当我发现两个不同的查询时，我看到一个奇怪的Mysql行为，其中一个小差异，一个左连接的位置。
慢查询：
SELECT i.id_affiliate, i.id_franchising, i.Codice
FROM network_configuration_affiliate AS c
INNER JOIN franchising AS fr ON fr.id = c.id_franchising
INNER JOIN network_selected_car AS i ON c.id_affiliate = i.id_affiliate
INNER JOIN (
select T1.id_car, T1.id_network, T1.id_franchising, T1.id_agencie
from network_car_destinations as T1
where T1.id_network='12' and ( T1.id_franchising = 968 or T1.id_franchising = 974 )
) AS n ON n.id_franchising=i.id_franchising AND n.id_car=i.id_car AND c.id_network=n.id_network
INNER JOIN affiliate_tipologies AS t ON t.id_tipology_ag=i.idCategory AND t.id_franchising=i.id_franchising
INNER JOIN network_assoc_tipologies AS p ON p.id_network=c.id_network AND p.id_default=t.id_tipology_net
LEFT JOIN network_conf_users as ce on ce.id_affiliate = i.id_affiliate and ce.id_user = i.id_user
WHERE c.id_network='12' and c.code_affiliate='69842' AND c.configured = 'yes' AND i.Code_car not like '' and p.code != '0'
GROUP BY i.Code_car
快速：
select T2.* from (
SELECT i.`id_affiliate`, i.id_franchising, i.Code_car, i.id_user
FROM network_configuration_affiliate AS c
INNER JOIN franchising AS fr ON fr.id = c.id_franchising
INNER JOIN network_selected_car AS i ON c.`id_affiliate` = i.`id_affiliate`
INNER JOIN (
select T1.id_car, T1.id_network, T1.id_franchising, T1.id_agencie
from network_car_destinations as T1
where T1.id_network='12' and ( T1.id_franchising = 968 or T1.id_franchising = 974 )
) AS n ON n.id_franchising=i.id_franchising AND n.id_car=i.id_car AND c.id_network=n.id_network
INNER JOIN affiliate_tipologies AS t ON t.id_tipology_ag=i.idCategory AND t.id_franchising=i.id_franchising
INNER JOIN network_assoc_tipologies AS p ON p.id_network=c.id_network AND p.id_default=t.id_tipology_net
WHERE c.id_network='12' and c.code_affiliate='69842' AND c.configured = 'yes' AND i.Code_car not like ''
GROUP BY i.Code_car
) as T2
LEFT JOIN network_conf_users as ce on ce.`id_affiliate` = T2.`id_affiliate` and ce.id_user = T2.id_user
WHERE c.id_network='12' and c.code_affiliate='69842' AND c.configured = 'yes' AND i.Code_car not like '' and p.code != '0'
GROUP BY i.Code_car
EXPLAIN几乎相同，两个查询的结果相同，但第一个查询结束时间为20秒，第二个查询结果为0.02，左连接的位置如何影响查询的执行？
EXPLAIN - 慢查询：
id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY c index_merge id_affiliate,id_network,configured,code_affiliate code_affiliate,id_network 203,5 NULL 1 100.00 Using intersect(code_affiliate,id_network); Using where; Using temporary; Using filesort
1 PRIMARY fr eq_ref PRIMARY PRIMARY 4 c.id_franchising 1 100.00 Using index
1 PRIMARY <derived2> ref <auto_key1> <auto_key1> 5 const 10 100.00 Using where
1 PRIMARY t ref id_franchising,id_tipology_ag,id_tipology_net id_franchising 5 n.id_franchising 13 100.00 Using where
1 PRIMARY p ref id_network,id_default id_default 5 t.id_tipology 26 100.00 Using where
1 PRIMARY i ref id_car,id_affiliate,id_franchising,idCategory id_car 5 n.id_car 3 100.00 Using where
1 PRIMARY ce ALL id_affiliate,id_user NULL NULL NULL 4 75.00 Using where; Using join buffer (Block Nested Loop)
2 DERIVED T1 ref id_franchising,id_network id_network 5 const 136952 100.00 Using where
显示警告：
Level Code Message
Note 1003 /* select#1 */
select `i`.`id_affiliate` AS `id_affiliate`,
`i`.`id_franchising` AS `id_franchising`,
`i`.`Code_car` AS `Code_car`
from `network_configuration_affiliate` `c`
join `franchising` `fr`
join `network_selected_car` `i`
join (/* select#2 */
select `T1`.`id_car` AS `id_car`,
`T1`.`id_network` AS `id_network`,
`T1`.`id_franchising` AS `id_franchising`,
`T1`.`id_agencie` AS `id_agencie`
from `network_car_destinations` `T1`
where ((`T1`.`id_network` = '12')
and ((`T1`.`id_franchising` = 968)
or (`T1`.`id_franchising` = 974)))
) `n`
join `affiliate_tipologies` `t`
join `network_assoc_tipologies` `p`
left join `network_conf_users` `ce` on(((`ce`.`id_user` = `i`.`id_user`)
and (`ce`.`id_affiliate` = `c`.`id_affiliate`))
)
where ((`fr`.`id` = `c`.`id_franchising`)
and (`i`.`id_affiliate` = `c`.`id_affiliate`)
and (`i`.`id_car` = `n`.`id_car`)
and (`t`.`id_franchising` = `n`.`id_franchising`)
and (`i`.`id_franchising` = `n`.`id_franchising`)
and (`i`.`idCategory` = `t`.`id_tipology_ag`)
and (`p`.`id_default` = `t`.`id_tipology_net`)
and (`n`.`id_network` = `c`.`id_network`)
and (`p`.`id_network` = `c`.`id_network`)
and (`c`.`configured` = 'yes')
and (`c`.`code_affiliate` = '69842')
and (`c`.`id_network` = '12')
and (not((`i`.`Code` like '')))
and (`p`.`code` <> '0')
)
group by `i`.`Code_car`
快速查询：
id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY <derived2> ALL NULL NULL NULL NULL 7280 100.00 NULL
1 PRIMARY ce ALL id_affiliate,id_user NULL NULL NULL 4 75.00 Using where; Using join buffer (Block Nested Loop)
2 DERIVED c index_merge id_affiliate,id_network,configured,code_affiliate code_affiliate,id_network 203,5 NULL 1 100.00 Using intersect(codice_affiliato,id_network); Using where; Using temporary; Using filesort
2 DERIVED fr eq_ref PRIMARY PRIMARY 4 c.id_franchising 1 100.00 Using index
2 DERIVED <derived3> ref <auto_key1> <auto_key1> 5 const 10 100.00 Using where
2 DERIVED i ref id_car,id_affiliate,id_franchising,idCategory id_car 5 n.id_car 2 100.00 Using where
2 DERIVED t ref id_franchising,id_tipology_ag,id_tipology_net id_franchising 5 n.id_franchising 14 100.00 Using where
2 DERIVED p ref id_network,id_default id_default 5 t.id_tipology_net 26 100.00 Using where
3 DERIVED T1 ref id_franchising,id_network id_network 5 const 133324 100.00 Using where
显示警告：
Level Code Message
Note 1003 /* select#1 */
select `T2`.id_affiliate AS id_affiliate,
`T2`.`id_franchising` AS `id_franchising`,
`T2`.Code_car AS Code_car,`T2`.id_user AS id_user
from (/* select#2 */
select `i`.id_affiliate AS id_affiliate,
`i`.`id_franchising` AS `id_franchising`,
`i`.Code_car AS Code_car,`i`.id_user AS id_user
from network_configuration_affiliate `c`
join `franchising` `fr`
join network_selected_car `i`
join (/* select#3 */
select `T1`.id_car AS id_car,
`T1`.`id_network` AS `id_network`,
`T1`.`id_franchising` AS `id_franchising`,
`T1`.`id_agencie` AS `id_agencie`
from network_car_destinations `T1`
where ((`T1`.`id_network` = '12')
and ((`T1`.`id_franchising` = 968)
or (`T1`.`id_franchising` = 974)))
) `n`
join affiliate_tipologies `t`
join network_assoc_tipologies `p`
where ((`fr`.`id` = `c`.`id_franchising`)
and (`i`.id_affiliate = `c`.id_affiliate)
and (`i`.id_car = `n`.id_car)
and (`i`.`id_franchising` = `n`.`id_franchising`)
and (`t`.`id_franchising` = `n`.`id_franchising`)
and (`t`.`id_tipology_ag` = `i`.`idCategory`)
and (`p`.`id_default` = `t`.`id_tipology_net`)
and (`n`.`id_network` = `c`.`id_network`)
and (`p`.`id_network` = `c`.`id_network`)
and (`c`.`configured` = 'yes')
and (`c`.`code_affiliate` = '69842')
and (`c`.`id_network` = '12')
and (not((`i`.Code_car like '')))
)
group by `i`.Code_car
) `T2`
left join network_conf_users `ce` on(((`ce`.id_affiliate = `T2`.id_affiliate)
and (`ce`.id_user = `T2`.id_user))
)
where 1
答案 0 :(得分：0)
左连接找到左边的那些字段并从右边找到那些字段，而内部连接将查找所有字段。通过不需要组织额外的列来保持简单，查询完成得更快。
答案 1 :(得分：0)
请告诉我们EXPLAINs。
如果优化程序确定它没有任何区别，则会将LEFT JOIN变为JOIN。请EXPLAIN EXTENDED SELECT ...然后立即SHOW WARNINGS;，以便我们查看是否发生了这种情况。
优化工具会尝试JOINing的各种订单（在没有LEFT或RIGHT的情况下）。因此，如果LEFT真的是多余的，我希望EXPLAINs能够使表格具有相同的顺序。
通常情况下，优化工具会以示例中的“派生”表n开头。但它可能隐藏在LEFT。
你有“复合”索引吗？这些可能是有益的：
T1: INDEX(id_network, id_franchising) -- in this order
c: INDEX(id_network, code_affiliate, configured)
<强>修订
随着查询的最近更改，我看到了这种模式：
慢速：
SELECT ...
FROM ...
JOIN ...
GROUP BY ...
快速：
SELECT ...
FROM ( SELECT ...
FROM ...
GROUP BY ... )
JOIN ...
（LEFT对问题只是部分重要;任何JOIN都可能出现此问题。）
我称之为慢速“膨胀 - 放气”。我的意思是，首先它确实加入了连接，从而增加了行数。然后它做了一个GROUP BY，它将结果缩小了。
在进行最终加入之前，快速放气，导致整体效果降低。
通过使用“复合”索引，几乎总能改进 index merge intersect。在这种情况下，我建议使用c。 （这会加速查询的两个版本。）