使用Subquery vs Left Joins进行MySQL索引优化

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,因为他们没有表现出色.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,c.store_id,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
麦克风

解决方法

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.另一种方法是使用一个联合,与MySQL可能会更快：

select a.id,c.id as store_id,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,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.

（编辑：李大同）

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