MDEV-23723: Crash when test_if_skip_sort_order() is checked for derived ...

The problem was caused by the following scenario:

Subquery's table has two indexes, KEY a(a), KEY a_b(a,b)

- LATERAL DERIVED optimization decides to use index a.
  = The subquery uses ref access over key a.
- test_if_skip_sort_order() sees that KEY a_b satisfies the
  subquery's GROUP BY clause, and attempts to switch to it.
  = It fails to do so, because KEYUSE objects for index a_b
    are switched off.

Fixed by disallowing to change the ref access key if it uses KEYUSE
objects injected by LATERAL DERIVED optimization.

mysql-test/main/derived_split_innodb.result

@@ -140,3 +140,37 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 2	DERIVED	t2	index	NULL	PRIMARY	4	NULL	3	
 drop view v1;
 drop table t1,t2;
+#
+#  MDEV-23723: Crash when test_if_skip_sort_order() is checked for derived table subject to split
+#
+CREATE TABLE t1 (a INT, b INT, KEY (a), KEY (a,b)) ENGINE=InnoDB;
+CREATE TABLE t2 (c INT, KEY (c)) ENGINE=InnoDB;
+SELECT * FROM t1 t1a JOIN t1 t1b;
+a	b	a	b
+INSERT INTO t2 VALUES (1),(2);
+INSERT INTO t1 VALUES (1,2),(3,4),(5,6),(7,8),(9,10),(11,12);
+set statement optimizer_switch='split_materialized=off' for EXPLAIN
+SELECT *
+FROM
+t1 JOIN
+(SELECT t1.a, t1.b FROM t1, t2 WHERE t1.b = t2.c GROUP BY t1.a, t1.b) as dt
+WHERE
+t1.a = dt.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	index	a,a_2	a_2	10	NULL	6	Using where; Using index
+1	PRIMARY	<derived2>	ref	key0	key0	5	test.t1.a	2	
+2	DERIVED	t1	index	NULL	a_2	10	NULL	6	Using where; Using index
+2	DERIVED	t2	ref	c	c	5	test.t1.b	1	Using index
+set statement optimizer_switch='split_materialized=on' for  EXPLAIN
+SELECT *
+FROM
+t1 JOIN
+(SELECT t1.a, t1.b FROM t1, t2 WHERE t1.b = t2.c GROUP BY t1.a, t1.b) as dt
+WHERE
+t1.a = dt.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	index	a,a_2	a_2	10	NULL	6	Using where; Using index
+1	PRIMARY	<derived2>	ref	key0	key0	5	test.t1.a	2	
+2	LATERAL DERIVED	t1	ref	a,a_2	a	5	test.t1.a	1	Using where; Using temporary; Using filesort
+2	LATERAL DERIVED	t2	ref	c	c	5	test.t1.b	1	Using index
+DROP TABLE t1, t2;

mysql-test/main/derived_split_innodb.test

@@ -124,3 +124,29 @@ eval set statement optimizer_switch='split_materialized=off' for explain $q;
 drop view v1;
 
 drop table t1,t2;
+
+--echo #
+--echo #  MDEV-23723: Crash when test_if_skip_sort_order() is checked for derived table subject to split
+--echo #
+CREATE TABLE t1 (a INT, b INT, KEY (a), KEY (a,b)) ENGINE=InnoDB;
+CREATE TABLE t2 (c INT, KEY (c)) ENGINE=InnoDB;
+
+SELECT * FROM t1 t1a JOIN t1 t1b;
+
+INSERT INTO t2 VALUES (1),(2);
+INSERT INTO t1 VALUES (1,2),(3,4),(5,6),(7,8),(9,10),(11,12);
+
+let $query=
+EXPLAIN
+SELECT *
+FROM
+  t1 JOIN
+  (SELECT t1.a, t1.b FROM t1, t2 WHERE t1.b = t2.c GROUP BY t1.a, t1.b) as dt
+WHERE
+  t1.a = dt.a;
+
+eval set statement optimizer_switch='split_materialized=off' for $query;
+eval set statement optimizer_switch='split_materialized=on' for  $query;
+
+DROP TABLE t1, t2;
+

sql/sql_select.cc

@@ -10102,6 +10102,7 @@ static bool create_ref_for_key(JOIN *join, JOIN_TAB *j,
   j->ref.disable_cache= FALSE;
   j->ref.null_ref_part= NO_REF_PART;
   j->ref.const_ref_part_map= 0;
+  j->ref.uses_splitting= FALSE;
   keyuse=org_keyuse;
 
   store_key **ref_key= j->ref.key_copy;
@@ -10150,6 +10151,7 @@ static bool create_ref_for_key(JOIN *join, JOIN_TAB *j,
         j->ref.null_rejecting|= (key_part_map)1 << i;
 
       keyuse_uses_no_tables= keyuse_uses_no_tables && !keyuse->used_tables;
+      j->ref.uses_splitting |= (keyuse->validity_ref != NULL);
       /*
         We don't want to compute heavy expressions in EXPLAIN, an example would
         select * from t1 where t1.key=(select thats very heavy);
@@ -22536,7 +22538,8 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
       todo: why does JT_REF_OR_NULL mean filesort? We could find another index
       that satisfies the ordering. I would just set ref_key=MAX_KEY here...
     */
-    if (tab->type == JT_REF_OR_NULL || tab->type == JT_FT)
+    if (tab->type == JT_REF_OR_NULL || tab->type == JT_FT ||
+        tab->ref.uses_splitting)
       goto use_filesort;
   }
   else if (select && select->quick)		// Range found by opt_range

sql/sql_select.h

@@ -178,6 +178,12 @@ typedef struct st_table_ref
   */
   bool          disable_cache;
 
+  /*
+    If true, this ref access was constructed from equalities generated by
+    LATERAL DERIVED (aka GROUP BY splitting) optimization
+  */
+  bool          uses_splitting;
+
   bool tmp_table_index_lookup_init(THD *thd, KEY *tmp_key, Item_iterator &it,
                                    bool value, uint skip= 0);
   bool is_access_triggered();