[英]How to avoid filesort when using left join and order by
我有3个表:accounts,contacts和accounts_contacts(一个映射表)。 我在每个表中都有1百万条记录。 该查询正在使用文件排序,并且需要一分钟以上的时间来运行:
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
如您所见,联系人表在联系人表上使用文件排序。
我尝试通过在“ ORDER BY”之前添加“ WHERE jt0_accounts.name <>”来摆脱文件排序,因此它变为:
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;
它确实摆脱了联系人表上的文件排序,但现在在映射表上使用了文件排序:
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
idx_account_contact索引由account_id和contacts_id组成。 我尝试将它们添加到WHERE子句中,但似乎没有什么区别。
任何建议,将不胜感激。 谢谢。
您可能无法对您的特定查询做很多事情。 但是,如果将查询更改为使用inner join
,则可能会有机会:
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,尝试以下索引: accounts(name, deleted, id)
, accounts_contacts(account_id, contact_id)
和concats(contact_id, deleted)
。
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