MySQL 8.0新特性--Descending Indexes(六)

Descending Indexes降序索引

降序索引主要是用來減小排序，去除filesort的。

MySQL支持降序索引:索引定義中的DESC再也不被忽略，而是按降序存儲鍵值。之前，索引能夠以相反的順序掃描，但會影響性能。能夠按前向順序掃描降序索引，效率更高。當最有效的掃描順序混合了某些列的升序和其餘列的降序時，降序索引還能夠使優化器使用多列索引。

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)

參考連接

8.3.13 Descending Indexes