因为mysql没有内置的关系除法运算符,所以程序员必须实现自己的关系除法运算符。有两个主要的实现示例,可以在this answer here.
为了子孙后代,我将把它们列在下面:
分组使用
SELECT t.documentid
FROM TABLE t
WHERE t.termid IN (1,2,3)
GROUP BY t.documentid
HAVING COUNT(DISINCT t.termid) = 3
需要注意的是,您必须使用having count(不同于
对于同一个documentid,termid的副本为2将是错误的
肯定的。计数必须等于
in子句。
使用联接
SELECT t.documentid
FROM TABLE t
JOIN TABLE x ON x.termid = t.termid
AND x.termid = 1
JOIN TABLE y ON y.termid = t.termid
AND y.termid = 2
JOIN TABLE z ON z.termid = t.termid
AND z.termid = 3
但这一次对于处理变化很大的标准来说可能是一件痛苦的事。
在这两种实现技术中,哪一种能提供最好的性能?
最佳答案
我在JOIN
版本中做了一些改进;见下文。
我投票赞成加速的联合方法。我是这样决定的:
有,版本1
mysql> FLUSH STATUS;
mysql> SELECT city
-> FROM us_vch200
-> WHERE state IN ('IL', 'MO', 'PA')
-> GROUP BY city
-> HAVING count(DISTINCT state) >= 3;
+-------------+
| city |
+-------------+
| Springfield |
| Washington |
+-------------+
mysql> SHOW SESSION STATUS LIKE 'Handler%';
+----------------------------+-------+
| Variable_name | Value |
+----------------------------+-------+
| Handler_external_lock | 2 |
| Handler_read_first | 1 |
| Handler_read_key | 2 |
| Handler_read_last | 1 |
| Handler_read_next | 4175 | -- full index scan
(etc)
+----+-------------+-----------+-------+-----------------------+------------+---------+------+------+--------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+-------+-----------------------+------------+---------+------+------+--------------------------------------------------+
| 1 | SIMPLE | us_vch200 | range | state_city,city_state | city_state | 769 | NULL | 4176 | Using where; Using index for group-by (scanning) |
+----+-------------+-----------+-------+-----------------------+------------+---------+------+------+--------------------------------------------------+
“额外”指出,它决定处理
GROUP BY
并使用INDEX(city, state)
,尽管INDEX(state, city)
可能有意义。有,版本2
切换到
INDEX(state, city)
会产生:mysql> FLUSH STATUS;
mysql> SELECT city
-> FROM us_vch200 IGNORE INDEX(city_state)
-> WHERE state IN ('IL', 'MO', 'PA')
-> GROUP BY city
-> HAVING count(DISTINCT state) >= 3;
+-------------+
| city |
+-------------+
| Springfield |
| Washington |
+-------------+
mysql> SHOW SESSION STATUS LIKE 'Handler%';
+----------------------------+-------+
| Variable_name | Value |
+----------------------------+-------+
| Handler_commit | 1 |
| Handler_external_lock | 2 |
| Handler_read_key | 401 |
| Handler_read_next | 398 |
| Handler_read_rnd | 398 |
(etc)
+----+-------------+-----------+-------+-----------------------+------------+---------+------+------+------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+-------+-----------------------+------------+---------+------+------+------------------------------------------+
| 1 | SIMPLE | us_vch200 | range | state_city,city_state | state_city | 2 | NULL | 397 | Using where; Using index; Using filesort |
+----+-------------+-----------+-------+-----------------------+------------+---------+------+------+------------------------------------------+
加入
mysql> SELECT x.city
-> FROM us_vch200 x
-> JOIN us_vch200 y ON y.city= x.city AND y.state = 'MO'
-> JOIN us_vch200 z ON z.city= x.city AND z.state = 'PA'
-> WHERE x.state = 'IL';
+-------------+
| city |
+-------------+
| Springfield |
| Washington |
+-------------+
2 rows in set (0.00 sec)
mysql> SHOW SESSION STATUS LIKE 'Handler%';
+----------------------------+-------+
| Variable_name | Value |
+----------------------------+-------+
| Handler_commit | 1 |
| Handler_external_lock | 6 |
| Handler_read_key | 86 |
| Handler_read_next | 87 |
(etc)
+----+-------------+-------+------+-----------------------+------------+---------+--------------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-----------------------+------------+---------+--------------------+------+--------------------------+
| 1 | SIMPLE | y | ref | state_city,city_state | state_city | 2 | const | 81 | Using where; Using index |
| 1 | SIMPLE | z | ref | state_city,city_state | state_city | 769 | const,world.y.city | 1 | Using where; Using index |
| 1 | SIMPLE | x | ref | state_city,city_state | state_city | 769 | const,world.y.city | 1 | Using where; Using index |
+----+-------------+-------+------+-----------------------+------------+---------+--------------------+------+--------------------------+
只需要
INDEX(state, city)
。对于这个公式,处理数是最小的,所以我推断它是最快的。注意优化器是如何决定从哪个表开始的,可能是由于
+-------+----------+
| state | COUNT(*) |
+-------+----------+
| IL | 221 |
| MO | 81 | -- smallest
| PA | 96 |
+-------+----------+
结论
JOIN
(没有不必要的t
表)可能是最快的。此外,还需要这个复合索引:INDEX(state, city)
。要转换回您的用例:
city --> documentid
state --> termid
注意:ymmv因为documentid和termid的值分布可能与我使用的测试用例有很大的不同。