mysql - SQL优化——慢查询

Question

给定 SQL 需要 1.2 秒：

SELECT DISTINCT contracts.id, jt0.id, jt1.id, jt2.id, jt3.id FROM contracts
LEFT JOIN accounts jt0 ON jt0.id = contracts.account_id AND jt0.deleted=0
LEFT JOIN manufacturers jt1 ON jt1.id = contracts.manufacturer_id AND jt1.deleted=0
LEFT JOIN products jt2 ON jt2.id = contracts.product_id AND jt2.deleted=0
LEFT JOIN users jt3 ON jt3.id = contracts.assigned_user_id AND jt3.deleted=0
WHERE contracts.deleted=0
ORDER BY contracts.application_number ASC 
LIMIT 0,21

这是解释扩展回报的原因：

id  select_type table   type    possible_keys   key key_len ref rows
1   SIMPLE  contracts   ref idx_contracts_deleted   idx_contracts_deleted   2   const   18968   100.00  Using where; Using temporary; Using filesort
1   SIMPLE  jt0 eq_ref  PRIMARY,idx_accnt_id_del,idx_accnt_assigned_del PRIMARY 108 xxx.contracts.account_id    1   100.00  
1   SIMPLE  jt1 eq_ref  PRIMARY,idx_manufacturers_id_deleted,idx_manufacturers_deleted  PRIMARY 108 xxx.contracts.manufacturer_id   1   100.00  
1   SIMPLE  jt2 eq_ref  PRIMARY,idx_products_id_deleted,idx_products_deleted    PRIMARY 108 xxx.contracts.product_id    1   100.00  
1   SIMPLE  jt3 eq_ref  PRIMARY,idx_users_id_del,idx_users_id_deleted,idx_users_deleted PRIMARY 108 xxx.contracts.assigned_user_id  1   100.00

我需要不同的，我需要留下所有的连接，我需要排序，我需要限制。
我可以以某种方式优化它吗？

score 0 · Accepted Answer

尝试一点参照完整性？我敢打赌，使用内部连接查询会快得多。应该是，因为查询优化器有更多的工作要做。您正在select为不更关心create.

我还将删除已删除的行到他们自己的表中，并删除deleted列。您的查询将更简单，并且可能运行得更快。

score 0 · Accepted Answer

尝试更改子表上的索引以包含该deleted列：

accounts(id, deleted)
manufacturers(id, deleted)
products(id, deleted)
users(id, deleted)

通过在索引中包含所有列，MySQL 有更好的机会利用索引。

另一个建议是找出导致值重复的原因，并使用子查询来消除重复项，而不是distinct.

例如，使用上述索引：

from contracts c left join
     (select id
      from accounts
      where deleted = 0
      group by id
     ) a
     on c.account_id = a.id
     . . .

子查询应该只使用索引，这可能会加快速度。

score 0 · Accepted Answer

首先在以下列上创建必要的索引。

contract.application_number，manufacturers.deleted，products.deleted，users.deleted

SELECT DISTINCT contracts.id, jt0.id, jt1.id, jt2.id, jt3.id 
FROM contracts
LEFT JOIN accounts jt0 
ON contracts.deleted=0 AND jt0.id = contracts.account_id
LEFT JOIN manufacturers jt1 
ON jt1.deleted=0 AND jt1.id = contracts.manufacturer_id
LEFT JOIN products jt2 
ON jt2.deleted=0 AND jt2.id = contracts.product_id
LEFT JOIN users jt3 
ON jt3.deleted=0 AND jt3.id = contracts.assigned_user_id
ORDER BY contracts.application_number ASC 
LIMIT 0,21

正如您所提到的，您已经在contracts.deleted上建立了索引

FROM 
  (SELECT * FROM contracts WHERE contracts.deleted = 0 USE INDEX(<deletedIndexName>))
LEFT JOIN 
  accounts jt0 
ON 
  jt0.id = contracts.account_id
LEFT JOIN  
  ...

score 0 · Accepted Answer

这些是我得到的唯一建议

我希望将 id 定义为主键和具有表之间关系的外键。
也许 application_number 可以被索引（然后排序会更快）
也许，如果您使用的是 MyISAM，如果您在选择之前锁定表，则 sql 可能会更快（不要忘记之后解锁）

mysql - SQL优化——慢查询

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