Bug #48419: another explain crash..

WHERE predicates containing references to empty tables in a
subquery were handled incorrectly by the optimizer when
executing EXPLAIN. As a result, the optimizer could try to
evaluate such predicates rather than just stop with
"Impossible WHERE noticed after reading const tables" as 
it would do in a non-subquery case. This led to valgrind 
errors and crashes.

Fixed the code checking the above condition so that subqueries
are not excluded and hence are handled in the same way as top
level SELECTs.

mysql-test/r/explain.result:
  Added a test case for bug #48419.
mysql-test/r/ps.result:
  Updated test results to take the new (and more correct)
  "Extra" comments in execution plans.
mysql-test/t/explain.test:
  Added a test case for bug #48419.
sql/sql_select.cc:
  There is no point in excluding subqueries from checking
  for identically false WHERE conditions.

mysql-test/r/explain.result

@@ -226,4 +226,16 @@ Warnings:
 Note	1276	Field or reference 'test.t1.c' of SELECT #2 was resolved in SELECT #1
 Note	1003	select (select 1 from `test`.`t2` where (`test`.`t2`.`d` = NULL)) AS `(SELECT 1 FROM t2 WHERE d = c)` from `test`.`t1`
 DROP TABLE t1, t2;
+#
+# Bug #48419: another explain crash..
+#
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (b BLOB, KEY b(b(100)));
+INSERT INTO t2 VALUES ('1'), ('2'), ('3');
+FLUSH TABLES;
+EXPLAIN SELECT 1 FROM t1 WHERE a = (SELECT 1 FROM t1 t JOIN t2 WHERE b <= 1 AND t.a);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+DROP TABLE t1, t2;
 End of 5.1 tests.

mysql-test/r/ps.result

@@ -155,24 +155,24 @@ execute stmt1 ;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 6	DERIVED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
-5	DEPENDENT SUBQUERY	t2	system	NULL	NULL	NULL	NULL	0	const row not found
-4	DEPENDENT SUBQUERY	t2	system	NULL	NULL	NULL	NULL	0	const row not found
+5	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+4	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 3	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 execute stmt1 ;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 6	DERIVED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
-5	DEPENDENT SUBQUERY	t2	system	NULL	NULL	NULL	NULL	0	const row not found
-4	DEPENDENT SUBQUERY	t2	system	NULL	NULL	NULL	NULL	0	const row not found
+5	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+4	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 3	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 explain SELECT (SELECT SUM(c1 + c12 + 0.0) FROM t2 where (t1.c2 - 0e-3) = t2.c2 GROUP BY t1.c15 LIMIT 1) as scalar_s, exists (select 1.0e+0 from t2 where t2.c3 * 9.0000000000 = t1.c4) as exists_s, c5 * 4 in (select c6 + 0.3e+1 from t2) as in_s, (c7 - 4, c8 - 4) in (select c9 + 4.0, c10 + 40e-1 from t2) as in_row_s FROM t1, (select c25 x, c32 y from t2) tt WHERE x * 1 = c25;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 6	DERIVED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
-5	DEPENDENT SUBQUERY	t2	system	NULL	NULL	NULL	NULL	0	const row not found
-4	DEPENDENT SUBQUERY	t2	system	NULL	NULL	NULL	NULL	0	const row not found
+5	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+4	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 3	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 deallocate prepare stmt1;

mysql-test/t/explain.test

@@ -198,4 +198,19 @@ INSERT INTO t2 VALUES (NULL), (0);
 EXPLAIN EXTENDED SELECT (SELECT 1 FROM t2 WHERE d = c) FROM t1;
 DROP TABLE t1, t2;
 
+
+--echo #
+--echo # Bug #48419: another explain crash..
+--echo #
+
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (b BLOB, KEY b(b(100)));
+INSERT INTO t2 VALUES ('1'), ('2'), ('3');
+
+FLUSH TABLES;
+
+EXPLAIN SELECT 1 FROM t1 WHERE a = (SELECT 1 FROM t1 t JOIN t2 WHERE b <= 1 AND t.a);
+
+DROP TABLE t1, t2;
+
 --echo End of 5.1 tests.

sql/sql_select.cc

@@ -1107,8 +1107,7 @@ JOIN::optimize()
   }
 
   if (conds && const_table_map != found_const_table_map &&
-      (select_options & SELECT_DESCRIBE) &&
-      select_lex->master_unit() == &thd->lex->unit) // upper level SELECT
+      (select_options & SELECT_DESCRIBE))
   {
     conds=new Item_int((longlong) 0,1);	// Always false
   }