MDEV-6402: Optimizer doesn't choose best execution plan when composite...

Fix test_if_skip_sort_order() logic:
WHEN
we use index X, which doesn't produce needed ordering, but there is
an index Y which does and has the same prefix as used prefix of X
THEN
don't just switch to using ref access on Y. If range(Y) would use more
key parts, use range(Y).

mysql-test/r/order_by_optimizer_innodb.result

@@ -0,0 +1,48 @@
+drop table if exists t0,t1,t2,t3;
+#
+# MDEV-6402: Optimizer doesn't choose best execution plan when composite key is used
+#
+create table t0(a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t1(a int);
+insert into t1 select A.a + B.a* 10 + C.a * 100 from t0 A, t0 B, t0 C;
+CREATE TABLE t2 (
+pk1 int(11) NOT NULL,
+pk2 int(11) NOT NULL,
+fd5 bigint(20) DEFAULT NULL,
+filler1 char(200),
+filler2 char(200),
+PRIMARY KEY (pk1,pk2),
+UNIQUE KEY ux_pk1_fd5 (pk1,fd5)
+) ENGINE=InnoDB;
+insert into t2 
+select 
+round(log(2,t1.a+1)),
+t1.a,
+t1.a,
+REPEAT('filler-data-', 10),
+REPEAT('filler-data-', 10)
+from 
+t1;
+select pk1, count(*) from t2 group by pk1;
+pk1	count(*)
+0	1
+1	1
+2	3
+3	6
+4	11
+5	23
+6	45
+7	91
+8	181
+9	362
+10	276
+# The following should use range(ux_pk1_fd5), two key parts (key_len=5+8=13)
+EXPLAIN SELECT * FROM t2 USE INDEX(ux_pk1_fd5) WHERE pk1=9 AND fd5 < 500 ORDER BY fd5 DESC LIMIT 10;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	range	ux_pk1_fd5	ux_pk1_fd5	13	NULL	137	Using where
+# This also must use range, not ref. key_len must be 13
+EXPLAIN SELECT * FROM t2                       WHERE pk1=9 AND fd5 < 500 ORDER BY fd5 DESC LIMIT 10;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	range	PRIMARY,ux_pk1_fd5	ux_pk1_fd5	13	NULL	137	Using where
+drop table t0,t1, t2;