Added a test case for bug #18940:in 5.0 the optimizer chose

a worse execution plan than in 4.1 for some queries.
It happened due the fact that at some conditions the 
optimizer always preferred range or full index scan access
methods to lookup access methods even when the latter were much
cheaper. 
The problem was not observed in 4.1 for the reported query
because the WHERE condition was not of a form that could
cause the problem.
Equality propagation introduced on 5.0 added an extra 
predicate and changed the WHERE condition. The new condition
provoked the optimizer to make a bad choice.

The problem was fixed by the patch for bug 17379.

mysql-test/r/select.result

@@ -3427,3 +3427,22 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	A	range	PRIMARY	PRIMARY	12	NULL	3	Using where
 1	SIMPLE	B	ref	PRIMARY	PRIMARY	8	const,test.A.e	10	
 drop table t1, t2;
+CREATE TABLE t1 (a int PRIMARY KEY, b int, INDEX(b));
+INSERT INTO t1 VALUES (1, 3), (9,4), (7,5), (4,5), (6,2),
+(3,1), (5,1), (8,9), (2,2), (0,9);
+CREATE TABLE t2 (c int, d int, f int, INDEX(c,f));
+INSERT INTO t2 VALUES
+(1,0,0), (1,0,1), (2,0,0), (2,0,1), (3,0,0), (4,0,1),
+(5,0,0), (5,0,1), (6,0,0), (0,0,1), (7,0,0), (7,0,1),
+(0,0,0), (0,0,1), (8,0,0), (8,0,1), (9,0,0), (9,0,1);
+EXPLAIN
+SELECT a, c, d, f FROM t1,t2 WHERE a=c AND b BETWEEN 4 AND 6;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	PRIMARY,b	b	5	NULL	3	Using where
+1	SIMPLE	t2	ref	c	c	5	test.t1.a	2	Using where
+EXPLAIN
+SELECT a, c, d, f FROM t1,t2 WHERE a=c AND b BETWEEN 4 AND 6 AND a > 0;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	PRIMARY,b	b	5	NULL	3	Using where
+1	SIMPLE	t2	ref	c	c	5	test.t1.a	2	Using where
+DROP TABLE t1, t2;

mysql-test/t/select.test

@@ -2899,3 +2899,24 @@ select 'In next EXPLAIN, B.rows must be exactly 10:' Z;
 explain select * from t2 A, t2 B where A.a=5 and A.b=5 and A.C<5
           and B.a=5 and B.b=A.e and (B.b =1 or B.b = 3 or B.b=5);
 drop table t1, t2;
+
+#
+#Bug #18940: selection of optimal execution plan caused by equality
+#            propagation (the bug was fixed by the patch for bug #17379)
+
+CREATE TABLE t1 (a int PRIMARY KEY, b int, INDEX(b));
+INSERT INTO t1 VALUES (1, 3), (9,4), (7,5), (4,5), (6,2),
+                      (3,1), (5,1), (8,9), (2,2), (0,9);
+
+CREATE TABLE t2 (c int, d int, f int, INDEX(c,f));
+INSERT INTO t2 VALUES
+ (1,0,0), (1,0,1), (2,0,0), (2,0,1), (3,0,0), (4,0,1),
+ (5,0,0), (5,0,1), (6,0,0), (0,0,1), (7,0,0), (7,0,1),
+ (0,0,0), (0,0,1), (8,0,0), (8,0,1), (9,0,0), (9,0,1);
+
+EXPLAIN
+SELECT a, c, d, f FROM t1,t2 WHERE a=c AND b BETWEEN 4 AND 6;
+EXPLAIN
+SELECT a, c, d, f FROM t1,t2 WHERE a=c AND b BETWEEN 4 AND 6 AND a > 0;
+
+DROP TABLE t1, t2;