `v2`.`g` = `v2`.`a`
+drop view v1,v2;
+drop table t1;
 # End of 10.2 tests

mysql-test/t/derived_cond_pushdown.test

@@ -2212,4 +2212,29 @@ select * from v1 where m > 0;
 drop view v1;
 drop table t1;
 
+--echo #
+--echo # MDEV-25635: pushdown into grouping view using aggregate functions
+--echo #             with constant arguments via a mergeable derived table
+--echo #
+
+create table t1 (a int);
+insert into t1 values (3), (7), (1), (3), (7), (7), (3);
+
+create view v1 as select a, sum(1) as f, sum(1) as g from t1 group by a;
+select * from v1;
+let $q1=
+select * from (select * from v1) as dt where a=f and a=g;
+eval $q1;
+eval explain extended $q1;
+
+create view v2 as select a, min(1) as f, min(1) as g from t1 group by a;
+select * from v2;
+let $q2=
+select * from (select * from v2) as dt where a=f and a=g;
+eval $q2;
+eval explain extended $q2;
+
+drop view v1,v2;
+drop table t1;
+
 --echo # End of 10.2 tests

sql/item.h

@@ -4952,7 +4952,10 @@ class Item_direct_view_ref :public Item_direct_ref
   table_map used_tables() const;
   void update_used_tables();
   table_map not_null_tables() const;
-  bool const_item() const { return used_tables() == 0; }
+  bool const_item() const
+  {
+    return (*ref)->const_item() && (null_ref_table == NO_NULL_TABLE);
+  }
   TABLE *get_null_ref_table() const { return null_ref_table; }
   bool walk(Item_processor processor, bool walk_subquery, void *arg)
   {