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) |