爲什麼我的MySQL加入緩慢？

Question

我有一個連接表的查詢。這是select語句：

SELECT *, A.id AS id, 
     A.username AS username, 
     G.desc AS customer_desc, 
     H.id AS counter, 
     SUM(F.amount_paid) 
FROM tblcheckin AS A 
LEFT OUTER JOIN tblrate AS B ON B.id = A.rate_id 
LEFT OUTER JOIN tblrefaccom_type AS C ON C.id = A.id 
LEFT OUTER JOIN tblcust_bill AS D ON D.check_in_id = A.id 
LEFT OUTER JOIN tblroom AS E ON E.room_no = A.room_id 
LEFT OUTER JOIN tblpayment AS F ON F.check_in_id = A.id 
LEFT OUTER JOIN tblrefcust_type AS G ON G.id = A.customer_desc 
LEFT OUTER JOIN tblindex AS H ON A.id = H.check_in_id 
WHERE A.check_out = "0000-00-00 00:00:00" 
GROUP BY F.check_in_id 
ORDER BY check_out, check_in 

結果：Showing rows 0 - 11 (12 total, Query took 7.4061 sec)

這是Explain SQL

這是我的就是我所做的迄今： 我設置optimizer_search_depth = 0因爲我想它會像這裏提到的那樣工作Joining many tables in mySQL。

我在這裏做錯了什麼？我感謝每一個反饋。謝謝。

UPDATE。問題解決了。我在tblpayment，tblcust_bill和tblindex的check_in_id列中添加了索引。這裏是新Explain SQL：

Answer 1

看你的解釋，我可以看到幾個的連接需要全表掃描。您可能沒有A.check_out，D.check_in_id，F.check_in_id（這是一個基於相對行數的殺手）和H.check_in_id的索引。這意味着它必須掃描整個表格A，D，F和H才能找到相關的行。

索引這4行，查詢次數應該會飛漲。

那個已經說了，我很驚訝這已經不快了，甚至沒有優化。我在每個表格中看到幾萬行 - 除非這些行真的很大（順便說一下，壞的做法 - 將文本/ blob和大列分成不同的表格），您應該能夠得到這些結果少於7秒。更像我期望的70毫秒。我懷疑你必須調整你的MySQL服務器設置 - 使用更多的內存，增加緩衝區大小等，但這些索引應該有很大的幫助。

Answer 2

您似乎要連接很多不需要的表來滿足您的查詢。這是否會工作，省去了額外的表格？

SELECT A.id AS id, 
     A.username AS username, 
     G.desc AS customer_desc, 
     H.id AS counter, 
     SUM(F.amount_paid) 
FROM tblcheckin AS A 
LEFT OUTER JOIN tblpayment AS F ON F.check_in_id = A.id 
LEFT OUTER JOIN tblrefcust_type AS G ON G.id = A.customer_desc 
LEFT OUTER JOIN tblindex AS H ON A.id = H.check_in_id 
WHERE A.check_out = "0000-00-00 00:00:00" 
GROUP BY F.check_in_id 
ORDER BY check_out, check_in 

您正在使用令人討厭的MySQL hackstension來進行GROUP BY。代之以做。

GROUP BY A.id, A.username, G.desc, H.id 

是tblcheckin一個表有多行嗎？如果是這樣，請確保check_out上有一個索引，並將其聲明爲NOT NULL。

確保您有在ON條款中提到的列的索引在你JOIN指令，即：

tblpayment.check_in_id 
    tblrefcust_type.id (maybe already a primary key) 
    tblindex.check_in_id 

考慮將覆蓋索引（多列索引）在你的表。這樣，您的查詢所需的數據就可以完全從索引中完全滿足，這將節省查詢查詢磁盤驅動器的時間。儘管如此，很多索引可能會降低插入和更新的速度。

tblcheckin (cneck_out, id, customer_desc, username) 
    tblpayment (check_in_id, amount_paid) 
    tblrefcust_type (id, desc) 
    tblindex  (check_in_id, id)