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在数据库查询中不走索引的情况与压力测试

时间:2019-01-16 00:20:11      阅读:349      评论:0      收藏:0      [点我收藏+]

标签:log   一个   prim   plain   orm   userinfo   函数   localhost   DBName   

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

在数据库查询中不走索引的情况与压力测试

标签:log   一个   prim   plain   orm   userinfo   函数   localhost   DBName   

原文地址:https://www.cnblogs.com/zhaijihai/p/10274913.html

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