首先,提前为文本墙道歉。我确实阅读了我能找到的每个类似的问题/答案,但是答案似乎不适用于我的查询,或者我需要更清晰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。所有日期都是UNIX纪元时间整数,以秒为单位:
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更改,以检测文件在任何48小时内是否缩小> 20%。通常情况下,我会尝试使用MySQL Window函数执行此操作,但我的服务器上的MySQL版本不支持它们(5.6.37 - 我无法控制,因为服务器不是由我的团队管理)。目前,我通过查找当前行中文件大小的外部查询以及在前48小时内找到最大文件大小的内部子查询(172,800秒)获取当前大小和最大大小(在过去48小时内) )值行:
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)
只检索那10行需要16秒,而在生产中,此查询将需要检索超过150行。内部查询正在对所有300万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)
有问题的部分似乎是这样的:
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_servers_outside.time”列引用来运行类似的查询,则会按预期使用索引并且查询速度很快:
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)
谢谢你和我一起读书。我再次为巨大的文本墙道歉,但我想确保我提供了足够的解释性细节来明确定义问题。
现在,我试图解决的问题是我需要在每行前48小时内检索file_size的最大值。因此,每行具有其自己的“max(file_size)”计算的唯一时间范围。然后,这将用于计算文件大小更改百分比。如上所述,我通常想要使用窗口函数,但我的MySQL版本(5.6.37)不支持它们,并且我不能更新到8.0,因为我没有这个服务器。
一如既往,任何建议都表示赞赏。谢谢你的阅读!
我首先尝试将file_size添加到index_time_servername索引,但我怀疑真正的问题是你必须在你的子查询中使用name_servers_outside.time,这来自不同的别名可能会使查询规划者感到困惑。
那么,在48小时之前的时间和时间之间,丢失子查询并将表连接到自身怎么样?
就像是...
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;该解决方案最终涉及多个组件,并且需要花费时间来完成并测试它们。
我试图解决的主要问题是查询性能。严格地说,我的原始查询返回了预期的数据,但是花了这么长时间才完成,这是不切实际的。所以解决方案就是尽可能多地寻找减少执行时间的方法。
以下是最终解决方案: