Mysql + 大表 = 慢查询？

我在 Mysql 上的大表上遇到一些性能问题： 该表有 3800 万行，大小为 3GB。 我想通过测试 2 列来选择： 我尝试了很多索引（每列一个索引，两列一个索引），但查询仍然很慢：如下所示，超过 4 秒才能获取 1644 行：

SELECT `twstats_twwordstrend`.`id`, `twstats_twwordstrend`.`created`, `twstats_twwordstrend`.`freq`, `twstats_twwordstrend`.`word_id` FROM `twstats_twwordstrend` WHERE (`twstats_twwordstrend`.`word_id` = 1001 AND `twstats_twwordstrend`.`created` > '2011-11-07 14:01:34' );
...
...
...
1644 rows in set (4.66 sec)

EXPLAIN SELECT `twstats_twwordstrend`.`id`, `twstats_twwordstrend`.`created`, `twstats_twwordstrend`.`freq`, `twstats_twwordstrend`.`word_id` FROM `twstats_twwordstrend` WHERE (`twstats_twwordstrend`.`word_id` = 1001 AND `twstats_twwordstrend`.`created` > '2011-11-07 14:01:34' );
+----+-------------+----------------------+-------+-----------------------------------------------------+-----------------------+---------+------+------+-------------+
| id | select_type | table                | type  | possible_keys                                       | key                   | key_len | ref  | rows | Extra       |
+----+-------------+----------------------+-------+-----------------------------------------------------+-----------------------+---------+------+------+-------------+
|  1 | SIMPLE      | twstats_twwordstrend | range | twstats_twwordstrend_4b95d890,word_id_created_index | word_id_created_index | 12      | NULL | 1643 | Using where |
+----+-------------+----------------------+-------+-----------------------------------------------------+-----------------------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> describe twstats_twwordstrend;
+---------+----------+------+-----+---------+----------------+
| Field   | Type     | Null | Key | Default | Extra          |
+---------+----------+------+-----+---------+----------------+
| id      | int(11)  | NO   | PRI | NULL    | auto_increment |
| created | datetime | NO   |     | NULL    |                |
| freq    | double   | NO   |     | NULL    |                |
| word_id | int(11)  | NO   | MUL | NULL    |                |
+---------+----------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

mysql> show index from twstats_twwordstrend;
+----------------------+------------+-------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table                | Non_unique | Key_name                      | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------------------+------------+-------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| twstats_twwordstrend |          0 | PRIMARY                       |            1 | id          | A         |    38676897 |     NULL | NULL   |      | BTREE      |         |               |
| twstats_twwordstrend |          1 | twstats_twwordstrend_4b95d890 |            1 | word_id     | A         |      655540 |     NULL | NULL   |      | BTREE      |         |               |
| twstats_twwordstrend |          1 | word_id_created_index         |            1 | word_id     | A         |      257845 |     NULL | NULL   |      | BTREE      |         |               |
| twstats_twwordstrend |          1 | word_id_created_index         |            2 | created     | A         |    38676897 |     NULL | NULL   |      | BTREE      |         |               |
+----------------------+------------+-------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
4 rows in set (0.03 sec)

我还发现仅获取表中较远的一行非常慢：

mysql> SELECT `twstats_twwordstrend`.`id`, `twstats_twwordstrend`.`created`, `twstats_twwordstrend`.`freq`, `twstats_twwordstrend`.`word_id` FROM `twstats_twwordstrend` limit 10000000,1;
+----------+---------------------+--------------------+---------+
| id       | created             | freq               | word_id |
+----------+---------------------+--------------------+---------+
| 10000001 | 2011-09-09 15:59:18 | 0.0013398539559188 |   41295 |
+----------+---------------------+--------------------+---------+
1 row in set (1.73 sec)

...并且在表格开始时并不慢：

mysql> SELECT `twstats_twwordstrend`.`id`, `twstats_twwordstrend`.`created`, `twstats_twwordstrend`.`freq`, `twstats_twwordstrend`.`word_id` FROM `twstats_twwordstrend` limit 1,1;
+----+---------------------+---------------------+---------+
| id | created             | freq                | word_id |
+----+---------------------+---------------------+---------+
|  2 | 2011-06-16 10:59:06 | 0.00237777777777778 |       2 |
+----+---------------------+---------------------+---------+
1 row in set (0.00 sec)

该表使用Innodb引擎。如何加快大表的查询速度？

您可以做的主要事情是添加索引。

每当您在 where 子句中使用列时，请确保它有索引。您创建的栏目中没有该栏目。

包括创建的列的多重索引本质上不是创建的索引，因为创建的列不是多重索引中的第一个。

使用多索引时，您几乎应该始终将基数较高的列放在前面。因此，索引为：(created, word_id), (word_id)会给你带来显着的提升。