MySQL Index & execution plan

One , Indexes

1. What is index
An index is a combination of one or more columns in a database table , Its function is to improve the query speed of data in the table .
A catalogue similar to a book , Easy and fast positioning , Find the specified content .
2. Advantages and disadvantages of index
advantage ： Speed up data query .
shortcoming ： The time to create and maintain indexes has increased , At the same time, it takes up hard disk space .

3. Index classification

1. General index ： Is the most basic index , It has no restrictions ;

2. unique index ： Similar to the previous normal index , The difference is that ： The value of the index column must be unique , But you can have an empty value . If it's a composite index , The combination of column values must be unique ;

3. primary key ： Is a special unique index , A table can only have one primary key , No null values are allowed ;

4. Composite index ： An index created on multiple fields , Only the first field that created the index is used in the query criteria , Indexes are used . When using a composite index, follow the leftmost prefix set ;

5. Full-text index ： Use FULLTEXT Parameters can be set , Full text index can only be created in CHAR,VARCHAR,TEXT Type field on . The main function is to improve the speed of querying larger string types ; Only MyISAM The engine supports the index ,MySQL The default engine does not support ;mysql5.7+

4. Create index
CREATE [UNIQUE|FULLTEXT] INDEX Index name ON Table name ( Field name [( length )][ASC|DESC])

5. Modify the index
ALTER TABLE Table name ADD [UNIQUE|FULLTEXT] INDEX Index name ( Field name [( length )][ASC|DESC])

6. Delete index
DROP INDEX Index name ON Table name
7. Case study
Additional explanation ：

Bulk import ：LOAD DATA INFILE（ recommend ）

You can go through first SELECT INTO OUTFILE The way , Export data to Mysql Of C:\ProgramData\MySQL\MySQL Server 5.5\data Under the table of contents , Re pass LOAD DATA INFILE Mode import .

1. select * from Table name into outfile ‘/ file name .sql’;
2. load data infile ‘/ file name .sql’ into table Table name ( Name 1,…);

1)  Common index cases 
create index  Index name  on  Table name ( Field name )
 
select * from t_log where moduleid='100401666';
 
create index idx_moduleid on t_log(moduleid);
 
2)  Unique index case ( The corresponding value cannot be the same )
create unique index  Index name  on  Table name ( Field name )
 
create unique index idx_url on t_log(url)
 
3)  Primary key index case 
create table  Table name (
   id int not null auto_increment primary key,
   ...
)
 
-- use id Query efficiency will be faster 
select * from t_log where id='212121213131'
 
 
4)  Combined index cases   The leftmost segment matching principle 
create index  Index name  on  Table name ( Field name 1, Field name 2,...)
 
 Some fields in brackets will match the index 
create index idx_userid_moduleid_url on t_log(userid,moduleid,url);
 
EXPLAIN SELECT * from t_log -- This line of code does not execute the index  ( Because there is no matching index )
EXPLAIN SELECT * from t_log where userid='';
EXPLAIN SELECT * from t_log where userid='' and moduleid='';
EXPLAIN SELECT * from t_log where url='';

Two , Import and export

Add ：delete And turncate The difference between

delete Time consuming 6. More than a second
delete from t_log ----- Long time consuming

TRUNCATE Time consuming 0.147 second （ Fast efficiency ）
The index will be deleted at the same time
TRUNCATE TABLE t_log# 3、 ... and , View

1. Using tools
（ similar Sqlyog、Navicat Wait to import and export data ） Longer time consuming

step ：

Right click Run SQL file
Select the file you want to import
Long import time

If there is no connection tool or the connection tool cannot be used under certain circumstances , Then we can import and export data by command

– notes ： Short time , Efficient

2. Use mysqldump Import and export
（1） export
1.1 Export table data and table structure
mysqldump -u user name -p password Database name > Database name .sql ( The name is optional )

     There is no need to enter the client when executing commands , But the database user and password are required to execute the command 
    
 Execute statement ：#/usr/local/mysql/bin mysqldump -uroot -pabc >abc.sql
 After typing enter, you will be prompted to output the password 

1.2  Export only table structure 

mysqldump -u user name  -p password  -d  Database name  >  Database name .sql

#mysqldump -uroot -p -d abc > abc.sql


 notes ： The exported data is in mysql Of bin Under the table of contents 

（2） Import

 Be careful ： First, create an empty database 
mysql>create database abc;

2.1 Method 1

mysql>use abc;                   # Select database 
mysql>set names utf8;            # Set database encoding 
mysql>source /home/abc/abc.sql;  # Import data 

2.2 Method 2

mysql -u user name  -p password   Database name  <  Database name .sql
#mysql -uabc_f -p abc < abc.sql

3.LOAD DATA INFILE

You can go through first SELECT INTO OUTFILE The way , Export data to Mysql Of C:\ProgramData\MySQL\MySQL Server 5.5\data Under the table of contents , Re pass LOAD DATA INFILE Mode import .

1. select * from Table name into outfile ‘/ file name .sql’;
2. load data infile ‘/ file name .sql’ into table Table name ( Name 1,…);

select * from t_log into outfile ‘E:/989898.sql’;
select * from t_log into outfile ‘D:/989898.sql’;

load data infile ‘E:/989898.sqll’ into table t_log(id,ip,userid,moduleid,content,createdate,url);
summary ： Take less time

3、 ... and , View

1. What is a view
（1) A view is a virtual table , A table derived from one or more tables in a database .
（2) The definition of view is stored in the database , There is no data stored in view , The data is stored in the original table .
（3) When using views to query data , The database system will take the corresponding data from the original table .
2. The function of view
（1) Make the operation simple .
（2) Increase data security .
（3) Improve the logical independence of tables .
3. Basic grammar
sentence ：CREATE VIEW View name AS SELECT
Tips ： The premise of successful view creation is that duplicate columns cannot appear

      for example ：CREATE VIEW
     
     CREATE VIEW v_student as
     select s.*,sc.score,c.*,t.tname from t_student s,t_score sc,t_course c,t_teacher t
     where s.sid=sc.sid and sc.cid-c.cid and c.tid=t.tid
     
     select * from v_student
      

Four , Implementation plan

Example 1 ：explain

EXPLAIN select * from t_user
EXPLAIN select * from t_user u left join t_role r on u.roleid = r.roleid
EXPLAIN select * from t_user u where u.roleid = (select roleid from t_role where rolename = ' Administrators ')

Example 2 ：id

id identical , From top to bottom
Purpose ： Lies in knowing sql What is the execution step of the statement left outreach

EXPLAIN select * from t_user u left join t_role r on u.roleid = r.roleid

Right outreach

EXPLAIN select * from t_user u right join t_role r on u.roleid = r.roleid

Example 3 ：select_type

（1）SIMPLE( Simple SELECT, Don't use UNION Or subquery, etc )

EXPLAIN select * from t_user

（2）PRIMARY( If the query contains any complex sub parts , The outermost select Marked as PRIMARY)
SUBQUERY( First in subquery SELECT)

EXPLAIN select * from t_user u where u.roleid = (select roleid from t_role where rolenmae = ' Administrators ')

（3）UNION(UNION The second or later of SELECT sentence )
UNION RESULT: from union Table to get the result SELECT

EXPLAIN select * from t_user where id=3
union
select * from t_user where roleid = 3

Example 4 ： Use index correctly

（1） Use like When the sentence is ,% The index is only used on the right .
It works
Improve query efficiency

EXPLAIN select * from t_user where username like ' Zhang '

（2）or The index will become invalid only if there is an unindexed column in the condition
Invalid

EXPLAIN select * from t_user where password = 123456 or username=' Zhang San '

It works

EXPLAIN select * from t_user where password = 123456 and username=' Zhang San '

（3） The type of condition is inconsistent
Invalid

EXPLAIN select * from t_user where username = 1

It works

EXPLAIN select * from t_user where username = '1'