Mysql 此子查询是否可以使用索引?
首先,提前为文字墙道歉。我确实通读了我能找到的每一个类似的问题/答案,但要么答案似乎不适用于我的问题,要么我需要更清楚地理解潜在的问题和解决方案 我有一个文件大小表以及相关的文件日期和观察时间戳。所有日期均为UNIX历元时间整数(以秒为单位):Mysql 此子查询是否可以使用索引?,mysql,mysql-5.6,Mysql,Mysql 5.6,首先,提前为文字墙道歉。我确实通读了我能找到的每一个类似的问题/答案,但要么答案似乎不适用于我的问题,要么我需要更清楚地理解潜在的问题和解决方案 我有一个文件大小表以及相关的文件日期和观察时间戳。所有日期均为UNIX历元时间整数(以秒为单位): mysql> describe name_servers; +-----------------------+------------------+------+-----+---------+----------------+ | Field
mysql> describe name_servers;
+-----------------------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------------------+------------------+------+-----+---------+----------------+
| server_name | varchar(255) | YES | | NULL | |
| file_date | int(10) unsigned | YES | | NULL | |
| file_size | int(10) unsigned | YES | | NULL | |
| time | int(10) unsigned | YES | MUL | NULL | |
| poll_id | int(11) | NO | PRI | NULL | auto_increment |
+-----------------------+------------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
mysql> show index from name_servers;
+--------------+------------+--------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+--------------+------------+--------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| name_servers | 0 | PRIMARY | 1 | poll_id | A | 3523218 | NULL | NULL | | BTREE | | |
| name_servers | 0 | index_time_servername | 1 | time | A | 503316 | NULL | NULL | YES | BTREE | | |
| name_servers | 0 | index_time_servername | 2 | server_name | A | 3523218 | NULL | NULL | YES | BTREE | | |
+--------------+------------+--------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)
我必须跟踪文件大小的变化,以检测文件是否在任何48小时内收缩>20%。通常我会尝试使用MySQL窗口函数来实现这一点,但我的服务器上的MySQL版本不支持这些功能(5.6.37——我无法控制,因为服务器不是由我的团队管理的)。目前,我通过查找当前行中文件大小的外部查询和查找前48小时(172800秒)中最大文件大小的内部子查询来获取当前大小和最大大小(在过去48小时内):
mysql> select name_servers_outside.server_name,
-> name_servers_outside.file_size,
-> name_servers_outside.file_date,
-> name_servers_outside.time,
-> (select max(file_size) from name_servers where time > (name_servers_outside.time - 172800) and server_name = 'example_server') as max_file_size
-> from name_servers as name_servers_outside
-> where name_servers_outside.server_name = 'example_server'
-> and name_servers_outside.time > (UNIX_TIMESTAMP() - 172800)
-> limit 10;
+-------------------+-------------------+-------------------+------------+-----------------------+
| server_name | file_size | file_date | time | max_file_size |
+-------------------+-------------------+-------------------+------------+-----------------------+
| example_server | 1159544 | 1550382945 | 1550382985 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383195 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383255 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383316 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383376 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383435 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383496 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383555 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383616 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383676 | 1159580 |
+-------------------+-------------------+-------------------+------------+-----------------------+
10 rows in set (16.11 sec)
仅检索这10行需要16秒,在生产环境中,此查询必须检索150多行。内部查询正在对所有300多万个表行进行完整扫描,并显示消息“为每个记录检查范围(索引映射:0x2)”:
mysql> explain
-> select name_servers_outside.server_name,
-> name_servers_outside.file_size,
-> name_servers_outside.file_date,
-> name_servers_outside.time,
-> (select max(file_size) from name_servers where time > (name_servers_outside.time - 172800) and server_name = 'example_server') as max_file_size
-> from name_servers as name_servers_outside
-> where name_servers_outside.server_name = 'example_server'
-> and name_servers_outside.time > (UNIX_TIMESTAMP() - 172800);
+----+--------------------+----------------------+-------+--------------------------+--------------------------+---------+------+---------+------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+----------------------+-------+--------------------------+--------------------------+---------+------+---------+------------------------------------------------+
| 1 | PRIMARY | name_servers_outside | range | index_time_servername | index_time_servername | 5 | NULL | 47302 | Using index condition; Using MRR |
| 2 | DEPENDENT SUBQUERY | name_servers | ALL | index_time_servername | NULL | NULL | NULL | 3533883 | Range checked for each record (index map: 0x2) |
+----+--------------------+----------------------+-------+--------------------------+--------------------------+---------+------+---------+------------------------------------------------+
2 rows in set (0.01 sec)
问题似乎在于:
time > (name_servers_outside.time - 172800)
如果我使用静态整数值而不是子查询中的“name\u servers\u outside.time”列引用运行类似查询,则索引将按预期使用,并且查询速度很快:
time > (UNIX_TIMESTAMP() - 172800)
修改后的查询:
mysql> select name_servers_outside.server_name,
-> name_servers_outside.file_size,
-> name_servers_outside.file_date,
-> name_servers_outside.time,
-> (select max(file_size) from name_servers where time > (UNIX_TIMESTAMP() - 172800) and server_name = 'example_server') as max_file_size
-> from name_servers as name_servers_outside
-> where name_servers_outside.server_name = 'example_server'
-> and name_servers_outside.time > (UNIX_TIMESTAMP() - 172800)
-> limit 10;
+--------------------+-------------------+-------------------+------------+-----------------------+
| server_name | file_size | file_date | time | max_file_size |
+--------------------+-------------------+-------------------+------------+-----------------------+
| example_server | 1159544 | 1550382945 | 1550382985 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383195 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383255 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383316 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383376 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383435 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383496 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383555 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383616 | 1159580 |
| example_server | 1159544 | 1550382945 | 1550383676 | 1159580 |
+--------------------+-------------------+-------------------+------------+-----------------------+
10 rows in set (0.01 sec)
mysql> explain
-> select name_servers_outside.server_name,
-> name_servers_outside.file_size,
-> name_servers_outside.file_date,
-> name_servers_outside.time,
-> (select max(file_size) from name_servers where time > (UNIX_TIMESTAMP() - 172800) and server_name = 'example_server') as max_file_size
-> from name_servers as name_servers_outside
-> where name_servers_outside.server_name = 'example_server'
-> and name_servers_outside.time > (UNIX_TIMESTAMP() - 172800)
-> limit 10;
+----+-------------+----------------------+-------+--------------------------+--------------------------+---------+------+-------+----------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------------------+-------+--------------------------+--------------------------+---------+------+-------+----------------------------------+
| 1 | PRIMARY | name_servers_outside | range | index_time_servername | index_time_servername | 5 | NULL | 49042 | Using index condition; Using MRR |
| 2 | SUBQUERY | name_servers | range | index_time_servername | index_time_servername | 5 | NULL | 49042 | Using index condition; Using MRR |
+----+-------------+----------------------+-------+--------------------------+--------------------------+---------+------+-------+----------------------------------+
2 rows in set (0.00 sec)
谢谢你和我一起读到现在。我再次为巨大的文本墙道歉,但我想确保我包含了足够的解释细节,以清楚地定义问题
现在,我试图解决的问题是,我需要检索每行之前48小时内文件大小的最大值。因此,每一行都有自己独特的“最大(文件大小)”计算时间范围。然后,这将用于计算文件大小更改的百分比。如上所述,我通常希望为此使用窗口函数,但我的MySQL版本(5.6.37)不支持这些函数,而且我无法更新到8.0,因为我没有这个服务器
如往常一样,任何建议都将不胜感激。谢谢你的阅读 我会首先尝试将文件大小添加到索引\u time\u servername索引中,但我怀疑真正的问题是,您必须在子查询中使用name\u servers\u outside.time,因为它来自不同的别名,这可能会让查询计划器感到困惑 那么,如果时间介于时间和时间(48小时前)之间,如何丢失子查询并将表连接到自身 类似于
SELECT
name_servers_outside.server_name,
name_servers_outside.file_size,
name_servers_outside.file_date,
name_servers_outside.time,
MAX(previous.file_size) AS max_file_size
FROM
name_servers AS ns
INNER JOIN name_servers AS previous
ON previous.time BETWEEN (ns.time - 172800) AND (ns.time - 1)
WHERE
ns.server_name = 'example_server'
AND ns.time > (UNIX_TIMESTAMP() - 172800)
GROUP BY
ns.server_name,
ns.file_size,
ns.file_date,
ns.time
LIMIT 10;
我为延迟回复道歉;解决方案最终涉及多个组件,并且需要花时间来完成和测试它们 我试图解决的主要问题是查询性能。严格地说,我最初的查询返回了预期的数据,但它花费了很长时间才完成,因此不实用。因此,解决方案就是找到尽可能多的方法来减少执行时间 以下是解决方案的最终结果:
欢迎来到SO,不要为文字墙道歉。这通常是一个很好的格式化问题,包含解决该问题所需的所有细节。证明:你得到了投票。当你尝试添加额外的单列索引时,比如在
time
或者甚至在file\u size
列上,你会得到不同的性能吗?或者在列序列上有一些奇怪的索引(服务器名称、文件大小)
?你能为它创建一个游乐场吗?在或中选择。使用CREATETABLE语句,插入足够的行以进行适当的测试。然后,我们可以轻松地测试我们想要的任何东西。