[英]Does MySQL optimize selected aggregations on related tables to avoid N+1?
这个查询在MySQL中最优吗? 我的意思是:是否有恒定数量的查询正在执行?
还是它属于N + 1问题? 在官方MySQL文档中没有发现关于优化的任何细节。
SELECT t.*, (SELECT COUNT(1) from related_table rt where rt.t_id = t.id)
FROM table t
天真地看,有一个查询和N个查询,因此它属于N + 1问题。
MySQL 5.5+是否会在内部自动地+改进此查询以进行恒定数量的查询? 也许将其内部转换为:
SELECT t.*, COUNT(rt.id)
FROM table t LEFT OUTER JOIN related_table rt
GROUP BY t.id
我的意思是:我知道如何手动进行改进,但是我之所以这样问是因为:
不,MySQL没有优化选择列表中的相关子查询。
您可以使用EXPLAIN来获取优化计划的报告来确认这一点。 这是一个使用测试数据库的类似查询:
mysql> explain select *, (SELECT COUNT(*) FROM cast_info where cast_info.role_id = role_type.id) AS c
from role_type\G
*************************** 1. row ***************************
id: 1
select_type: PRIMARY
table: role_type
partitions: NULL
type: index
possible_keys: NULL
key: role
key_len: 98
ref: NULL
rows: 12
filtered: 100.00
Extra: Using index
*************************** 2. row ***************************
id: 2
select_type: DEPENDENT SUBQUERY
table: cast_info
partitions: NULL
type: ref
possible_keys: cr
key: cr
key_len: 4
ref: imdb.role_type.id
rows: 2534411
filtered: 100.00
Extra: Using index
DEPENDENT SUBQUERY的选择类型意味着该子查询将执行多次,可能对外部查询的每一行执行一次。
与EXPLAIN进行比较,以进行手动优化的查询:
mysql> explain select r.*, COUNT(c.id) AS c from role_type AS r left outer join cast_info as c on r.id = c.role_id group by r.id\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: r
partitions: NULL
type: index
possible_keys: PRIMARY,role
key: PRIMARY
key_len: 4
ref: NULL
rows: 12
filtered: 100.00
Extra: NULL
*************************** 2. row ***************************
id: 1
select_type: SIMPLE
table: c
partitions: NULL
type: ref
possible_keys: cr
key: cr
key_len: 4
ref: imdb.r.id
rows: 2534411
filtered: 100.00
Extra: Using index
这表明对第二张表的访问只是一个简单的引用联接。
您也可以使用MySQL查询分析器进行测试。 这是第二个查询,它使用join:
+----------------------+----------+
| Status | Duration |
+----------------------+----------+
| starting | 0.000167 |
| checking permissions | 0.000015 |
| checking permissions | 0.000016 |
| Opening tables | 0.000050 |
| init | 0.000059 |
| System lock | 0.000044 |
| optimizing | 0.000011 |
| statistics | 0.000151 |
| preparing | 0.000099 |
| Sorting result | 0.000019 |
| executing | 0.000010 |
| Sending data | 9.700879 |
| end | 0.000024 |
| query end | 0.000022 |
| closing tables | 0.000017 |
| freeing items | 0.000243 |
| cleaning up | 0.000056 |
+----------------------+----------+
这是带有依赖子查询的子查询:
+----------------------+----------+
| Status | Duration |
+----------------------+----------+
| starting | 0.000152 |
| checking permissions | 0.000014 |
| checking permissions | 0.000013 |
| Opening tables | 0.000050 |
| init | 0.000067 |
| System lock | 0.000042 |
| optimizing | 0.000010 |
| statistics | 0.000367 |
| preparing | 0.000033 |
| optimizing | 0.000015 |
| statistics | 0.000032 |
| preparing | 0.000020 |
| executing | 0.000010 |
| Sending data | 0.000191 |
| executing | 0.000010 |
| Sending data | 4.103899 |
| executing | 0.000018 |
| Sending data | 2.413570 |
| executing | 0.000018 |
| Sending data | 0.043924 |
| executing | 0.000022 |
| Sending data | 0.037834 |
| executing | 0.000020 |
| Sending data | 0.014127 |
| executing | 0.000021 |
| Sending data | 0.089977 |
| executing | 0.000023 |
| Sending data | 0.045968 |
| executing | 0.000024 |
| Sending data | 0.000044 |
| executing | 0.000005 |
| Sending data | 0.190935 |
| executing | 0.000034 |
| Sending data | 1.046394 |
| executing | 0.000018 |
| Sending data | 0.017567 |
| executing | 0.000021 |
| Sending data | 0.882959 |
| end | 0.000046 |
| query end | 0.000023 |
| closing tables | 0.000018 |
| freeing items | 0.000248 |
| cleaning up | 0.000025 |
+----------------------+----------+
您可以看到子查询导致多次执行。 以我为例,role_type表中只有几行,但是如果您有成百上千的子查询执行次数,则可能会很长,以至于探查器会截断该报告。
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