今天数据库负载就直线上升，数据库连接数撑爆。把语句抓出来一看，罪魁祸首是一条很简单的语句：SELECT * FROM eload_promotion_code WHERE 1 AND exp_time<1478782591 AND cishu=0 order by id desc limit 454660,20; 二话不说先把这个语句kill了，然后慢慢看怎么优化。
先看一下这个表的索引：
>show index from eload_promotion_code\G
*************************** 1. row ***************************
Table: eload_promotion_code
Non_unique: 0
Key_name: PRIMARY
Seq_in_index: 1
Column_name: id
Collation: A
Cardinality: 921642
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
Index_comment:
*************************** 2. row ***************************
Table: eload_promotion_code
Non_unique: 1
Key_name: idx_cishu_exp
Seq_in_index: 1
Column_name: cishu
Collation: A
Cardinality: 15
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
Index_comment:
*************************** 3. row ***************************
Table: eload_promotion_code
Non_unique: 1
Key_name: idx_cishu_exp
Seq_in_index: 2
Column_name: exp_time
Collation: A
Cardinality: 921642
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
Index_comment:
可以看到id为主键，idx_cishu_exp为（cishu,exp_time）的唯一索引
看一下这个语句的执行计划，可以看到排序没有用到索引
explain SELECT * FROM eload_promotion_code WHERE 1 AND exp_time<1478782591 AND cishu=0 order by id desc limit 454660,20\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: eload_promotion_code
type: ref
possible_keys: idx_cishu_exp
key: idx_cishu_exp
key_len: 4
ref: const
rows: 460854
Extra: Using where; Using filesort
1 row in set (0.00 sec)
将select * 换成select id后再看执行计划，可以用索引覆盖
>explain select id FROM eload_promotion_code WHERE 1 AND exp_time<1478782591 AND cishu=0 order by id desc limit 454660,20 \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: eload_promotion_code
type: range
possible_keys: idx_cishu_exp
key: idx_cishu_exp
key_len: 8
ref: NULL
rows: 460862
Extra: Using where; Using index; Using filesort
1 row in set (0.00 sec)
好吧，这个语句有救了，采用延时关联先取出id,然后根据id获取原表所需要的行，改写后的语句来了：select * from eload_promotion_code inner join (select id from eload_promotion_code where exp_time<1478782591 AND cishu=0 order by id desc limit 454660,20) as x on eload_promotion_code.id=x.id;
执行一下，0.3s出结果。
这样就算完了。
本文出自 “一直在路上” 博客，请务必保留此出处http://chenql.blog.51cto.com/8732050/1871575
原文地址：http://chenql.blog.51cto.com/8732050/1871575
