MDEV-16104 Server crash in JOIN::fix_all_splittings_in_plan

upon select with view and subqueries

This bug occurred when a splittable materialized derived/view
were used inside another splittable materialized derived/view.
The bug happened because the function JOIN::fix_all_splittings_in_plan()
was called at the very beginning of the optimization phase 2 at
the moment when the plan structure of the embedding derived/view
were not valid. The proper position for this call is the very
end of the optimization phase 1.

mysql-test/main/derived_cond_pushdown.result

@@ -15164,3 +15164,36 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 3	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	5	Using temporary
 2	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	5	Using temporary; Using filesort
 drop table t1;
+#
+# MDEV-16104: embedded splittable materialized derived/views
+#
+CREATE TABLE t1 (f int PRIMARY KEY) ENGINE=MyISAM;
+INSERT INTO t1
+VALUES (3), (7), (1), (4), (8), (5), (9);
+CREATE ALGORITHM=MERGE VIEW v1 AS
+SELECT a2.*
+FROM
+( SELECT f, COUNT(*) as c FROM t1 GROUP BY f ) AS a1
+JOIN
+t1 AS a2
+USING (f);
+EXPLAIN EXTENDED
+SELECT * FROM ( SELECT STRAIGHT_JOIN f, COUNT(*) as c FROM v1 GROUP BY f ) AS s;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	7	100.00	
+2	DERIVED	<derived4>	ALL	NULL	NULL	NULL	NULL	7	100.00	Using temporary; Using filesort
+2	DERIVED	a2	eq_ref	PRIMARY	PRIMARY	4	a1.f	1	100.00	Using index
+4	DERIVED	t1	index	PRIMARY	PRIMARY	4	NULL	7	100.00	Using index; Using temporary; Using filesort
+Warnings:
+Note	1003	/* select#1 */ select `s`.`f` AS `f`,`s`.`c` AS `c` from (/* select#2 */ select straight_join `a2`.`f` AS `f`,count(0) AS `c` from ((/* select#4 */ select `test`.`t1`.`f` AS `f`,count(0) AS `c` from `test`.`t1` group by `test`.`t1`.`f`)) `a1` join `test`.`t1` `a2` where `a2`.`f` = `a1`.`f` group by `a2`.`f`) `s`
+SELECT * FROM ( SELECT STRAIGHT_JOIN f, COUNT(*) as c FROM v1 GROUP BY f ) AS s;
+f	c
+1	1
+3	1
+4	1
+5	1
+7	1
+8	1
+9	1
+DROP VIEW v1;
+DROP TABLE t1;

mysql-test/main/derived_cond_pushdown.test

@@ -2690,3 +2690,26 @@ eval $q;
 eval explain $q;
 
 drop table t1;
+
+--echo #
+--echo # MDEV-16104: embedded splittable materialized derived/views
+--echo #
+
+CREATE TABLE t1 (f int PRIMARY KEY) ENGINE=MyISAM;
+INSERT INTO t1
+  VALUES (3), (7), (1), (4), (8), (5), (9);
+
+CREATE ALGORITHM=MERGE VIEW v1 AS
+SELECT a2.*
+FROM
+  ( SELECT f, COUNT(*) as c FROM t1 GROUP BY f ) AS a1
+  JOIN
+  t1 AS a2
+  USING (f);
+
+EXPLAIN EXTENDED
+SELECT * FROM ( SELECT STRAIGHT_JOIN f, COUNT(*) as c FROM v1 GROUP BY f ) AS s;
+SELECT * FROM ( SELECT STRAIGHT_JOIN f, COUNT(*) as c FROM v1 GROUP BY f ) AS s;
+
+DROP VIEW v1;
+DROP TABLE t1;

sql/sql_select.cc

@@ -1853,6 +1853,18 @@ JOIN::optimize_inner()
     DBUG_RETURN(1);
   }
 
+  /*
+    If a splittable materialized derived/view dt_i is embedded into
+    into another splittable materialized derived/view dt_o then
+    splitting plans for dt_i and dt_o are evaluated independently.
+    First the optimizer looks for the best splitting plan sp_i for dt_i.
+    It happens when non-splitting plans for dt_o are evaluated.
+    The cost of sp_i is considered as the cost of materialization of dt_i
+    when evaluating any splitting plan for dt_o.
+  */
+  if (fix_all_splittings_in_plan())
+    DBUG_RETURN(1);
+
 setup_subq_exit:
   with_two_phase_optimization= check_two_phase_optimization(thd);
   if (with_two_phase_optimization)
@@ -9374,9 +9386,6 @@ bool JOIN::get_best_combination()
   full_join=0;
   hash_join= FALSE;
 
-  if (fix_all_splittings_in_plan())
-    DBUG_RETURN(TRUE);
-
   fix_semijoin_strategies_for_picked_join_order(this);
    
   JOIN_TAB_RANGE *root_range;