订单加入需要很长时间

Question

当我添加从JOINED表派生的ORDER BY时，执行查询的时间从0.008秒上升到0.50秒，这虽然在order by字段上有一个索引。以下是查询：

SELECT a.productid 
FROM   products a 
   JOIN products_lng b FORCE INDEX (primary)
     ON a.productid = b.productid 
        AND b.code = 'US' 
   JOIN pricing c 
     ON c.productid = a.productid 
   JOIN thumbnails d 
     ON d.productid = a.productid 
   JOIN bigthumbnails e 
     ON e.productid = a.productid 
   JOIN products_categories f 
     ON f.productid = a.productid 
WHERE  a.forsale = 'Y' 
GROUP  BY b.productid 
ORDER  BY b.product 

解释计划：

b具有以下索引：

CREATE TABLE `products_lng` (
 `code` varchar(2) CHARACTER SET latin1 NOT NULL DEFAULT '',
 `productid` int(11) NOT NULL DEFAULT '0',
 `product` varchar(255) CHARACTER SET latin1 NOT NULL DEFAULT '',
 `descr` varchar(512) CHARACTER SET latin1 NOT NULL,
 `full_descr` varchar(1024) CHARACTER SET latin1 NOT NULL,
 `processed` varchar(1) CHARACTER SET latin1 NOT NULL,
 PRIMARY KEY (`code`,`productid`,`product`),
 KEY `ad` (`code`,`productid`,`product`,`descr`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_bin

Answer 1

最后，看起来ORDER BY和GROUP BY的组合是查询执行时间超过正常的原因。然而，通过将查询放在子查询中并因此将ORDER BY和GROUP BY分开，我设法将执行时间大幅限制为0.0015秒而不是原始的0.5秒。我仍然需要根据我的要求调整查询，但以下是它应该是什么样子。基本上，它归结为在子查询中定义WHERE参数和周围查询要选择的值。

    SELECT z.*,d.*,e.*
FROM products_lng z
JOIN thumbnails d ON d.productid = z.productid
JOIN bigthumbnails e ON e.productid = z.productid
WHERE EXISTS
    ( SELECT b.productid
     FROM products b
     JOIN pricing c ON c.productid = b.productid
     JOIN products_lng i USE INDEX (PRIMARY) ON i.productid = b.productid
     AND i.code = 'US'
     JOIN products_categories f ON f.productid = b.productid
     WHERE b.productid = z.productid
     GROUP BY b.productid )
ORDER BY z.product