重點關註:

1) 沒有查詢條件，或者查詢條件沒有建立索引

例如:

select * from tab; 全表掃描。
select * from tab where 1=1;

在業務數據庫中，特別是數據量比較大的表。
是沒有全表掃描這種需求。　　

　　1、對用戶查看是非常痛苦的。
　　2、對服務器來講毀滅性的。

例如:

select * from tab;
SQL改寫成以下語句：
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列沒有索引

改：
    
 
1、換成有索引的列作為查詢條件
    2、將name列建立索引

2) 查詢結果集是原表中的大部分數據，應該是25％以上。

查詢的結果集，超過了總數行數25%，優化器覺得就沒有必要走索引了。

假如：

tab表 id，name id:1-100w ，id列有索引

select * from tab where id>500000;

如果業務允許，可以使用limit控制。

怎麽改寫 ？
結合業務判斷，有沒有更好的方式。如果沒有更好的改寫方案
盡量不要在mysql存放這個數據了。放到redis裏面。

3) 索引本身失效，統計數據不真實

索引有自我維護的能力。
對於表內容變化比較頻繁的情況下，有可能會出現索引失效。

4) 查詢條件使用函數在索引列上，或者對索引列進行運算，運算包括(+，-，*，/，! 等)

例子：

錯誤的例子：select * from test where id-1=9; 
正確的例子：select * from test where id=10;

算術運算
函數運算

desc select * from blog_userinfo where DATE_FORMAT(last_login,‘%Y-%m-%d‘) >= ‘2019-01-01‘;

子查詢

5)隱式轉換導致索引失效.這一點應當引起重視.也是開發中經常會犯的錯誤.

select * from t1 where telnum=110;


這樣會導致索引失效. 錯誤的例子：
 
------------------------
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‘;


-----
單獨的>,<,in 有可能走，也有可能不走，和結果集有關，盡量結合業務添加limit
or或in  盡量改成union

EXPLAIN  SELECT * FROM teltab WHERE telnum   IN (‘110‘,‘119‘);
改寫成：

EXPLAIN SELECT * FROM teltab WHERE telnum=‘110‘
UNION ALL
SELECT * FROM teltab WHERE telnum=‘119‘

-----------------------------------

7) like "%_" 百分號在最前面不走

EXPLAIN SELECT * FROM teltab WHERE telnum LIKE ‘31%‘   走range索引掃描

EXPLAIN SELECT * FROM teltab WHERE telnum LIKE ‘%110‘  不走索引


%linux%類的搜索需求，可以使用elasticsearch 專門做搜索服務的數據庫產品

8) 單獨引用聯合索引裏非第一位置的索引列.作為條件查詢時不走索引.

列子：
復合索引：

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

走索引的情況測試：
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‘;

壓力測試

1、模擬數據庫數據

為了測試我們創建一個oldboy的庫創建一個t1的表，然後導入50萬行數據，腳本如下：

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}"

執行腳本：
sh slap.sh

2、檢查數據可用性

mysql -uroot -p123
select count(*) from oldboy.t1;

3、在沒有優化之前我們使用mysqlslap來進行壓力測試

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

在數據庫查詢中不走索引的情況與壓力測試