先前一直對sql語句優化沒有太在乎，最近**使用者量增加，發現資料庫壓力很大自己寫的語句啊，新來的架構師對我們語句一一優化，我總結如下（不斷更新。。。）
（1）優化前的語句：
# query_time: 5.967435 lock_time: 0.000129 rows_sent: 1 rows_examined: 803401
set timestamp=1286843575;
select livemessag0_.id as id38_, livemessag0_.isactive as isactive38_, livemessag0_.content as content38_, livemessag0_.createtime as createtime38_, livemessag0_.userid as userid38_, livemessag0_.objectid as objectid38_, livemessag0_.recordid as recordid38_, livemessag0_.type as type38_ from live_message livemessag0_ where (livemessag0_.objectid in (select livescrip1_.id from live_scrip livescrip1_ where livescrip1_.senderid='ff8080812aebac2d012aef6491b3666d')) and livemessag0_.type=2 limit 6;
優化後的語句：
select livemessag0_.id as id38_,
livemessag0_.isactive as isactive38_,
livemessag0_.content as content38_,
livemessag0_.createtime as createtime38_,
livemessag0_.userid as userid38_,
livemessag0_.objectid as objectid38_,
livemessag0_.recordid as recordid38_,
livemessag0_.type as type38_
from live_scrip livescrip1_ left join
live_message livemessag0_
on livescrip1_.id=livemessag0_.objectid
where livescrip1_.senderid = 'ff8080812aebac2d012aef6491b3666d' and
livemessag0_.type = 2
limit 6;
[b]總結：盡量少用子查詢用表連線的方式代替（如果表連線不是太複雜的話），這樣優化後大概能減少1/3的時間，後來發現livemessag0_.objectid竟然沒有建立索引，暈菜[/b]
（2）優化前的慢查詢：
select r1.id,r1.username,r1.nickname,r1.img_name,r1.datecreated from acegi_users as r1 join (select round(rand() *(select max(num_id) from acegi_users)) as id) as r2 where r1.num_id >= r2.id and r1.id not in(select r.user_id from user_relations r where r.relation_user_id='ff8080812ad5ad7f012ad5aee5150005') and r1.id!='ff8080812ad5ad7f012ad5aee5150005' and r1.role='expert' order by r1.num_id limit 3;
優化後的sql：
select r1.id,
r1.username,
r1.nickname,
r1.img_name,
r1.datecreated
from acegi_users as r1
join
(select round(rand() *(select max(num_id) from acegi_users)) as id) as r2
where r1.num_id >= r2.id
andnot exists (select r0.user_id from (select r.user_id from user_relations r where
r.relation_user_id = 'ff8080812ad5ad7f012ad5aee5150005' union select 'ff8080812ad5ad7f012ad5aee5150005') r0 where r0.user_id=r1.id)
andr1.role = 'expert'
order by r1.num_id
limit 3;
[b]總結：少用not in 用not exists代替之[/b]
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