Question

我创建了两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，但我无法进行。

查询1 - 给我我想要的结果。慢（~0.700秒）

查询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。

QUERY 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

QUERY 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中为我提供我需要的行？

由于麦克

Answer 1

我认为此查询会为您提供所需内容：

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);

涉及NULL，你可能会有一些左连接。另一种方法是使用一个联合，与MySQL可能会更快：

 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);

你可能会使用union的后半部分作为左外连接而不是'not in'子查询 - 取决于你的MySQL版本如何优化。

Answer 2

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where子句排除数据。我会在几行（如标记）中取出LEFT，并看看这是如何改进的：

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`

您也可以考虑从连接中取出＆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，但您可能需要它。我尝试使用子查询来获取ID并限制：

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

使用Subquery vs Left Joins进行MySQL索引优化

2 个答案: