Brief introduction to clustered index, secondary index, index push down

1、 Cluster index

InnoDB There are two kinds of indexes :

1. Cluster index : Generally, the primary key when creating a table will be mysql As cluster index , If there is no primary key, select a non empty unique index as the cluster index , If none, an index is implicitly created as a clustered index
2. Secondary index : That is, non clustered index or secondary index , Usually, the index we add is the auxiliary index ;

Cluster index Not a single index type , It's a kind of Data storage mode , It is to construct one according to the primary key of each table B+ Trees , At the same time, the leaf node stores the row record data of the whole table , therefore primary key The default is to use Cluster index .

2、 Secondary indexes ( Secondary index )

When creating a table, the primary key will be created by default Cluster index , Clustering ( Primary key ) The leaf node of the index stores the whole row of data .

Except clustering ( Primary key ) All indexes other than indexes are called secondary indexes, that is, non primary key indexes , The leaf node content of the secondary index is the value of the primary key , The smaller the primary key length , The smaller the leaf node of the secondary index , The less space it takes up ;

The secondary index needs to scan one more index tree in the query , That is to say, back to the table , adopt Overlay index and Default index push down The mechanism can avoid returning tables ;

3、 Back to the table

Back to the table ： Is to scan the row where the data is located through the index , Then get other field data through the row primary key index .

In short, it is ： There are existing index fields in the query fields , If there are non indexed fields, it will happen back to the table .

For example, the index field is name

#  The query uses an index , And there are only name The data of , But all the fields are queried , At this time, you will go back to the table to get the values of other fields 
#  Through the index name Find the data row , Then find other field values through the primary key 
select * from table where name = '';   

Primary key index query : Cluster index is used as primary key , All column data is saved in the index , You can directly locate the data through the primary key and return ;

Auxiliary index query : The auxiliary index includes the field value corresponding to the index and the primary key value , If the query fields are all index fields , Just return the data directly through the index ; If the field of the query is other than the index field , It also contains other fields , Find the primary key through the secondary index , Then find the corresponding data in the cluster index through the primary key ==( That is to say, back to the table )==

Can pass Overlay index and Default index push down The mechanism can avoid returning tables ;

4、 Index push down (CIP)

ICP Is to combine index scanning and index filtering , An index optimization strategy after the filtered record data is pushed down to the storage engine .

Push is enabled by default under index conditions , You can use system parameters optimizer_switch To check whether the controller is on .

The advantages of index push down are as follows ：

• 1. Reduce the number of operations back to the table .

• 2. Reduce uploading to MySQL SERVER Layer data .

Index push down usage conditions ：

• It can only be used for range、 ref、 eq_ref、ref_or_null Access method ;

• It can only be used for InnoDB and MyISAM Storage engine and its partition table ;

• Yes InnoDB For storage engines , Index push down is only applicable to secondary indexes （ Non primary key index ）;

• Conditions that reference subqueries cannot be pushed down ;

• A condition that references a storage function cannot be pushed down , Because the storage engine cannot call the storage function .

MySQL Service layer ： Used to resolve SQL The grammar of 、 semantics 、 Generate query plan 、 Take over from MySQL Secondary filtering of data pushed on the storage engine layer, etc .

MySQL Storage engine layer ： according to MySQL Requests from the service layer , Upload data to by indexing or full table scanning MySQL Service layer .

MySQL An index scan ： Filter according to the specified index , Traverse the index, find the primary key value corresponding to the index key, and return to the table to filter the remaining filter conditions .

MySQL Index filtering ： After index scanning and secondary conditional filtering based on the index, return to the table .