8

我创建了 2 个查询，我可以使用它们来执行相同的功能。它们都包含我想合并到一个查询中但我无法合并的属性。

QUERY 1 - 给了我我想要的结果。慢（~0.700 秒）

QUERY 2 - 给了我很多我忽略和跳过的行。快速（~0.005 秒）

我的目标是修改 QUERY 2 以删除除每个项目的 1 之外的所有空价格行。如果不考虑性能，我似乎无法做到这一点。这是由于我对 MySQL 中的索引使用缺乏经验和理解。

查询 1

使用设计不佳的子查询，该子查询不允许跨包含 10k 行的 tbl_sale (e) 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

这是查询 1 的解释

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（除了 ORDER BY）。每次连接都使用索引。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

这是查询的解释：

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）以提供我需要的行，就像在 QUERY 1 中使用的那样？

谢谢迈克

4

2 回答 2

1

我认为这个查询会给你你想要的：

select a.id, a.sv, a.description, c.id, c.name, b.price
  from 
    tbl_item a left outer join tbl_sale b on (a.id=b.item_id)
      left outer join tbl_storeflyer d on (b.flyer_id=d.flyer_id and d.date_beg = '20120801')
      left outer join tbl_store c on (d.store_id = c.id)
      left outer join tbl_storewatch x on (c.id = x.store_id)
      left outer join tbl_watch y on (a.id = y.item_id);

如果涉及 NULL，您可能会有一些左连接。另一种方法是使用联合，使用 MySQL 可能更快：

 select a.id, a.sv, a.description, c.id as store_id, c.name, b.price
  from
    tbl_item a,
    tbl_sale b,
    tbl_storeflyer d,
    tbl_store c,
    tbl_storewatch x,
    tbl_watch y
  where
    a.id = b.item_id and
    b.flyer_id = d.flyer_id and
    d.store_id = c.id and
    c.id = x.store_id and
    a.id = y.item_id and
    d.date_beg = '20120801'
union
 select a.id, a.sv, a.description, null as store_id, null as name, null as price
  from
    tbl_item a
  where
    a.id not in (select b.item_id from tbl_sale b);

您可能会使用联合的后半部分作为左外连接而不是“不在”子查询 - 取决于您的 MySQL 版本如何优化。

于 2012-12-08T11:07:15.847 回答
0

QUERY 1 中的子选择使用隐式内连接，而查询 2 使用所有左连接显式连接。因此，在查询 2 中没有排除数据的 where 子句。我会在几行中取出 LEFT（如标记），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`

您还可以考虑将 and 子句从连接中取出并将它们移动到 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，使用外连接可以避免很多，但您可能需要它。我会尝试使用子查询来获取 id 并通过它来限制：

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
于 2012-12-08T07:01:11.197 回答