三张关联表，大表；单次查询耗时400s，有group by order by 如何优化

问题SQL：

select

p.person_id as personId,

p.person_name as personName,

p.native_place as nativePlace,

ci.company_name as companyName,

pp.seal_number as sealNumber,

GROUP_CONCAT(pp.major) as major,

pp.register_name as registerName

from qyt_person p

left join qyt_person_practising pp on p.person_id=pp.person_id

left join qyt_company_info ci on p.company_id=ci.company_id

group by p.person_id,pp.register_name

order by p.create_time desc

limit 1,10

SQL总耗时393秒，通过Explain分析，发现为200万数据的表建立了临时表，且做了一次排序操作

通过查看SQL运行分析，也看出来，构造临时表耗时106秒，排序用了285秒（没索引的排序慢）

解决思路：根据业务需求再次审视如何减少数据量

1、业务需求：最新人员可以先取出来10名

2、取出来后再关联查询他们所在企业，所获证书（有索引，查询快）

3、这种小的临时表排序的耗时就可以接受了

4、SQL语句有子查询功能，可以把200万的表数据缩减为10人的小表

SQL语句如下：

select

p.person_id as personId,

p.person_name as personName,

p.native_place as nativePlace,

p.create_time as createTime,

ci.company_name as companyName,

pp.seal_number as sealNumber,

GROUP_CONCAT(pp.major) as major,

pp.register_name as registerName

from (SELECT person_id, person_name, native_place,company_id ,create_time from qyt_person order by create_time desc limit 0,10) p

left join qyt_person_practising pp on p.person_id=pp.person_id

left join qyt_company_info ci on p.company_id=ci.company_id

group by p.person_id,pp.register_name

ORDER BY p.create_time desc

limit 0,10

结果完美！响应时间为0.017秒

通过Explain分析，临时表就10条记录，所以处理耗时非常少

大块的时间还是损耗在构造临时表和排序上，但是这个时间必须得损失

原文地址：https://www.cnblogs.com/njxuyufeng/p/11394058.html