为什么MySQL不使用JOIN上的主键加上ORDER?
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这对你来说很简洁(显然是MySQL):
# Setting things up DROP DATABASE IF EXISTS index_test_gutza; CREATE DATABASE index_test_gutza; USE index_test_gutza; CREATE TABLE customer_order ( id MEDIUMINT UNSIGNED NOT NULL AUTO_INCREMENT, invoice MEDIUMINT UNSIGNED NOT NULL DEFAULT 0, PRIMARY KEY(id) ); INSERT INTO customer_order (id, invoice) VALUES (1, 1), (2, 2), (3, 3), (4, 4), (5, 5); CREATE TABLE customer_invoice ( id MEDIUMINT UNSIGNED NOT NULL AUTO_INCREMENT, invoice_no MEDIUMINT UNSIGNED DEFAULT NULL, invoice_pdf LONGBLOB, PRIMARY KEY(id) ); INSERT INTO customer_invoice (id, invoice_no) VALUES (1, 1), (2, 2), (3, 3), (4, 4), (5, 5); # Ok, here's the beef EXPLAIN SELECT co.id FROM customer_order AS co; EXPLAIN SELECT co.id FROM customer_order AS co ORDER BY co.id; EXPLAIN SELECT co.id, ci.invoice_no FROM customer_order AS co LEFT JOIN customer_invoice AS ci ON ci.id=co.invoice; EXPLAIN SELECT co.id, ci.invoice_no FROM customer_order AS co LEFT JOIN customer_invoice AS ci ON ci.id=co.invoice ORDER BY co.id;
底部有四个EXPLAIN语句.前两个完全符合您的期望:
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | 1 | SIMPLE | co | index | NULL | PRIMARY | 3 | NULL | 5 | Using index | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
第三个已经很有趣 – 请注意customer_order中的主键如何不再使用:
+----+-------------+-------+--------+---------------+---------+---------+-----------------------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+--------+---------------+---------+---------+-----------------------------+------+-------------+ | 1 | SIMPLE | co | ALL | NULL | NULL | NULL | NULL | 5 | | | 1 | SIMPLE | ci | eq_ref | PRIMARY | PRIMARY | 3 | index_test_gutza.co.invoice | 1 | Using index | +----+-------------+-------+--------+---------------+---------+---------+-----------------------------+------+-------------+
然而,第四个是zinger – 简单地在主键上添加ORDER BY导致customer_order上的filesort(这是预期的,因为它已经在上面被弄糊涂了):
+----+-------------+-------+--------+---------------+---------+---------+-----------------------------+------+----------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+--------+---------------+---------+---------+-----------------------------+------+----------------+ | 1 | SIMPLE | co | ALL | NULL | NULL | NULL | NULL | 5 | Using filesort | | 1 | SIMPLE | ci | eq_ref | PRIMARY | PRIMARY | 3 | index_test_gutza.co.invoice | 1 | Using index | +----+-------------+-------+--------+---------------+---------+---------+-----------------------------+------+----------------+
文件排序!而且我从不使用除customer_order表中的主键以外的任何内容进行排序,以及在customer_invoice表中使用JOIN的主键.那么,在所有好的和正确的名义中,为什么它会突然切换到filesort?!更重要的是,我该如何避免这种情况?为了记录,我很乐意接受一个记录在案的答案,解释为什么这是无法避免的(如果是这样的话.)
正如您现在可能怀疑的那样,这实际上是在生产中发生的,虽然这些表并不是很大(只有数百条记录),但发票表上的文件排序(包含PDF文件)在运行时会终止服务器类似于上面的查询(我需要知道哪些订单已经发出发票,哪些不是).
在你问之前,我设计了数据库,我认为我可以安全地将PDF文件存储在该表中,因为我从来不需要任何搜索查询 – 我总是有它的主键!
更新(评论概要)
以下是下面评论中建议的概要,因此您不必阅读所有内容:
> *你应该在customer_order.invoice *上添加一个键 – 我实际上在生产中尝试过,它没有区别(因为它不应该)
>您应该使用USE INDEX – 尝试过,不起作用.我也试过FORCE INDEX – 没有结果(没有任何改变)
>你过度简化了用例,我们需要实际的生产查询 – 我可能在第一次迭代中剥离了一些,所以我更新了它(我刚刚在SELECT中添加了ci.invoice_no,用于最后几次查询).为了记录,如果有人真的很好奇,这里是生产查询,完全一样(这将检索订单的最后一页):
SELECT corder.id, corder.public_id, CONCAT(buyer.fname," ",buyer.lname) AS buyer_name, corder.status, corder.payment, corder.reserved AS R, corder.tracking_id!="" as A, corder.payment_received as pay_date, invoice.invoice_no AS inv, invoice.receipt_no AS rec, invoice.public AS pub_inv, proforma.proforma_no AS prof, proforma.public AS pub_pf, corder.rating, corder.rating_comments!="" AS got_comment FROM corder LEFT JOIN user as buyer ON buyer.id=corder.buyer LEFT JOIN invoice as invoice ON invoice.id=corder.invoice LEFT JOIN invoice as proforma ON proforma.id=corder.proforma ORDER BY id DESC LIMIT 400, 20;
上面的查询(这也是我在生产中运行的那个)大约需要14秒才能运行.这是在生产中执行的简化查询,如上面的用例所示:
SELECT corder.id, invoice.invoice_no FROM corder LEFT JOIN invoice ON invoice.id=corder.invoice ORDER BY corder.id DESC LIMIT 400, 20;
这个需要13秒才能运行.请注意,只要我们谈论结果的最后一页(我们是),LIMIT就没有任何区别.也就是说,在涉及filesort时,检索最后12个结果或所有412个结果之间绝对没有显着差异.
结论
ypercube的答案不仅正确,而且不幸的是它似乎是唯一合法的答案.我试图进一步将条件与字段分开,因为SELECT * FROM corder子查询最终可能涉及大量数据,如果corder本身包含LONGBLOB(并且在子查询中复制主查询中的字段是不优雅的),但不幸的是它没有似乎工作:
SELECT corder.id, corder.public_id, CONCAT(buyer.fname," ",buyer.lname) AS buyer_name, corder.status, corder.payment, corder.reserved AS R, corder.tracking_id != "" AS A, corder.payment_received AS pay_date, invoice.invoice_no AS inv, invoice.receipt_no AS rec, invoice.public AS pub_inv, proforma.proforma_no AS prof, proforma.public AS pub_pf, corder.rating, corder.rating_comments!="" AS got_comment FROM corder LEFT JOIN user as buyer ON buyer.id = corder.buyer LEFT JOIN invoice AS invoice ON invoice.id = corder.invoice LEFT JOIN invoice AS proforma ON proforma.id = corder.proforma WHERE corder.id IN ( SELECT id FROM corder ORDER BY id DESC LIMIT 400,20 ) ORDER BY corder.id DESC;
此操作失败,并显示以下错误消息:
ERROR 1235 (42000): This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'
我使用的是MySQL 5.1.61,这在5.1系列中是最近才出现的(显然这在5.5.x中也不支持).
解决方法:
你可以尝试这个版本(它基本上首先得到的是corder表的420行,保留其中的20行然后执行3个外连接):
SELECT
corder.id,
corder.public_id,
CONCAT(buyer.fname," ",buyer.lname) AS buyer_name,
corder.status,
corder.payment,
corder.reserved AS R,
corder.tracking_id != "" AS A,
corder.payment_received AS pay_date,
invoice.invoice_no AS inv,
invoice.receipt_no AS rec,
invoice.public AS pub_inv,
proforma.proforma_no AS prof,
proforma.public AS pub_pf,
corder.rating,
corder.rating_comments!="" AS got_comment
FROM
( SELECT *
FROM corder
ORDER BY
id DESC
LIMIT 400, 20
)
AS corder
LEFT JOIN user as buyer ON buyer.id = corder.buyer
LEFT JOIN invoice AS invoice ON invoice.id = corder.invoice
LEFT JOIN invoice AS proforma ON proforma.id = corder.proforma
ORDER BY
corder.id DESC ;
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