[英]Does MySQL optimize selected aggregations on related tables to avoid N+1?
這個查詢在MySQL中最優嗎? 我的意思是:是否有恆定數量的查詢正在執行?
還是它屬於N + 1問題? 在官方MySQL文檔中沒有發現關於優化的任何細節。
SELECT t.*, (SELECT COUNT(1) from related_table rt where rt.t_id = t.id)
FROM table t
天真地看,有一個查詢和N個查詢,因此它屬於N + 1問題。
MySQL 5.5+是否會在內部自動地+改進此查詢以進行恆定數量的查詢? 也許將其內部轉換為:
SELECT t.*, COUNT(rt.id)
FROM table t LEFT OUTER JOIN related_table rt
GROUP BY t.id
我的意思是:我知道如何手動進行改進,但是我之所以這樣問是因為:
不,MySQL沒有優化選擇列表中的相關子查詢。
您可以使用EXPLAIN來獲取優化計划的報告來確認這一點。 這是一個使用測試數據庫的類似查詢:
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的選擇類型意味着該子查詢將執行多次,可能對外部查詢的每一行執行一次。
與EXPLAIN進行比較,以進行手動優化的查詢:
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
這表明對第二張表的訪問只是一個簡單的引用聯接。
您也可以使用MySQL查詢分析器進行測試。 這是第二個查詢,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 |
+----------------------+----------+
這是帶有依賴子查詢的子查詢:
+----------------------+----------+
| 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 |
+----------------------+----------+
您可以看到子查詢導致多次執行。 以我為例,role_type表中只有幾行,但是如果您有成百上千的子查詢執行次數,則可能會很長,以至於探查器會截斷該報告。
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.