使用Subquery vs Left Joins进行MySQL索引优化

Question

我创建了2个可以使用相同功能的查询.它们都包含我想要合并到单个查询中的属性,但我无法做到.

QUERY 1 - 给我我想要的结果.慢(~0.700秒)

QUERY 2 - 给了我很多我忽略并跳过的行.快(~0.005秒)

我的目标是修改QUERY 2以删除除每个项目1之外的所有空价格行.我似乎无法做到这一点,而不是在表现上喋喋不休.这是由于我缺乏对MySQL中索引使用的经验和理解.

查询1

使用设计不佳的子查询,该子查询不允许在包含10k行的tbl_sale(e)中使用索引.

SELECT b.id, b.sv, b.description, der.store_id, f.name, der.price
FROM tbl_watch AS a
    LEFT JOIN tbl_item AS b ON a.item_id = b.id
LEFT JOIN (
    SELECT c.store_id, d.flyer_id, e.item_id, e.price
    FROM tbl_storewatch AS c, tbl_storeflyer AS d
    FORCE INDEX ( storebeg_ndx ) , tbl_sale AS e
    WHERE c.user_id = '$user_id'
    AND (
        d.store_id = c.store_id
        AND d.date_beg = '20121206'
        )
    AND e.flyer_id = d.flyer_id
        ) AS der ON a.item_id = der.item_id
LEFT JOIN tbl_store as f ON der.store_id = f.id
WHERE a.user_id = '$user_id'
ORDER BY b.description ASC

这是EXPLAIN for QUERY 1

id  select_type table       type    possible_keys   key             key_len     ref     rows    Extra
1   PRIMARY     a           ref     user_item_ndx   user_item_ndx   4           const   30  Using index; Using temporary; Using filesort
1   PRIMARY     b           eq_ref  PRIMARY         PRIMARY         4           a.item_id   1   
1   PRIMARY     <derived2>  ALL     NULL            NULL            NULL        NULL    300     
1   PRIMARY     f           eq_ref  PRIMARY         PRIMARY         4           der.store_id    1   
2   DERIVED     c           ref     user_ndx        user_ndx        4                   6   
2   DERIVED     e           ALL     NULL            NULL    NULL    NULL                9473    Using join buffer
2   DERIVED     d           eq_ref  storebeg_ndx    storebeg_ndx    8           c.store_id  1   Using where

查询2

使用非常有效的所有左连接(ORDER BY除外).索引用于每个连接.此查询返回tbl_watch中每个项目的所有可能匹配项.这是查询:

SELECT b.id, b.sv, b.description, c.store_id, f.name, e.price
FROM tbl_watch AS a
LEFT JOIN tbl_item AS b ON a.item_id = b.id
LEFT JOIN tbl_storewatch AS c ON c.user_id = '$user_id'
LEFT JOIN tbl_storeflyer AS d ON d.store_id = c.store_id
    AND d.date_beg = '$s_date'
LEFT JOIN tbl_sale AS e ON e.item_id = a.item_id
    AND e.flyer_id = d.flyer_id 
LEFT JOIN tbl_store as f ON d.store_id = f.id
WHERE a.user_id = '$user_id'
ORDER BY b.description ASC

以下是查询的EXPLAIN:

id  select_type     table   type    possible_keys           key             key_len     ref                     rows    Extra
1   SIMPLE          a       ref     user_item_ndx           user_item_ndx   4           const                   6       Using index; Using temporary; Using filesort
1   SIMPLE          b       eq_ref  PRIMARY                 PRIMARY         4           a.item_id               1   
1   SIMPLE          c       ref     user_ndx                user_ndx        4           const                   2   
1   SIMPLE          d       eq_ref  storebeg_ndx,storendx   storebeg_ndx    8           c.store_id,const        1   
1   SIMPLE          e       eq_ref  itemflyer_ndx           itemflyer_ndx   8           a.item_id,d.flyer_id    1   
1   SIMPLE          f       eq_ref  PRIMARY                 PRIMARY         4           d.store_id              1   

如何修改QUERY 2(更高效)以便在QUERY 1中使用我需要的行？

谢谢迈克

Answer 1

查询 1 中的子选择使用隐式内部联接，而查询 2 使用所有左联接显式联接。因此，没有 where 子句可以排除查询 2 中的数据。我会取出几行中的 LEFT（如标记的那样），看看这如何改进：

SELECT b.id, b.sv, b.description, c.store_id, f.name, e.price
FROM tbl_watch AS a
LEFT JOIN tbl_item AS b ON a.item_id = b.id
LEFT JOIN tbl_storewatch AS c ON c.user_id = '$user_id'
-- Left removed below
JOIN tbl_storeflyer AS d ON d.store_id = c.store_id
    AND d.date_beg = '$s_date'
-- Left removed below
JOIN tbl_sale AS e ON e.item_id = a.item_id
    AND e.flyer_id = d.flyer_id 
LEFT JOIN tbl_store as f ON d.store_id = f.id
WHERE a.user_id = '$user_id'
ORDER BY b.description ASC`

您还可以考虑从连接中取出 and 子句并将它们移动到 WHERE：

SELECT b.id, b.sv, b.description, c.store_id, f.name, e.price
FROM tbl_watch AS a
LEFT JOIN tbl_item AS b ON a.item_id = b.id
LEFT JOIN tbl_storewatch AS c ON c.user_id = '$user_id'
JOIN tbl_storeflyer AS d ON d.store_id = c.store_id
JOIN tbl_sale AS e ON e.item_id = a.item_id
LEFT JOIN tbl_store as f ON d.store_id = f.id
WHERE a.user_id = '$user_id'
AND d.date_beg = '$s_date'
AND e.flyer_id = d.flyer_id 
ORDER BY b.description ASC

最后，日期数学相当密集。在查询 2 中，使用外连接，您可以避免很多连接，但您可能需要它。我尝试使用子查询来获取 ids 并进行限制：

SELECT b.id, b.sv, b.description, c.store_id, f.name, e.price
FROM tbl_watch AS a
LEFT JOIN tbl_item AS b ON a.item_id = b.id
LEFT JOIN tbl_storewatch AS c ON c.user_id = '$user_id'
JOIN tbl_storeflyer AS d ON d.store_id = c.store_id
JOIN tbl_sale AS e ON e.item_id = a.item_id
LEFT JOIN tbl_store as f ON d.store_id = f.id
WHERE a.user_id = '$user_id'
AND e.flyer_id = d.flyer_id 
AND d.id in  (select d.id from d where date_beg = '$s_date')
ORDER BY b.description ASC