联合快速MySQL查询的性能下降

Question

我有两个查询在单独运行时运行得非常快，但是当通过使用其中一个作为子查询进行组合时，性能会急剧下降。

FAST QUERY I：

SELECT DISTINCT p.products_image, 
       p.products_subimage1, 
       pd.products_name, 
       p.products_quantity, 
       p.products_model, 
       p.products_ordered, 
       p.products_id, 
       p.products_price, 
       p.products_weight, 
       p.products_length, 
       p.products_width, 
       p.products_height, 
       p.products_tax_class_id, 
       p.products_status, 
       IF(s.status, s.specials_new_products_price, NULL)    AS 
       specials_new_products_price, 
       IF(s.status, s.specials_new_products_price, p.products_price) AS 
       final_price 
FROM products p 
     LEFT JOIN specials s 
       ON p.products_id = s.products_id 
     LEFT JOIN products_to_categories p2c 
       ON p.products_id = p2c.products_id 
     LEFT JOIN products_description pd 
       ON p.products_id = pd.products_id 

     LEFT JOIN products_attributes pa 
       ON p.products_id = pa.products_id 
     LEFT JOIN (SELECT `products_id`, 
       `products_stock_attributes`, 
       `products_stock_quantity` 
     FROM products_stock 
     WHERE `products_stock_quantity` > 0) t 
     ON t.products_id = p.products_id 
     WHERE t.products_stock_attributes IN ('1-31', '1-25', '1-18', '1-7') 
     AND p.products_status = '1' 
     AND Date_sub(Curdate(), INTERVAL 30 day) >= p.products_date_added 
     AND pa.options_values_id IN (31, 25, 18, 7) 
GROUP BY p.products_id 
HAVING Count(DISTINCT pa.options_values_id) = 4 
     AND Count(DISTINCT t.products_stock_attributes) = 4 

FAST QUERY II：

SELECT sul2.* 
FROM stock_update_log sul2 
INNER JOIN (SELECT products_id, 
    Max(date_time) AS maxDateTime 
FROM stock_update_log 
WHERE stock_qty_change > 1 
AND id > 154700 
    AND Date_sub(Curdate(), INTERVAL 360 day)<= From_unixtime(date_time) 
GROUP BY products_id)gsul 
     ON sul2.products_id = gsul.products_id 
AND sul2.date_time = gsul.maxdatetime 

合并查询（非常慢的）：

SELECT DISTINCT p.products_image, 
       p.products_subimage1, 
       pd.products_name, 
       p.products_quantity, 
       p.products_model, 
       p.products_ordered, 
       p.products_id, 
       p.products_price, 
       sul.date_time, 
       p.products_weight, 
       p.products_length, 
       p.products_width, 
       p.products_height, 
       p.products_tax_class_id, 
       p.products_status, 
       IF(s.status, s.specials_new_products_price, NULL)    AS 
       specials_new_products_price, 
       IF(s.status, s.specials_new_products_price, p.products_price) AS 
       final_price 
FROM products p 
     LEFT JOIN specials s 
       ON p.products_id = s.products_id 
     LEFT JOIN products_to_categories p2c 
       ON p.products_id = p2c.products_id 
     LEFT JOIN products_description pd 
       ON p.products_id = pd.products_id 
     LEFT JOIN (SELECT sul2.* 
        FROM stock_update_log sul2 
         INNER JOIN (SELECT products_id, 
              Max(date_time) AS maxDateTime 
            FROM stock_update_log 
            WHERE stock_qty_change > 1 
              AND id > 154700 
              AND Date_sub(Curdate(), 
               INTERVAL 360 day) 
               <= 
               From_unixtime(date_time) 
            GROUP BY products_id)gsul 
           ON sul2.products_id = gsul.products_id 
            AND sul2.date_time = gsul.maxdatetime) sul 
       ON p.products_id = sul.products_id 
     LEFT JOIN products_attributes pa 
       ON p.products_id = pa.products_id 
     LEFT JOIN (SELECT `products_id`, 
       `products_stock_attributes`, 
       `products_stock_quantity` 
     FROM products_stock 
     WHERE `products_stock_quantity` > 0) t 
     ON t.products_id = p.products_id 
     WHERE t.products_stock_attributes IN ('1-31', '1-25', '1-18', '1-7') 
     AND p.products_status = '1' 
     AND Date_sub(Curdate(), INTERVAL 360 day) <= From_unixtime(sul.date_time) 
     AND Date_sub(Curdate(), INTERVAL 30 day) >= p.products_date_added 
     AND sul.id > 154700 
     AND sul.stock_qty_change > 1 
     AND pa.options_values_id IN (31, 25, 18, 7) 
GROUP BY p.products_id 
HAVING Count(DISTINCT pa.options_values_id) = 4 
     AND Count(DISTINCT t.products_stock_attributes) = 4 
ORDER BY sul.date_time DESC 

解释的组合查询结果：

我一直在尝试几个小时，弄清楚为什么它结合他们这么慢，并试图重写组合查询无济于事，所以我伸出援手来自这里的专家。

什么原因导致它变得如此缓慢，我该怎么做才能使它变得更快？

Answer 1

查询II感觉它有一百万行;得到的tmp表没有索引;它隐藏在LEFT JOIN的后面。尽管如此，优化器似乎足够聪明，可以从混乱开始，并为联合查询提供7904行。

你能避免LEFT？

然后代码继续进行，但最终必须从表扫描中检查所有这些行与18057行。优化器再一次通过使用“连接缓冲区”来做一件聪明的事情。尽管如此，7904 * 923 * 18057还是有很多行。

这似乎是邪恶部分：

LEFT JOIN 
    (SELECT `products_id`, 
       `products_stock_attributes`, 
       `products_stock_quantity` 
     FROM products_stock 
     WHERE `products_stock_quantity` > 0 
    ) t 
    ON t.products_id = p.products_id 

    WHERE t.products_stock_attributes IN ('1-31', '1-25', '1-18', '1-7') 

所有这些似乎是红旗：（？）

• 不必要LEFT;
• “where attributes in ...”在子查询之外时，它们可能在里面;
• 你或许可以将HAVING Count(DISTINCT t.products_stock_attributes) = 4折叠到子查询中。

Answer 2

您正在连接7个表格，这肯定会很慢。我建议避免加入超过3张桌子。

你有没有试过自己做两个查询，然后结合他们的结果？