Question

我有一个包含事件的数据库表。

mysql> describe events;
+-------------+------------------+------+-----+---------------------+----------------+
| Field       | Type             | Null | Key | Default             | Extra          |
+-------------+------------------+------+-----+---------------------+----------------+
| device      | varchar(32)      | YES  | MUL | NULL                |                |
| psu         | varchar(32)      | YES  | MUL | NULL                |                |
| event       | varchar(32)      | YES  | MUL | NULL                |                |
| down_time   | timestamp        | NO   | MUL | CURRENT_TIMESTAMP   |                |
| up_time     | timestamp        | NO   | MUL | 0000-00-00 00:00:00 |                |
| id          | int(10) unsigned | NO   | PRI | NULL                | auto_increment |
+-------------+------------------+------+-----+---------------------+----------------+
6 rows in set (0.01 sec)

我想查找时间重叠的事件并使用以下查询：

SELECT *

FROM link_events a 
JOIN link_events b 

ON  ( a.down_time <= b.up_time )
AND ( a.up_time >= b.down_time )

WHERE (a.device = 'd1' AND b.device = 'd2')
AND   (a.psu = 'p1' AND b.psu = 'p2')
AND   (a.event = 'e1' AND b.event = 'e2');

+-------------+-----------+------------+---------------------+---------------------+--------+-------------+-----------+------------+---------------------+---------------------+--------+
| device      | psu       | event      | down_time           | up_time             | id     | device      | psu       | event      | down_time           | up_time             | id     |
+-------------+-----------+------------+---------------------+---------------------+--------+-------------+-----------+------------+---------------------+---------------------+--------+
| d1          | p1        | e1         | 2013-01-14 16:42:10 | 2013-01-14 16:43:00 | 374529 | d2          | p2        | e2         | 2013-01-14 16:42:14 | 2013-01-14 16:42:18 | 211570 |
| d1          | p1        | e1         | 2013-05-29 18:49:26 | 2013-05-30 12:31:15 | 374569 | d2          | p2        | e2         | 2013-05-30 08:48:20 | 2013-05-30 08:48:27 | 211787 |
| d1          | p1        | e1         | 2013-05-29 18:49:26 | 2013-05-30 12:31:15 | 374569 | d2          | p2        | e2         | 2013-05-30 08:48:54 | 2013-05-30 08:48:58 | 211788 |
+-------------+-----------+------------+---------------------+---------------------+--------+-------------+-----------+------------+---------------------+---------------------+--------+
3 rows in set (35.88 sec)

events表包含以下行数：

mysql> select count(*) from events;
+----------+
| count(*) |
+----------+
|   977759 |
+----------+
1 row in set (0.01 sec)

mysql> select count(*) from events where device = 'd1' and psu = 'p1' and event = 'e1';
+----------+
| count(*) |
+----------+
|    11397 |
+----------+
1 row in set (0.12 sec)

mysql> select count(*) from events where device = 'd2' and psu = 'p2' and event = 'e2';
+----------+
| count(*) |
+----------+
|      243 |
+----------+
1 row in set (0.00 sec)

数据库安装在Windows 7笔记本电脑上并使用MyISAM引擎。有没有办法更好地组织数据库或更改索引改善第一次查询的查询时间为35秒。重复相同的查询给出了立即的结果，但是如果我冲洗表格＆＃39;和第三次重复查询所花费的时间再次为35秒。任何帮助表示赞赏！

以下是ADD KEY后的EXPLAIN输出：

mysql> EXPLAIN
    -> SELECT *
    ->
    -> FROM link_events a
    -> JOIN link_events b
    ->
    -> ON       ( a.down_time <= b.up_time )
    -> AND      ( a.up_time >= b.down_time )
    ->
    -> WHERE (a.device = 'd1' AND b.device = 'd2')
    -> AND (a.psu = 'l1' AND b.psu = 'l2')
    -> AND (a.event = 'e1' AND b.event = 'e2');
+----+-------------+-------+------+--------------------------------------------------------------------------------+---------------+---------+-------------------+------+-----------------------+
| id | select_type | table | type | possible_keys                                                                  | key           | key_len | ref               | rows | Extra                 |
+----+-------------+-------+------+--------------------------------------------------------------------------------+---------------+---------+-------------------+------+-----------------------+
|  1 | SIMPLE      | b     | ref  | device,psu,event,down_time,up_time,device_2,device_3                           | device_2      | 297     | const,const,const |  180 | Using index condition |
|  1 | SIMPLE      | a     | ref  | device,psu,event,down_time,up_time,device_2,device_3                           | device_2      | 297     | const,const,const | 7744 | Using index condition |
+----+-------------+-------+------+--------------------------------------------------------------------------------+---------------+---------+-------------------+------+-----------------------+
2 rows in set (0.07 sec)

新专栏：

mysql> describe link_events;
    +-------------+------------------+------+-----+---------------------+-----------------------------+
    | Field       | Type             | Null | Key | Default             | Extra                       |
    +-------------+------------------+------+-----+---------------------+-----------------------------+
    | device_name | varchar(32)      | YES  | MUL | NULL                |                             |
    | link_name   | varchar(32)      | YES  | MUL | NULL                |                             |
    | event_type  | varchar(32)      | YES  | MUL | NULL                |                             |
    | down_time   | timestamp        | NO   | MUL | CURRENT_TIMESTAMP   | on update CURRENT_TIMESTAMP |
    | up_time     | timestamp        | NO   | MUL | 0000-00-00 00:00:00 |                             |
    | span        | geometry         | NO   | MUL | NULL                |                             |
    | id          | int(10) unsigned | NO   | PRI | NULL                | auto_increment              |
    +-------------+------------------+------+-----+---------------------+-----------------------------+
    7 rows in set (0.03 sec)

说明：

mysql> EXPLAIN
    ->
    ->  SELECT
    ->
    ->  CONCAT('Link1','-', 'Link2') overlaps,
    ->  GREATEST(a.down_time,b.down_time) AS downtime,
    ->  LEAST(a.up_time,b.up_time) AS uptime,
    ->  TIME_TO_SEC(TIMEDIFF( LEAST(a.up_time,b.up_time),
    ->          GREATEST(a.down_time,b.down_time))) AS duration
    ->
    ->  FROM link_events a
    ->  JOIN link_events b
    ->
    ->  ON      Intersects (a.span, b.span)
    ->
    ->  WHERE (a.device_name = 'd1' AND b.device_name = 'd2')
    ->  AND (a.link_name = 'l1' AND b.link_name = 'l2')
    ->  AND (a.event_type = 'e1' AND b.event_type = 'e1');
    +----+-------------+-------+------+-------------------------------------------------------------------+---------------+---------+-------------------+-------+------------------------------------+
    | id | select_type | table | type | possible_keys                                                     | key           | key_len | ref               | rows  | Extra                              |
    +----+-------------+-------+------+-------------------------------------------------------------------+---------------+---------+-------------------+-------+------------------------------------+
    |  1 | SIMPLE      | a     | ref  | span,device_name,link_name,event_type,device_name_2,device_name_3 | device_name_2 | 297     | const,const,const |   383 | Using index condition              |
    |  1 | SIMPLE      | b     | ref  | span,device_name,link_name,event_type,device_name_2,device_name_3 | device_name_2 | 297     | const,const,const | 14580 | Using index condition; Using where |
    +----+-------------+-------+------+-------------------------------------------------------------------+---------------+---------+-------------------+-------+------------------------------------+
    2 rows in set (0.09 sec)

使用相交需要1分12秒？

Answer 1

对于此查询：

SELECT *
FROM link_events a JOIN
     link_events b 
     ON  (a.down_time <= b.up_time) AND (a.up_time >= b.down_time)
WHERE (a.device = 'd1' AND b.device = 'd2') AND
      (a.psu = 'p1' AND b.psu = 'p2') AND
      (a.event = 'e1' AND b.event = 'e2');

您需要link_events(device, psu, event, up_time, down_time)上的索引。为清楚起见，我会更像这样表达查询：

SELECT *
FROM link_events a JOIN
     link_events b 
     ON  (a.down_time <= b.up_time) AND (a.up_time >= b.down_time)
WHERE (a.device, a.psu, a.event) IN (('d1', 'p1', 'e1')) AND
      (b.device, a.psu, a.event) IN (('d2', 'p2', 'e2'));

Answer 2

尝试：

ALTER TABLE link_events ADD KEY(device,psu,event,up_time),
  ADD KEY(device,psu,event,down_time)

希望这足够有选择性。如果这没有帮助，请发布EXPLAIN的结果，这样我们就可以确保优化器尽力而为，如果需要，我们将从那里开始。

编辑：

重要的是要理解并非所有索引对于特定查询都具有相同的值。一个常见的错误是将索引视为一个魔术工作者，如果您只引用索引中的列，它将自动加速查询。情况并非如此。需要设计密钥，并且需要以允许记录的最佳访问路径的方式编写查询。更改可能显得无关紧要的内容（例如索引中列的顺序或编写SQRT(x) = 4.4而不是x = 4.4 * 4.4）可能会使索引无法使用，并使查询速度降低一千甚至一百万或更多。

我强烈建议您阅读：

http://dev.mysql.com/doc/refman/5.7/en/mysql-indexes.html

了解MySQL如何使用密钥可以在将来为您节省很多麻烦。

编辑2 - 另一个想法是添加一个包含span GEOMETRY NOT NULL, SPATIAL KEY (span)的列linestring(point(up_time,0),point(down_time,0)) - 时间需要是数字（例如，您可以使用UNIX_TIMESTAMP()转换） - 并使用{{1在查询中。通过一些微调，这有可能比改进的查询快得多，因为使用专门为此类事物设计的基于几何的算法来检测跨度交叉。

改进慢速Mysql查询

2 个答案: