使用group by优化MySQL范围查询

2019年8月6日 139次阅读

我有一张桌子,上面有每天的温度(巨大的桌子)和一张有周期开始和结束日期的桌子(小桌子).现在我想知道每个时期的平均温度,但查询需要很长时间.可以改进吗？

注意：升级到版本5.6.19-1~exp1ubuntu2后,长响应时间消失,并且可能是由5.6.8之前的MySQL版本中的错误引起的(请参阅Quassnoi的评论)

要使用随机数据重建日期和期间表：

create table days (
  day int not null auto_increment primary key,
  temperature float not null);

insert into days values(null,rand()),(null,rand()),
  (null,rand()),(null,rand()),(null,rand()),(null,rand()),
  (null,rand()),(null,rand()); # 8 rows

insert into days select null, d1.temperature
  from days d1, days d2, days d3, days d4,
  days d5, days d6, days d7; # 2M rows

create table periods(id int not null auto_increment primary key,
  first int not null,
  last int not null,
  index(first) using btree,
  index(last) using btree,
  index(first,last) using btree);

# add 10 periods of 1-11 days each
insert into periods(first,last)
  select floor(rand(day)*2000000), floor(rand(day)*2000000 + rand()*10)
  from days limit 10;

列出每个时期的全天温度都没有问题(以1ms为单位返回)：

select id, temperature
  from periods join days on day >= first and day <= last;

现在,使用GROUP BY,它实际上非常慢(~1750ms)

# ALT1
select id, avg(temperature)
  from periods join days on day >= first and day <= last group by id;

用BETWEEN替换< =和> =将其加速(~1600ms)：

# ALT2
select id, avg(temperature)
  from periods join days on day between first and last group by id;

事实证明,单个句点的结果会立即返回(1ms)：

select id, (select avg(temperature)
  from days where day >= first and day <= last) from periods
  where id=1;

但是,如果没有WHERE,则需要4200 ms,平均每个周期为420 ms！

# ALT3
select id,
  (select avg(temperature) from days where day >= first and day <= last)
  from periods;

是什么让查询如此缓慢 – 甚至(很多)比单个句点的结果慢10倍以上,尽管句号表只有10行？有没有办法优化这个查询？

编辑：更多信息：

mysql> select @@version;
+-------------------------+
| @@version               |
+-------------------------+
| 5.5.41-0ubuntu0.14.04.1 |
+-------------------------+

# ALT1
mysql> explain select id, avg(temperature) from periods join days on day >= first and day <= last group by id;
+----+-------------+---------+-------+--------------------+---------+---------+------+---------+----------------------------------------------+
| id | select_type | table   | type  | possible_keys      | key     | key_len | ref  | rows    | Extra                                        |
+----+-------------+---------+-------+--------------------+---------+---------+------+---------+----------------------------------------------+
|  1 | SIMPLE      | periods | index | first,last,first_2 | first_2 | 8       | NULL |      10 | Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | days    | ALL   | PRIMARY            | NULL    | NULL    | NULL | 2097596 | Using where; Using join buffer               |
+----+-------------+---------+-------+--------------------+---------+---------+------+---------+----------------------------------------------+

# ALT1 without GROUP BY
mysql> explain select id, temperature from periods join days on day >= first and day <= last;
+----+-------------+---------+-------+--------------------+---------+---------+------+---------+------------------------------------------------+
| id | select_type | table   | type  | possible_keys      | key     | key_len | ref  | rows    | Extra                                          |
+----+-------------+---------+-------+--------------------+---------+---------+------+---------+------------------------------------------------+
|  1 | SIMPLE      | periods | index | first,last,first_2 | first_2 | 8       | NULL |      10 | Using index                                    |
|  1 | SIMPLE      | days    | ALL   | PRIMARY            | NULL    | NULL    | NULL | 2097596 | Range checked for each record (index map: 0x1) |
+----+-------------+---------+-------+--------------------+---------+---------+------+---------+------------------------------------------------+

# ALT2
mysql> explain select id, avg(temperature) from periods join days on day between first and last group by id;
+----+-------------+---------+-------+--------------------+---------+---------+------+---------+----------------------------------------------+
| id | select_type | table   | type  | possible_keys      | key     | key_len | ref  | rows    | Extra                                        |
+----+-------------+---------+-------+--------------------+---------+---------+------+---------+----------------------------------------------+
|  1 | SIMPLE      | periods | index | first,last,first_2 | first_2 | 8       | NULL |      10 | Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | days    | ALL   | PRIMARY            | NULL    | NULL    | NULL | 2097596 | Using where; Using join buffer               |
+----+-------------+---------+-------+--------------------+---------+---------+------+---------+----------------------------------------------+

# ALT3
mysql> explain select id, (select avg(temperature) from days where day >= first and day <= last) from periods;
+----+--------------------+---------+-------+---------------+---------+---------+------+---------+-------------+
| id | select_type        | table   | type  | possible_keys | key     | key_len | ref  | rows    | Extra       |
+----+--------------------+---------+-------+---------------+---------+---------+------+---------+-------------+
|  1 | PRIMARY            | periods | index | NULL          | first_2 | 8       | NULL |      10 | Using index |
|  2 | DEPENDENT SUBQUERY | days    | ALL   | PRIMARY       | NULL    | NULL    | NULL | 2097596 | Using where |
+----+--------------------+---------+-------+---------------+---------+---------+------+---------+-------------+

# ALT3 with where
mysql> explain select id, (select avg(temperature) from days where day >= first and day <= last) from periods where id = 1;
+----+--------------------+---------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type        | table   | type  | possible_keys | key     | key_len | ref   | rows | Extra       |
+----+--------------------+---------+-------+---------------+---------+---------+-------+------+-------------+
|  1 | PRIMARY            | periods | const | PRIMARY       | PRIMARY | 4       | const |    1 |             |
|  2 | DEPENDENT SUBQUERY | days    | range | PRIMARY       | PRIMARY | 4       | NULL  |   10 | Using where |
+----+--------------------+---------+-------+---------------+---------+---------+-------+------+-------------+

EDIT2：FROM中嵌套查询的执行计划,由Lennart建议(查询执行时间3ms)

mysql> explain select id,avg(temperature) from (select id,temperature from periods join days on day between first and last) as t group by id;
+----+-------------+------------+-------+--------------------+---------+---------+------+----------+------------------------------------------------+
| id | select_type | table      | type  | possible_keys      | key     | key_len | ref  | rows     | Extra                                          |
+----+-------------+------------+-------+--------------------+---------+---------+------+----------+------------------------------------------------+
|  1 | PRIMARY     | <derived2> | ALL   | NULL               | NULL    | NULL    | NULL |       50 | Using temporary; Using filesort                |
|  2 | DERIVED     | periods    | index | first,last,first_2 | first_2 | 8       | NULL |       10 | Using index                                    |
|  2 | DERIVED     | days       | range | PRIMARY,day        | PRIMARY | 4       | NULL |        5 | Range checked for each record (index map: 0x3) |
+----+-------------+------------+-------+--------------------+---------+---------+------+----------+------------------------------------------------+

最佳答案这是一个丑陋的伎俩,因为：

select id, temperature 
from periods join days 
    on day between first and last;

很快,我们可以尝试激发优化器来首先评估它.仅使用子查询是不够的：

select id, avg(temperature) 
from (
    select id, temperature 
    from periods 
    join days 
        on day between first and last
) as t 
group by id;
[...]
10 rows in set (1.67 sec)

但是,在子查询中调用非确定性函数似乎可以解决这个问题：

select id, avg(temperature) 
from (
    select id, temperature, rand() 
    from periods 
    join days 
        on day between first and last
) as t 
group by id;
[...]
10 rows in set (0.00 sec)

除非是关键和必要的,否则我会远离这些伎俩.随着优化器变得更好(可能是下一个修复),它可能会跳过对rand()的调用,突然之间您的旧计划和性能又重新开始.

如果您使用此类技巧,请务必在代码中仔细记录它们,以便在不再需要时对其进行清理.

MariaDB [test]> select @@version;
+-----------------+
| @@version       |
+-----------------+
| 10.0.20-MariaDB |
+-----------------+
1 row in set (0.00 sec)

explain select id, avg(temperature) from periods join days on day between first and last group by id;
| id   | select_type | table   | type  | possible_keys      | key     | key_len | ref  | rows    | Extra                                           |
|    1 | SIMPLE      | periods | index | first,last,first_2 | first_2 | 8       | NULL |      10 | Using index; Using temporary; Using filesort    |
|    1 | SIMPLE      | days    | ALL   | PRIMARY            | NULL    | NULL    | NULL | 2094315 | Using where; Using join buffer (flat, BNL join) |

explain select id, avg(temperature) from (select id, temperature from periods join days on day between first and last) as t group by id;
| id   | select_type | table   | type  | possible_keys      | key     | key_len | ref  | rows    | extra                                           |
|    1 | SIMPLE      | periods | index | first,last,first_2 | first_2 | 8       | NULL |      10 | Using index; Using temporary; Using filesort    |
|    1 | SIMPLE      | days    | ALL   | PRIMARY            | NULL    | NULL    | NULL | 2094315 | Using where; Using join buffer (flat, BNL join) |


explain select id, avg(temperature) from (select id, temperature, rand() from periods join days on day between first and last) as t group by id;
| id   | select_type | table      | type  | possible_keys      | key     | key_len | ref  | rows    | Extra                                          |
|    1 | PRIMARY     | <derived2> | ALL   | NULL               | NULL    | NULL    | NULL |       2 | Using temporary; Using filesort                |
|    2 | DERIVED     | periods    | index | first,last,first_2 | first_2 | 8       | NULL |      10 | Using index                                    |
|    2 | DERIVED     | days       | ALL   | PRIMARY            | NULL    | NULL    | NULL | 2094315 | Range checked for each record (index map: 0x1) |