MDEV-435: Expensive subqueries may be evaluated during optimization in merge_key_fields

Fix by Sergey Petrunia.

This patch only prevents the evaluation of expensive subqueries during optimization.
The crash reported in this bug has been fixed by some other patch.
The fix is to call value->is_null() only when  !value->is_expensive(), because is_null()
may trigger evaluation of the Item, which in turn triggers subquery evaluation if the
Item is a subquery.

1 files changed, 21 insertions, 0 deletions

diff --git a/mysql-test/r/subselect_no_scache.result b/mysql-test/r/subselect_no_scache.result
index e68f5990c08..f648385f67a 100644
--- a/mysql-test/r/subselect_no_scache.result
+++ b/mysql-test/r/subselect_no_scache.result
@@ -6853,6 +6853,27 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 2	SUBQUERY	t2	ref	b	b	5	test.t1.a	2	Using index
 DROP TABLE t1,t2;
 #
+# MDEV-435: Expensive subqueries may be evaluated during optimization in merge_key_fields
+#
+CREATE TABLE t1 (a INT, KEY(a)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (8),(0);
+CREATE TABLE t2 (b INT, c VARCHAR(1)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (4,'j'),(6,'v');
+CREATE TABLE t3 (d VARCHAR(1)) ENGINE=MyISAM;
+INSERT INTO t3 VALUES ('b'),('c');
+EXPLAIN
+SELECT * FROM t1
+WHERE a = (SELECT MAX(b) FROM t2 WHERE c IN (SELECT MAX(d) FROM t3)) OR a = 10;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	index	a	a	5	NULL	2	Using where; Using index
+2	SUBQUERY	<subquery3>	ALL	distinct_key	NULL	NULL	NULL	1	
+2	SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where; Using join buffer (flat, BNL join)
+3	MATERIALIZED	t3	ALL	NULL	NULL	NULL	NULL	2	
+SELECT * FROM t1
+WHERE a = (SELECT MAX(b) FROM t2 WHERE c IN (SELECT MAX(d) FROM t3)) OR a = 10;
+a
+drop table t1, t2, t3;
+#
 # MDEV-405: Server crashes in test_if_skip_sort_order on EXPLAIN with GROUP BY and HAVING in EXISTS subquery
 #
 CREATE TABLE t1 (a INT, KEY(a));