重点关注:
1) 没有查询条件，或者查询条件没有建立索引
例如:
select * from tab; 全表扫描。 select * from tab where 1=1;
在业务数据库中，特别是数据量比较大的表。
是没有全表扫描这种需求。
1、对用户查看是非常痛苦的。
2、对服务器来讲毁灭性的。
例如:
select * from tab; SQL改写成以下语句： selec * from tab order by price limit 10 需要在price列上建立索引
select * from tab where name=‘zhangsan‘ name列没有索引
改：
1、换成有索引的列作为查询条件
2、将name列建立索引
2) 查询结果集是原表中的大部分数据，应该是25％以上。
查询的结果集，超过了总数行数25%，优化器觉得就没有必要走索引了。
假如：
tab表 id，name id:1-100w ，id列有索引
select * from tab where id>500000;
如果业务允许，可以使用limit控制。
怎么改写 ？
结合业务判断，有没有更好的方式。如果没有更好的改写方案
尽量不要在mysql存放这个数据了。放到redis里面。
3) 索引本身失效，统计数据不真实
索引有自我维护的能力。
对于表内容变化比较频繁的情况下，有可能会出现索引失效。
4) 查询条件使用函数在索引列上，或者对索引列进行运算，运算包括(+，-，*，/，! 等)
例子：
错误的例子：select * from test where id-1=9; 正确的例子：select * from test where id=10;
算术运算
函数运算
desc select * from blog_userinfo where DATE_FORMAT(last_login,‘%Y-%m-%d‘) >= ‘2019-01-01‘;
子查询
5)隐式转换导致索引失效.这一点应当引起重视.也是开发中经常会犯的错误.
select * from t1 where telnum=110; 这样会导致索引失效. 错误的例子： ------------------------ mysql> alter table tab add index inx_tel(telnum); Query OK, 0 rows affected (0.03 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> mysql> desc tab; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | | telnum | varchar(20) | YES | MUL | NULL | | +--------+-------------+------+-----+---------+-------+ 3 rows in set (0.01 sec) mysql> select * from tab where telnum=‘1333333‘; +------+------+---------+ | id | name | telnum | +------+------+---------+ | 1 | a | 1333333 | +------+------+---------+ 1 row in set (0.00 sec) mysql> select * from tab where telnum=1333333; +------+------+---------+ | id | name | telnum | +------+------+---------+ | 1 | a | 1333333 | +------+------+---------+ 1 row in set (0.00 sec) mysql> explain select * from tab where telnum=‘1333333‘; +----+-------------+-------+------+---------------+---------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+---------+---------+-------+------+-----------------------+ | 1 | SIMPLE | tab | ref | inx_tel | inx_tel | 63 | const | 1 | Using index condition | +----+-------------+-------+------+---------------+---------+---------+-------+------+-----------------------+ 1 row in set (0.00 sec) mysql> explain select * from tab where telnum=1333333; +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | tab | ALL | inx_tel | NULL | NULL | NULL | 2 | Using where | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec) mysql> explain select * from tab where telnum=1555555; +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | tab | ALL | inx_tel | NULL | NULL | NULL | 2 | Using where | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec) mysql> explain select * from tab where telnum=‘1555555‘; +----+-------------+-------+------+---------------+---------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+---------+---------+-------+------+-----------------------+ | 1 | SIMPLE | tab | ref | inx_tel | inx_tel | 63 | const | 1 | Using index condition | +----+-------------+-------+------+---------------+---------+---------+-------+------+-----------------------+ 1 row in set (0.00 sec) mysql> ---------------------------------------
6) <> ，not in 不走索引
EXPLAIN SELECT * FROM teltab WHERE telnum <> ‘110‘; EXPLAIN SELECT * FROM teltab WHERE telnum NOT IN (‘110‘,‘119‘); ------------ mysql> select * from tab where telnum <> ‘1555555‘; +------+------+---------+ | id | name | telnum | +------+------+---------+ | 1 | a | 1333333 | +------+------+---------+ 1 row in set (0.00 sec) mysql> explain select * from tab where telnum <> ‘1555555‘; ----- 单独的>,<,in 有可能走，也有可能不走，和结果集有关，尽量结合业务添加limit or或in 尽量改成union EXPLAIN SELECT * FROM teltab WHERE telnum IN (‘110‘,‘119‘); 改写成： EXPLAIN SELECT * FROM teltab WHERE telnum=‘110‘ UNION ALL SELECT * FROM teltab WHERE telnum=‘119‘ -----------------------------------
7) like "%_" 百分号在最前面不走
EXPLAIN SELECT * FROM teltab WHERE telnum LIKE ‘31%‘ 走range索引扫描 EXPLAIN SELECT * FROM teltab WHERE telnum LIKE ‘%110‘ 不走索引 %linux%类的搜索需求，可以使用elasticsearch 专门做搜索服务的数据库产品
8) 单独引用联合索引里非第一位置的索引列.作为条件查询时不走索引.
列子： 复合索引： DROP TABLE t1 CREATE TABLE t1 (id INT,NAME VARCHAR(20),age INT ,sex ENUM(‘m‘,‘f‘),money INT); ALTER TABLE t1 ADD INDEX t1_idx(money,age,sex); DESC t1 SHOW INDEX FROM t1 走索引的情况测试： EXPLAIN SELECT NAME,age,sex,money FROM t1 WHERE money=30 AND age=30 AND sex=‘m‘; EXPLAIN SELECT NAME,age,sex,money FROM t1 WHERE money=30 AND age=30 ; EXPLAIN SELECT NAME,age,sex,money FROM t1 WHERE money=30 AND sex=‘m‘; ----->部分走索引 不走索引的： EXPLAIN SELECT NAME,age,sex,money FROM t1 WHERE age=20 EXPLAIN SELECT NAME,age,sex,money FROM t1 WHERE age=30 AND sex=‘m‘; EXPLAIN SELECT NAME,age,sex,money FROM t1 WHERE sex=‘m‘;
压力测试
1、模拟数据库数据
为了测试我们创建一个oldboy的库创建一个t1的表，然后导入50万行数据，脚本如下：
vim slap.sh
#!/bin/bash
HOSTNAME="localhost"
PORT="3306"
USERNAME="root"
PASSWORD="123"
DBNAME="oldboy"
TABLENAME="t1"
#create database
mysql -h ${HOSTNAME} -P${PORT} -u${USERNAME} -p${PASSWORD} -e "drop database if exists ${DBNAME}"
create_db_sql="create database if not exists ${DBNAME}"
mysql -h ${HOSTNAME} -P${PORT} -u${USERNAME} -p${PASSWORD} -e "${create_db_sql}"
#create table
create_table_sql="create table if not exists ${TABLENAME}(stuid int not null primary key,stuname varchar(20) not null,stusex char(1)
not null,cardid varchar(20) not null,birthday datetime,entertime datetime,address varchar(100)default null)"
mysql -h ${HOSTNAME} -P${PORT} -u${USERNAME} -p${PASSWORD} ${DBNAME} -e "${create_table_sql}"
#insert data to table
i="1"
while [ $i -le 500000 ]
do
insert_sql="insert into ${TABLENAME} values($i,‘alexsb_$i‘,‘1‘,‘110011198809163418‘,‘1990-05-16‘,‘2017-09-13‘,‘oldboyedu‘)"
mysql -h ${HOSTNAME} -P${PORT} -u${USERNAME} -p${PASSWORD} ${DBNAME} -e "${insert_sql}"
let i++
done
#select data
select_sql="select count(*) from ${TABLENAME}"
mysql -h ${HOSTNAME} -P${PORT} -u${USERNAME} -p${PASSWORD} ${DBNAME} -e "${select_sql}"
执行脚本：
sh slap.sh
2、检查数据可用性
mysql -uroot -p123 select count(*) from oldboy.t1;
3、在没有优化之前我们使用mysqlslap来进行压力测试
mysqlslap --defaults-file=/etc/my.cnf --concurrency=100 --iterations=1 --create-schema=‘oldboy‘ --query="select * from oldboy.t1 where stuname=‘alexsb_100‘" engine=innodb --number-of-queries=2000 -uroot -p123 -verbose
原文地址：https://www.cnblogs.com/zhaijihai/p/10274913.html
时间： 2024-11-05 15:53:08