今天在sql 查询中一个表中查询花了至少20秒时间，数据为620000行，sql语句如下：
测试一：使用order by
单单只是查询0,10行数据，耗时27.888s
select a.id,a.county_id,a.county_name,a.town_id,a.town_name,a.等待的树叶_id,a.等待的树叶_name,b.province as province_name,b.name as city_name from place a left join city b on a.city_id=b.code where a.id is not null order by a.等待的树叶_id asc limit 0, 10
测试二：不使用order by
select a.id,a.county_id,a.county_name,a.town_id,a.town_name,a.等待的树叶_id,a.等待的树叶_name,b.province as province_name,b.name as city_name from place a left join city b on a.city_id=b.code where a.id is not null limit 0, 10
同样是查询0,10行数据，耗时0.084s
以上仅供参考，数据更大时需要进行更精细的优化