[MySQL learning notes 22] index

Advantages and disadvantages

advantage	Inferiority
Improve the efficiency of data retrieval , Reduce the io cost	The index column takes up space
Sort data by indexing Columns , Reduce the cost of sorting data , Reduce CPU Consumption of	Indexing greatly improves query efficiency , It also reduces the efficiency of updating tables , Because you need to update the index while updating the table

Index structure

Index structure	describe
B+Tree Indexes ( Default )	The most common type of index , Most engines support
Hash Indexes	The underlying principle is the hash table , Range query is not supported , Can only match exactly
R-Tree	Commonly used for geospatial data types
Full-text( Full-text index )	It's a way of building inverted indexes , How to quickly match documents

Each engine supports indexing

Index classification

classification	meaning	characteristic	keyword
primary key	The index created for the primary key in the table	Automatically created by default , There can only be one	PRIMARY
unique index	Avoid a column with duplicate values	There can be multiple	UNIQUE, You can specify when creating a table , It can also be added manually later
General index	Quickly locate data	There can be multiple	nothing , You need to create... Manually
Full-text index	Fast Text Retrieval	There can be multiple	FULLTEXT

expand : The uniqueness of column value depends on the unique index .

classification	meaning	characteristic
Clustered index	Put data storage and storage together , The leaf node of the index structure holds the row data	There has to be , And there is only one , When creating a table, the primary key is automatically used as a clustered index , If there is no primary key , Use the first unique index as the clustered index , If not ,InnoDB Will automatically a hidden rowid As a clustered index
Secondary indexes	Separate data from index , The leaf node of the index structure stores the corresponding primary key	There can be multiple

example

Such as execution select * from user where name =‘Arm’;
Then we'll go first name Search in the index Arm, Query to Arm The corresponding id yes 10, Then I'll take this 10 Go to the clustered index to find specific data , Then return . This process is called “ Return to the table for query ”.

Create index

create [unique|fulltext] index index_name on table_name (index_col_name1,…);

Look at the index

show index from table_name;

Delete index

drop index index_name on table_name;