MySQL-01. Summary of database optimization and explain field knowledge in work

Database optimization - topic of conversation

Why optimize ？

• The throughput bottleneck of the system often appears in the access speed of the database
• There are more and more data in the database , Processing time will slow down accordingly
• Optimization principle ： Reduce resource use , Increase the reaction speed of the system

0.Redis cache

• For this kind of information that does not change frequently and has a large amount of data , Add it to the cache , Improve the access efficiency of the system
• Reducing database connections is an optimization tool , Some queries do not need to access the database , You can cache the server redis,elasticsearch Add cache , Reduce database connections .

1. Master slave copy , Read / write separation

• The program uses more databases and updates less , Consider using when there are many queries , Reduce database pressure , Improve performance .
• adopt MySQL Master slave copy ,curd go master The server , Inquiry slaver From the server , So it's reduced to just one MySQL Server pressure .

2. Database parameter configuration optimization

• For example, the default maximum number of connections is 100, Even if SQL No matter how well the statement is optimized , No matter how high the hardware configuration is , Request exceeds 100 Have to wait , This is the result of unreasonable configuration .
• https://cloud.tencent.com/developer/article/1582406

3. Optimize the design of database table structure

• Select a reasonable field data type （ The database will eventually be written to disk , The length of the field also affects the size of the disk I/O operation ）. such as ： The age of a person is indicated by an unsigned unsigned tinyint that will do , It is not necessary to use integer Length of data type ; User's mobile number 11 Bit length , It is not necessary to 255 Length .
• SQL Optimize , Use index

3.1 Indexes

3.1.1. In which case the index is used

• Frequent fields as query criteria
• Fields sorted in the query , Access speeds up
• The fields counted or grouped in the query
• The data column with foreign key must be indexed

3.1.2. Which case does not require indexing

• Frequently updated fields
• where Fields not used in conditions
• There are too few records
• Fields with duplicate data

4.MySQL Performance optimization artifact -Explain

4.1 brief introduction

MySQL Provides a Explain command , It's right select Statement analysis , And the output select Details of execution , For developers to optimize .

4.2explain Introduction to main formats

4.2.1 id

select Identifier , Query serial number , by sql The order in which statements are executed .

4.2.2 select_type

select_type	meaning
simple	ordinary SELECT sentence （ barring UNION Operation or subquery operation ）
primary	Indicates that this query is the outermost query
union	UNION In operation , Inside the query SELECT
subquery	First in subquery select
dependent union	UNION In operation , Inside the query SELECT（ The inner layer of the SELECT Sentence and outer SELECT Statement has dependencies ）
subquery	First in subquery select
union result	UNION Results of operation ,id The value is usually NULL

4.2.3 type

Best to worst

type value	meaning
const	It's found by index once , At most one matching row in a single table ,primary key perhaps unique index Search for
eq_ref	Uniqueness index , In a multi table connection, the connection column of the driven table has primary key perhaps unique index Search for
ref	And eq_ref similar , But not using primary key perhaps unique index, It's a normal index . It can also be on a single table non-unique Index search
range	Range query in single table index , Use the index to query out some row data in a single table .ref The column will become null.
index	Than all Better
all	Full table scan

4.2.4 key

• possible_key Index list , Not necessarily used in practice
• key Actual index used
• key_len Maximum possible length of index field

4.2.5 rows

• According to table statistics and index selection , Roughly estimate the number of rows to read to find the required record

4.2.6 extra

Contains additional information that is not suitable for display in other columns but is important

• using filesort Access is not in the order of indexes in the table
• using temporary Increased the burden of the database , Temporary tables are used to store intermediate result sets , Apply to group by,distinct, or order by Columns for different tables .
• using index well , Override index used