MySQL 使用 JOIN 和 COUNT 進行查詢優化

Question

我有以下 MySQL 查詢：

SELECT t1.id, t1.releaseid, t1.site, t1.date, t2.pos FROM `tracers` as t1
LEFT JOIN (
    SELECT `releaseid`, `date`, COUNT(*) AS `pos` 
    FROM `tracers` GROUP BY `releaseid`
) AS t2 ON t1.releaseid = t2.releaseid AND t2.date <= t1.date 
ORDER BY `date` DESC , `pos` DESC LIMIT 0 , 100

想法是將 select 發布並計算在記錄日期之前有多少其他網站也發布了它，以獲得 position。

解釋說：

id  select_type table   type    possible_keys   key key_len ref rows    Extra
1   PRIMARY t1  ALL NULL    NULL    NULL    NULL    498422  Using temporary; Using filesort
1   PRIMARY <derived2>  ALL NULL    NULL    NULL    NULL    91661    
2   DERIVED tracers index   NULL    releaseid   4   NULL    498422   

關於如何消除 Using 臨時的任何建議； 使用文件排序？ 這需要很長時間。 我想到並嘗試過的索引沒有任何幫助。

Answer 1

下面的這個答案可能不會改變解釋 output，但是如果您的主要問題是排序數據，它通過刪除 order 子句確定將使您的查詢運行得更快，請先嘗試對您的子查詢連接表進行排序，您的查詢將是：

SELECT t1.id, t1.releaseid, t1.site, t1.date, t2.pos FROM `tracers` as t1
LEFT JOIN (
    SELECT `releaseid`, `date`, COUNT(*) AS `pos` 
    FROM `tracers` GROUP BY `releaseid`
    ORDER BY `pos` DESC -- additional order
) AS t2 ON t1.releaseid = t2.releaseid AND t2.date <= t1.date 
ORDER BY `date` DESC , `pos` DESC LIMIT 0 , 100

注意：我的數據庫版本是 mysql-5.0.96-x64，也許在另一個版本中你會得到不同的結果。

Answer 2

嘗試在tracers.date上添加一個索引，在tracers.releaseid上添加一個

Answer 3

1. 確保你有一個關於 releaseid 的索引。
2. 翻轉您的 JOIN，子查詢必須在 LEFT JOIN 的左側。
3. 將 ORDER BY 和 LIMIT 子句放在子查詢中。

Answer 4

嘗試有兩個索引，一個在(date)上，一個在(releaseid, date)上。

另一件事是您的查詢似乎沒有按照您的描述進行。 它實際上計算正確嗎？

嘗試將其重寫為：

SELECT t1.id, t1.releaseid, t1.site, t1.`date`
     , COUNT(*) AS pos
FROM tracers AS t1
  JOIN tracers AS t2
    ON  t2.releaseid = t1.releaseid
    AND t2.`date` <= t1.`date` 
GROUP BY t1.releaseid
ORDER BY t1.`date` DESC
       , pos DESC
LIMIT 0 , 100

或作為：

SELECT t1.id, t1.releaseid, t1.site, t1.`date`
     , ( SELECT COUNT(*)
         FROM tracers AS t2
         WHERE t2.releaseid = t1.releaseid
           AND t2.`date` <= t1.`date`
       ) AS pos
FROM tracers AS t1
ORDER BY t1.`date` DESC
       , pos DESC
LIMIT 0 , 100