SELECT COUNT指數

Question

我是一個MySQL數據庫優化的新手。

我有以下慢速查詢：

SELECT COUNT(DISTINCT p.product_id) AS total 
    FROM shs_product p 
LEFT JOIN shs_product_description pd 
     ON (p.product_id = pd.product_id) 
LEFT JOIN shs_product_to_store p2s 
     ON (p.product_id = p2s.product_id) 
LEFT JOIN shs_product_to_category p2c 
     ON (p.product_id = p2c.product_id) 
    WHERE pd.language_id = '1' 
     AND p.status = '1' 
     AND p.date_available <= NOW() 
     AND p2s.store_id = '$ 

誰能行動提供建議創建索引，以加快此查詢？ 您推薦哪張桌子和哪一列？

任何幫助將不勝感激......

Answer 1

三點意見：

1. 的where子句撤消外連接p2s和pd。所以你不妨稱這些內部連接。
2. p2c表未被使用。因爲您正在執行count(distinct)，您可以將其刪除。
3. 據推測，該ID是整數，所以你不需要引號

因此，查詢是等效

SELECT COUNT(DISTINCT p.product_id) AS total 
FROM shs_product p JOIN 
    shs_product_description pd 
    ON p.product_id = pd.product_id JOIN 
    shs_product_to_store p2s 
    ON p.product_id = p2s.product_id 
WHERE pd.language_id = 1 AND p.status = 1 AND p.date_available <= NOW() AND 
     p2s.store_id = '$' 

對於此查詢，您想對shs_product(status, date_available)的索引。您還需要用於加入的列和where子句中的索引。我會建議：shs_product_description(product_id, language_id)和shs_product_to_store(product_id, store_id)。

最後，假設product_id是一個獨特的指數product，您可以短語使用exists和count(*)，而不是count(distinct)此查詢：

select count(*) 
from shs_product p 
where p.status = 1 AND p.date_available <= NOW() and 
     exists (select 1 
       from shs_product_description pd 
       where p.product_id = pd.product_id and pd.language_id = 1 
      ) and 
     exists (select 1 
       from shs_product_to_store p2s 
       where p.product_id = p2s.product_id and p2s.store_id = '$' 
      );