MDEV-27426 Wrong result upon query using index_merge with DESC key

Make QUICK_RANGE_SELECT::cmp_next() aware of reverse-ordered key parts.

(QUICK_RANGE_SELECT::cmp_prev() uses key_cmp() and so it already works
correctly)

mysql-test/main/desc_index_range.result

@@ -179,3 +179,13 @@ json_detailed(json_extract(trace, '$**.potential_group_range_indexes'))
 ]
 drop table t1;
 set optimizer_trace=default;
+#
+# MDEV-27426: Wrong result upon query using index_merge with DESC key
+#
+CREATE OR REPLACE TABLE t1 (pk INT, a INT, b int, KEY(a), PRIMARY KEY(pk DESC)) ENGINE=InnoDB;
+INSERT INTO t1 VALUES (1,4,5),(2,9,6),(3,NULL,7),(4,NULL,8);
+SELECT * FROM t1 WHERE pk > 10 OR a > 0;
+pk	a	b
+1	4	5
+2	9	6
+DROP TABLE t1;

mysql-test/main/desc_index_range.test

@@ -86,3 +86,13 @@ from information_schema.optimizer_trace;
 drop table t1;
 
 set optimizer_trace=default;
+
+--echo #
+--echo # MDEV-27426: Wrong result upon query using index_merge with DESC key
+--echo #
+
+CREATE OR REPLACE TABLE t1 (pk INT, a INT, b int, KEY(a), PRIMARY KEY(pk DESC)) ENGINE=InnoDB;
+INSERT INTO t1 VALUES (1,4,5),(2,9,6),(3,NULL,7),(4,NULL,8);
+
+SELECT * FROM t1 WHERE pk > 10 OR a > 0;
+DROP TABLE t1;

sql/opt_range.cc

@@ -13079,24 +13079,25 @@ int QUICK_RANGE_SELECT::cmp_next(QUICK_RANGE *range_arg)
        key+= store_length, key_part++)
   {
     int cmp;
+    bool reverse= MY_TEST(key_part->flag & HA_REVERSE_SORT);
     store_length= key_part->store_length;
     if (key_part->null_bit)
     {
       if (*key)
       {
         if (!key_part->field->is_null())
-          return 1;
+          return reverse ? 0 : 1;
         continue;
       }
       else if (key_part->field->is_null())
-        return 0;
+        return reverse ? 1 : 0;
       key++;					// Skip null byte
       store_length--;
     }
     if ((cmp=key_part->field->key_cmp(key, key_part->length)) < 0)
-      return 0;
+      return reverse ? 1 : 0;
     if (cmp > 0)
-      return 1;
+      return reverse ? 0 : 1;
   }
   return (range_arg->flag & NEAR_MAX) ? 1 : 0;          // Exact match
 }