MySQL index

constraint ： Ensure data integrity .
Indexes ： Quickly locate specific data , Improve query efficiency .
Actually two different concepts , An index is equivalent to establishing a directory in a dictionary .

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stay MySQL in , There are usually two ways to access row data of a database table ：
1) Sequential access
Sequential access is a full table scan , Traverse line by line from beginning to end , Until the qualified target data is found .

When looking for a small amount of data in tens of millions of data , Using sequential access will traverse all data , It takes a lot of time , Obviously, it will affect the processing performance of the database .
2) Index access
Index access is to directly access the record rows in the table by traversing the index .

The premise is to build an index on the table , After the index is created on the column , When searching data, you can directly find the position of the corresponding record row according to the index on the column , So as to quickly find the data . The index stores a pointer to the specified column data value , Sorts the pointers according to the specified sort order .

for example , On the student list tb_students in , If based on student_id Column indexed , The system establishes a mapping table from the index column to the actual record . When users need to find where student_id = 12022 When , The system starts with student_id The record was found on the index , Then find the data row directly through the mapping table , And return this row of data . Just like the contents in the dictionary .

Because the speed of scanning the index is generally much faster than that of scanning the actual data rows , Therefore, the use of index can greatly improve the efficiency of the database .

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Indexes

Indexes are defined on columns .
If the index has already been created on the column as the search criteria ,MySQL No need to scan any record to get the location of the target record quickly .
General index
unique index ： The values on this column cannot be repeated , Null value allowed .
primary key ： No null values are allowed .

When creating a primary key constraint ,mysql An index will be created automatically by default
To realize that the value of the primary key does not duplicate , You need to retrieve data every time you insert a new record , So in order to improve the retrieval speed , At the same time, create an index on the primary key .
When creating a unique constraint ,mysql A unique index will be created automatically by default
Just like the above , Implement unique constraints through unique indexes

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