利用索引优化查询
? ? ? ? 聽說索引能優化查詢，今天我們嘗試一次，畢竟自己動手印象更深刻，還是先創建一個表。
mysql> create table test(-> id int not null,-> name varchar(20)-> ); Query OK, 0 rows affected (0.03 sec)mysql> show tables; +-----------------+ | Tables_in_mapan | +-----------------+ | student | | test | | user | +-----------------+ 3 rows in set (0.00 sec)mysql> insert into test value-> (12,"zhangsan"),(9,"lisi"),(25,"wangwu"),(15,"zhaoliu"); Query OK, 4 rows affected (0.00 sec) Records: 4 Duplicates: 0 Warnings: 0mysql> select * from test; +----+----------+ | id | name | +----+----------+ | 12 | zhangsan | | 9 | lisi | | 25 | wangwu | | 15 | zhaoliu | +----+----------+ 4 rows in set (0.00 sec)mysql> show index from test; Empty set (0.00 sec)show index from test是查看表是否含有索引。
接下來進行查詢：
mysql> explain select * from test where id=25; +----+-------------+-------+------+---------------+------+---------+------+----- -+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows| Extra | +----+-------------+-------+------+---------------+------+---------+------+----- -+-------------+ | 1 | SIMPLE | test | ALL | NULL | NULL | NULL | NULL | 4| Using where | +----+-------------+-------+------+---------------+------+---------+------+----- -+-------------+ 1 row in set (0.00 sec)type=ALL，全盤掃描，性能屬于最差的。接下來為表增加索引，在進行查詢。
mysql> alter table test add index(id); Query OK, 4 rows affected (0.05 sec) Records: 4 Duplicates: 0 Warnings: 0mysql> show index from test; +-------+------------+----------+--------------+-------------+-----------+------ -------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardi nality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +-------+------------+----------+--------------+-------------+-----------+------ -------+----------+--------+------+------------+---------+---------------+ | test | 1 | id | 1 | id | A |NULL | NULL | NULL | | BTREE | | | +-------+------------+----------+--------------+-------------+-----------+------ -------+----------+--------+------+------------+---------+---------------+ 1 row in set (0.01 sec) mysql> explain select * from test where id=25; +----+-------------+-------+------+---------------+------+---------+-------+---- --+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | row s | Extra | +----+-------------+-------+------+---------------+------+---------+-------+---- --+-------+ | 1 | SIMPLE | test | ref | id | id | 4 | const | 1 | | +----+-------------+-------+------+---------------+------+---------+-------+---- --+-------+ 1 row in set (0.00 sec)alter table test add index(id)使id成為表test的索引，可以看到type=ref，效率高于type=ALL。實際測試索引能優化查詢。總結
