MDEV-30975 Wrong result with cross Join given join order

For queries with derived tables populated having some side-effect, we
will fill such a derived table more than once, but without clearing
its rows.  Consequently it will have duplicate rows.
An example query exhibiting the problem is
  SELECT STRAIGHT_JOIN c1 FROM t1 JOIN (SELECT @A := 0) x;
Since mysql_derived_fill will, for UNCACHEABLE_DEPENDENT tables, drop
all rows and repopulate, we relax the condition at line 1204: rather
than assume all uncacheable values prevent early return, we now
allow an early return for uncacheable values other than
UNCACHEABLE_DEPENDENT.  In general, we only populate derived tables
once unless they're dependent tables.

mysql-test/main/join.result

@@ -3423,6 +3423,62 @@ SELECT COUNT(*) FROM t1 LEFT JOIN (t2 LEFT JOIN t3 ON t2.b = t3.c) ON t1.a = t2.
 COUNT(*)
 2
 DROP TABLE t1, t2, t3;
+#
+# MDEV-30975: Wrong result with cross Join given join order
+#
+CREATE TABLE `t1` (
+`t1_seq` INT NOT NULL,
+`c1` VARCHAR(10) NOT NULL ,
+PRIMARY KEY (`t1_seq`) USING BTREE
+);
+CREATE TABLE `t2` (
+`t2_seq` INT NOT NULL,
+`t1_seq` INT NOT NULL,
+`c2` VARCHAR(10) NOT NULL ,
+PRIMARY KEY (`t2_seq`, `t1_seq`) USING BTREE
+);
+INSERT INTO t1 VALUES(1, 'A');
+INSERT INTO t2 VALUES(1, 1, 'T2-1-1');
+INSERT INTO t2 VALUES(2, 1, 'T2-1-2');
+INSERT INTO t2 VALUES(3, 1, 'T2-1-3');
+SELECT LPAD(@rownum := @rownum + 1, 8, 0) AS str_num
+, t1.t1_seq
+, t2.t2_seq
+, t1.c1
+, t2.c2
+FROM t1
+INNER JOIN t2 ON (t1.t1_seq = t2.t1_seq)
+CROSS JOIN ( SELECT @rownum := 0 ) X;
+str_num	t1_seq	t2_seq	c1	c2
+00000001	1	1	A	T2-1-1
+00000002	1	2	A	T2-1-2
+00000003	1	3	A	T2-1-3
+SELECT STRAIGHT_JOIN LPAD(@rownum := @rownum + 1, 8, 0) AS str_num
+, t1.t1_seq
+, t2.t2_seq
+, t1.c1
+, t2.c2
+FROM t1
+INNER JOIN t2 ON (t1.t1_seq = t2.t1_seq)
+CROSS JOIN ( SELECT @rownum := 0 ) X;
+str_num	t1_seq	t2_seq	c1	c2
+00000001	1	1	A	T2-1-1
+00000002	1	2	A	T2-1-2
+00000003	1	3	A	T2-1-3
+SELECT STRAIGHT_JOIN * FROM t1 JOIN t2 ON (t1.t1_seq = t2.t1_seq) JOIN (SELECT @a := 0) x;
+t1_seq	c1	t2_seq	t1_seq	c2	@a := 0
+1	A	1	1	T2-1-1	0
+1	A	2	1	T2-1-2	0
+1	A	3	1	T2-1-3	0
+SELECT * FROM t1 JOIN t2 ON (t1.t1_seq = t2.t1_seq) JOIN (SELECT @a := 0) x;
+t1_seq	c1	t2_seq	t1_seq	c2	@a := 0
+1	A	1	1	T2-1-1	0
+1	A	2	1	T2-1-2	0
+1	A	3	1	T2-1-3	0
+SELECT STRAIGHT_JOIN c1 FROM t1 JOIN (SELECT @a := 0) x;
+c1
+A
+DROP TABLE t1, t2;
 # End of 10.5 tests
 #
 # MDEV-31449: Assertion s->table->opt_range_condition_rows <= s->found_records

mysql-test/main/join.test

@@ -1838,6 +1838,52 @@ SELECT * FROM t1 LEFT JOIN (t2 LEFT JOIN t3 ON t2.b = t3.c) ON t1.a = t2.b;
 SELECT COUNT(*) FROM t1 LEFT JOIN (t2 LEFT JOIN t3 ON t2.b = t3.c) ON t1.a = t2.b;
 DROP TABLE t1, t2, t3;
 
+--echo #
+--echo # MDEV-30975: Wrong result with cross Join given join order
+--echo #
+
+CREATE TABLE `t1` (
+	`t1_seq` INT NOT NULL,
+	`c1` VARCHAR(10) NOT NULL ,
+	PRIMARY KEY (`t1_seq`) USING BTREE
+);
+
+CREATE TABLE `t2` (
+	`t2_seq` INT NOT NULL,
+	`t1_seq` INT NOT NULL,
+	`c2` VARCHAR(10) NOT NULL ,
+	PRIMARY KEY (`t2_seq`, `t1_seq`) USING BTREE
+);
+
+INSERT INTO t1 VALUES(1, 'A');
+INSERT INTO t2 VALUES(1, 1, 'T2-1-1');
+INSERT INTO t2 VALUES(2, 1, 'T2-1-2');
+INSERT INTO t2 VALUES(3, 1, 'T2-1-3');
+
+SELECT LPAD(@rownum := @rownum + 1, 8, 0) AS str_num
+     , t1.t1_seq
+     , t2.t2_seq
+     , t1.c1
+     , t2.c2
+  FROM t1
+       INNER JOIN t2 ON (t1.t1_seq = t2.t1_seq)
+       CROSS JOIN ( SELECT @rownum := 0 ) X;
+
+SELECT STRAIGHT_JOIN LPAD(@rownum := @rownum + 1, 8, 0) AS str_num
+     , t1.t1_seq
+     , t2.t2_seq
+     , t1.c1
+     , t2.c2
+  FROM t1
+       INNER JOIN t2 ON (t1.t1_seq = t2.t1_seq)
+       CROSS JOIN ( SELECT @rownum := 0 ) X;
+
+SELECT STRAIGHT_JOIN * FROM t1 JOIN t2 ON (t1.t1_seq = t2.t1_seq) JOIN (SELECT @a := 0) x;
+SELECT * FROM t1 JOIN t2 ON (t1.t1_seq = t2.t1_seq) JOIN (SELECT @a := 0) x;
+SELECT STRAIGHT_JOIN c1 FROM t1 JOIN (SELECT @a := 0) x;
+
+DROP TABLE t1, t2;
+
 --echo # End of 10.5 tests
 
 --echo #

sql/sql_derived.cc

@@ -1201,8 +1201,12 @@ bool mysql_derived_fill(THD *thd, LEX *lex, TABLE_LIST *derived)
                        (derived->alias.str ? derived->alias.str : "<NULL>"),
                        derived->get_unit()));
 
-  if (unit->executed && !unit->uncacheable && !unit->describe &&
-      !derived_is_recursive)
+  /*
+    Only fill derived tables once, unless the derived table is dependent in
+    which case we will delete all of its rows and refill it below.
+  */
+  if (unit->executed && !(unit->uncacheable & UNCACHEABLE_DEPENDENT) &&
+      !unit->describe && !derived_is_recursive)
     DBUG_RETURN(FALSE);
   /*check that table creation passed without problems. */
   DBUG_ASSERT(derived->table && derived->table->is_created());
@@ -1261,6 +1265,7 @@ bool mysql_derived_fill(THD *thd, LEX *lex, TABLE_LIST *derived)
   }
   else
   {
+    DBUG_ASSERT(!unit->executed || (unit->uncacheable & UNCACHEABLE_DEPENDENT));
     SELECT_LEX *first_select= unit->first_select();
     unit->set_limit(unit->global_parameters());
     if (unit->lim.is_unlimited())