Why filtering, ordering and limiting does not use index?

[Marc Canaleta]

I have a table with more than one milion rows with an index not unique in a column. When I try to get first N rows less than a value (combination of WHERE column < constant ORDER BY column LIMIT n), it uses the index but rows_examined is very high. It should be N...:


mysql> explain SELECT score_alltimePoints FROM score WHERE score_alltimePoints < 4915 ORDER BY score_alltimePoints DESC LIMIT 30;
+----+-------------+-------+-------+---------------------+---------------------+---------+------+--------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------------+---------------------+---------+------+--------+--------------------------+
| 1 | SIMPLE | score | range | score_alltimePoints | score_alltimePoints | 4 | NULL | 768417 | Using where; Using index |
+----+-------------+-------+-------+---------------------+---------------------+---------+------+--------+--------------------------+
1 row in set (0.09 sec)



It seems obvious for me that it should use the index (btree) to get to the first row and browse descending through the index. It should only examine N rows.

Is this possible?

Thank you!