优化 MySQL 个索引以在一秒内查询

Question

一个非常简单的 min-max no-join no-nesting SQL 的查询时间超过 2 秒。

TABLE 结构:::

> DESCRIBE tbl;
+-------------+--------------+------+-----+---------+----------------+
| Field       | Type         | Null | Key | Default | Extra          |
+-------------+--------------+------+-----+---------+----------------+
| id          | int(11)      | NO   | PRI | NULL    | auto_increment |
| ...         | ...          | ...  | ... | ...     | ...            |
| created_at  | datetime     | YES  | MUL | NULL    |                |
+-------------+--------------+------+-----+---------+----------------+
7 rows in set (0.00 sec)

table 包含 10,000,000 多行

TABLE:::

中的索引

> SHOW INDEX IN tbl;
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name   | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| tbl   |          0 | PRIMARY    |            1 | id          | A         |    10000545 |     NULL | NULL   |      | BTREE      |         |               |
| tbl   |          1 | created_at |            1 | created_at  | A         |          18 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)

SQL 受关注::: 查找最后 10k 个条目的最小-最大日期时间

SELECT 
    min(created_at), 
    max(created_at) 
FROM tbl 
ORDER BY id DESC 
LIMIT 10000

问题::: 第一个查询总是需要 2 秒以上才能完成。在第一个 select 之后，所有后续查询调用只需不到 0.001 秒即可完成，除非将新行插入 table。

2.06 秒后第一次调用：

> SELECT min(created_at), max(created_at) FROM tbl USE INDEX (created_at) ORDER BY id DESC LIMIT 10000;
+---------------------+---------------------+
| min(created_at)     | max(created_at)     |
+---------------------+---------------------+
| 2010-01-01 00:00:00 | 2015-12-28 00:00:00 |
+---------------------+---------------------+
1 row in set (2.06 sec)

0.00 秒后的后续调用：

> SELECT min(created_at), max(created_at) FROM tbl USE INDEX (created_at) ORDER BY id DESC LIMIT 10000;
+---------------------+---------------------+
| min(created_at)     | max(created_at)     |
+---------------------+---------------------+
| 2010-01-01 00:00:00 | 2015-12-28 00:00:00 |
+---------------------+---------------------+
1 row in set (0.00 sec)

将新行添加到 table 后，它再次需要 2 秒以上才能完成，然后所有后续查询调用只需不到 0.001 秒即可完成。

我知道每次插入新行时索引都会重新排列。所以没关系。但是，我的目标是将第一次查询时间缩短到几毫秒以下，因为在频繁更新的系统中每次查询花费 2 秒以上会降低性能性能太重了。

查询计划的解释::: 解释语句显示查询几乎遍历了 table 的所有行。所以我猜我有通过索引改进的空间。但是我应该索引什么？

> EXPLAIN SELECT min(created_at), max(created_at) FROM tbl ORDER BY id DESC LIMIT 10000;
+----+-------------+-------+-------+---------------+------------+---------+------+----------+-------------+
| id | select_type | table | type  | possible_keys | key        | key_len | ref  | rows     | Extra       |
+----+-------------+-------+-------+---------------+------------+---------+------+----------+-------------+
|  1 | SIMPLE      | tbl   | index | NULL          | created_at | 9       | NULL | 10000545 | Using index |
+----+-------------+-------+-------+---------------+------------+---------+------+----------+-------------+
1 row in set (0.00 sec)

Answer 1

您的原始查询没有 return "the minimum/maximum date from the last 10000 entries"； LIMIT 在聚合函数处理后应用，所以你问的是 "Give me the max/min date, then limit that to the first 10k"... 而且只有一行。

您必须为此使用子查询：

SELECT min(created_at), max(created_at)
FROM (SELECT created_at
      FROM my_table
      ORDER BY id
      LIMIT 10000) subtable;

你最好的选择是 (id, created_at) 上的索引，因为子查询只需要遍历索引，然后 min/max 查询只需要对超过 10k 个元素进行排序。

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