Descending Indexes降序索引
降序索引主要是用来减少排序，去除filesort的。
MySQL支持降序索引:索引定义中的DESC不再被忽略，而是按降序存储键值。以前，索引可以以相反的顺序扫描，但会影响性能。可以按前向顺序扫描降序索引，效率更高。当最有效的扫描顺序混合了某些列的升序和其他列的降序时，降序索引还可以使优化器使用多列索引。
mysql> CREATE TABLE t ( -> c1 INT, c2 INT, -> INDEX idx1 (c1 ASC, c2 ASC), -> INDEX idx2 (c1 ASC, c2 DESC), -> INDEX idx3 (c1 DESC, c2 ASC), -> INDEX idx4 (c1 DESC, c2 DESC) -> ); Query OK, 0 rows affected (0.40 sec) mysql> insert into t values(1,1),(1,2),(1,3),(1,4),(1,5), -> (2,1),(2,2),(2,3),(2,4),(2,5), -> (3,1),(3,2),(3,3),(3,4),(3,5), -> (4,1),(4,2),(4,3),(4,4),(4,5), -> (5,1),(5,2),(5,3),(5,4),(5,5); Query OK, 25 rows affected (0.12 sec) Records: 25 Duplicates: 0 Warnings: 0 mysql> desc select * from t ORDER BY c1 ASC, c2 ASC; +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | t | NULL | index | NULL | idx1 | 10 | NULL | 25 | 100.00 | Using index | +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.01 sec) mysql> desc select * from t ORDER BY c1 DESC, c2 DESC; +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | t | NULL | index | NULL | idx4 | 10 | NULL | 25 | 100.00 | Using index | +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> desc select * from t ORDER BY c1 ASC, c2 DESC; +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | t | NULL | index | NULL | idx2 | 10 | NULL | 25 | 100.00 | Using index | +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> desc select * from t ORDER BY c1 DESC, c2 ASC; +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | t | NULL | index | NULL | idx3 | 10 | NULL | 25 | 100.00 | Using index | +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)
参考链接
8.3.13 Descending Indexes