[MySQL] index and transaction

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1. Indexes

What is index

Index is a special kind of file , Contains a reference pointer to all records in the data table . You can index one or more columns in a table , And specify the type of index , Each index has its own data structure implementation .

The function of index

• Tables in the database 、 data 、 The relationship between indexes , It's like a book on a shelf 、 The relationship between book content and book catalog .
• The function of the index is similar to that of a book catalogue , Can be used for Rapid positioning 、 Retrieving data .
• The index is for Improve database performance It helps a lot .

Index usage scenarios

Consider creating an index on a column or columns of a database table , The following points need to be considered ：

• Large amount of data , And we often make conditional queries on these columns .
• The insert operation of the database table , And modify these columns less frequently .
• Indexes take up extra disk space .
• according to where Conditional query to create the appropriate index .
• The more indexes you create, the better , When there is a large amount of data, updating the index also takes time

When the above conditions are met , Consider indexing these fields in the table , To improve query efficiency .
conversely , If the non conditional query column , Or insert it often 、 Modify the operating , Or when there is not enough disk space , Don't think about creating indexes

Simple use of index

Create a primary key constraint （PRIMARY KEY）、 Unique constraint （UNIQUE）、 Foreign key constraints （FOREIGN KEY） when , Meeting Automatically create the index of the corresponding column .

Look at the index

show index from Table name ;

Create index

create index Index name on Table name ( Field name );

Delete index

drop index Index name on Table name ;

2. Business

What is a transaction and why it is used

Concept of transactions

Business （ It's atomic ） A logical set of operations , The units that make up this set of operations , All or nothing , All or nothing . In different environments , You can have business . In the database , It's database transactions .

Reasons for using transactions

In some cases , If there are no transactions, there will be big problems . For example, the problem of bank transfer .

Prepare the test sheet ：

drop table if exists accout;
create table accout(
 id int primary key auto_increment,
 name varchar(20) comment ' title of account ',
 money decimal(11,2) comment ' amount of money '
);
insert into accout(name, money) values
(' Alibaba ', 5000),
(' The Forty Thieves ', 1000);

for instance , Forty thieves stole from Alibaba's account 2000 element

--  Alibaba accounts are down 2000
update accout set money=money-2000 where name = ' Alibaba ';
--  The accounts of the forty thieves have increased 2000
update accout set money=money+2000 where name = ' The Forty Thieves ';

If you are executing the first sentence above SQL when , There was a network error , Or the database hangs up , Alibaba's account will be reduced 2000, But there is no increase in the account of the forty thieves .

Solution ： Use transactions to control , Make sure the above two sentences SQL
Or it all works , Or all failed

How to start a transaction

Start the trilogy of affairs

1. Open transaction ：start transaction;
2. Execute more than one SQL sentence
3. Roll back or commit rollback（ All failed ）/commit（ All success ）.

explain ：rollback It's all failure ,commit It's all success .

An example of starting a transaction

start transaction;
--  Alibaba accounts are down 2000
update accout set money=money-2000 where name = ' Alibaba ';
--  The accounts of the forty thieves have increased 2000
update accout set money=money+2000 where name = ' The Forty Thieves ';

commit;

After opening the transaction , If not implemented to commit, Then the data in the database has not been modified , Although the current database shows that money has changed , But if you open another database port and re-enter, you will find that there is no change , This is the benefit of starting a transaction , If the execution is not successful, the data in the database will not be modified .

Four characteristics of transactions

1. Atomicity ： For a set of operations （ Mainly updates ）, All or nothing , All or nothing .
2. Uniformity ： In a business , The results of multiple queries are consistent .
3. Isolation, ： Different transactions , Inquire about / The modified data is isolated from each other . A transaction is not committed / Before rolling back , Only you can see the modified data .
4. persistence ： After the transaction is committed , The data will be persisted to the hard disk .

summary

Indexes ：
（1） For inserting 、 Delete tables with high data frequency , Not applicable index
（2） For a column with high modification frequency , This column is not applicable to index
（3） Those with high query frequency through the conditions of a certain column or several columns , You can create indexes on these columns

Business ：

start transaction;
...
rollback/commit;