MDEV-27419 [demo/check-ci] choose desc key for desc ordering

A key may be chosen in test_if_skip_sort_order(), after calling
test_if_cheaper_ordering(). The latter iterates over the keys, and
call test_if_order_by_key() to determine whether the key can be used
in ORDER BY.

We make it so that if the result of test_if_order_by_key() indicates
the current key under consideration would result in a better matching
ordering (e.g. ORDER BY DESC matches a desc key better than an asc
key), then we choose that key, given that the range_cost is the same.

TODO: consider whether it is possible to quantify the order matching
by passing direction to get_range_limit_read_cost.

mysql-test/main/mdev_27419.result

+create table t (a int, key aa(a), key ad(a desc)) engine=InnoDB;
+insert into t values (1),(5),(2),(8),(4),(6),(7),(9),(3);
+explain select * from t order by a desc;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t	index	NULL	ad	5	NULL	9	Using index
+flush status;
+select * from t order by a desc;
+a
+9
+8
+7
+6
+5
+4
+3
+2
+1
+show status like 'Handler_read%';
+Variable_name	Value
+Handler_read_first	1
+Handler_read_key	0
+Handler_read_last	0
+Handler_read_next	9
+Handler_read_prev	0
+Handler_read_retry	0
+Handler_read_rnd	0
+Handler_read_rnd_deleted	0
+Handler_read_rnd_next	0
+drop table t;

mysql-test/main/mdev_27419.test

+# Remove the include if you run the test on MySQL 8.0
+--source include/have_innodb.inc
+
+create table t (a int, key aa(a), key ad(a desc)) engine=InnoDB;
+insert into t values (1),(5),(2),(8),(4),(6),(7),(9),(3);
+
+explain select * from t order by a desc;
+flush status;
+select * from t order by a desc;
+show status like 'Handler_read%';
+
+# Cleanup
+drop table t;

sql/sql_select.cc

@@ -32369,11 +32369,10 @@ test_if_cheaper_ordering(const JOIN_TAB *tab, ORDER *order, TABLE *table,
 
         /*
           We will try use the key if:
-          - If there is no ref key and no usable keys has yet been found and
-            there is either a group by or a FORCE_INDEX
           - If the new cost is better than read_time
         */
-        if (range_cost < read_time)
+        if (range_cost < read_time ||
+            (range_cost == read_time && direction > best_key_direction))
         {
           read_time= range_cost;
           possible_key.add("chosen", true);