如何避免使用左连接时的文件和

Question

我有3个表：帐户，联系人和accounts_contacts（映射表）。 我在每张表中都有100万条记录。此查询使用文件排序，并需要超过一分钟，运行：

explain SELECT contacts.salutation salutation, contacts.first_name first_name, contacts.last_name last_name, contacts.title title, jt0_accounts.id account_id, jt0_accounts.name account_name 

FROM contacts 

LEFT JOIN accounts_contacts jt1_accounts_contacts ON (contacts.id = jt1_accounts_contacts.contact_id AND jt1_accounts_contacts.deleted = 0) 

LEFT JOIN accounts jt0_accounts ON (jt0_accounts.id = jt1_accounts_contacts.account_id AND jt0_accounts.deleted = 0) 

ORDER BY jt0_accounts.name DESC; 

这是解释输出：

id select_type table type possible_keys key key_len ref rows Extra 
1 SIMPLE contacts ALL NULL NULL NULL NULL 195634 Using temporary; Using filesort 
1 SIMPLE jt1_accounts_contacts ref idx_contid_del_accid idx_contid_del_accid 113 sugar7.contacts.id,const 1 
1 SIMPLE jt0_accounts eq_ref PRIMARY,idx_accounts_id_del,idx_accounts_date_entered,idx_accnt_assigned_del PRIMARY 108 sugar7.jt1_accounts_contacts.account_id 1 

正如你所看到的，接触表使用上的联系人表文件排序。

我试图通过添加摆脱文件排序的“WHERE jt0_accounts.name <>'”前“ORDER BY”，所以它变成了：

explain SELECT contacts.salutation salutation, contacts.first_name first_name, contacts.last_name last_name, contacts.title title, jt0_accounts.id account_id, jt0_accounts.name account_name 

FROM contacts 

LEFT JOIN accounts_contacts jt1_accounts_contacts ON (contacts.id = jt1_accounts_contacts.contact_id AND jt1_accounts_contacts.deleted = 0) 

LEFT JOIN accounts jt0_accounts ON (jt0_accounts.id = jt1_accounts_contacts.account_id AND jt0_accounts.deleted = 0) 

WHERE jt0_accounts.name <> '' 
ORDER BY jt0_accounts.name DESC; 

它得到上摆脱了文件排序的联系人表，但它现在使用的文件排序的映射表：

id select_type table type possible_keys key key_len ref rows Extra 
1 SIMPLE jt1_accounts_contacts ALL idx_account_contact,idx_contid_del_accid NULL NULL NULL 34994 Using where; Using temporary; Using filesort 
1 SIMPLE jt0_accounts eq_ref PRIMARY,idx_accounts_id_del,idx_accounts_date_entered,idx_accnt_name_del,idx_accnt_assigned_del PRIMARY 108 sugar7.jt1_accounts_contacts.account_id 1 Using where 
1 SIMPLE contacts eq_ref PRIMARY,idx_contacts_id_del,idx_contacts_date_entered PRIMARY 108 sugar7.jt1_accounts_contacts.contact_id 1 Using where 

的idx_account_contact指数由ACCOUNT_ID和contacts_id的。我试着将它们添加到WHERE子句中，但它似乎没有任何区别。

任何建议，将不胜感激。 谢谢。

Answer 1

对于您特别的查询，您可能没有太多的工作要做。但是，如果你改变使用inner join查询，你可能有机会：

SELECT c.salutation, c.first_name, c.last_name, c.title, 
     a.id as account_id, a.name as account_name 
FROM accounts a JOIN 
    accounts_contacts ac 
    ON a.id = ac.account_id AND a.deleted = 0 
    contacts c JOIN 
    ON c.id = ac.contact_id AND ac.deleted = 0  
ORDER BY a.name DESC; 

然后，请尝试以下指标：accounts(name, deleted, id)，accounts_contacts(account_id, contact_id)和concats(contact_id, deleted)。