Clustered and non clustered indexes

About clustered index and non clustered index .

Clustered index is not a separate index type , It's a way of storing data . Put data storage and index together , Find the index and find the data .

Non clustered indexes are also called secondary indexes , Separate data storage from indexing , The leaf node of the index structure points to the corresponding row address of the data , The corresponding data can only be found through the address .

InnoDB in , An index created on top of a clustered index is called a secondary index , Secondary index access data always needs secondary search , Non clustered indexes are secondary indexes , Like composite index 、 Prefix index 、 unique index , The secondary index leaf node no longer stores the physical location of the row , It's the primary key value .

When a table has a clustered index , Its data rows are actually stored in the leaf nodes of the index .

Cluster index is the primary key by default , If there is no primary key defined ,InnoDB Will choose a unique non empty index instead of . If there is no such index ,InnoDB Will implicitly define a primary key as the cluster index .InnoDB Only the records gathered on the same page .

Advantages and disadvantages of clustering index , advantage ：

1. You can save the data together . Fewer disks I/O The number of times ;
2. Data access is faster . Data and indexes are stored on the same node , Get faster than non clustered indexes ;
3. Queries scanned with overlay index can directly use the primary key value of the page node .

shortcoming ：

1. If all the data is in memory , The advantage of clustered index is not obvious ;
2. The insertion speed depends heavily on the insertion order . Insert in the order of the primary key to load the data into InnoDB The fastest way in the table ; If you do not insert in the primary key order , It is best to use after loading OPTIMIZE TABLE Order to reorganize the table ;
3. Updating the clustered index is costly ;
4. Insert new row based on clustered index , Or when the primary key is updated and the row needs to be moved , May face “ Page splitting ” The problem of ;
5. Clustered indexes can slow down full table scans , Especially the rows are sparse , Or the data storage is discontinuous due to row splitting ;
6. The secondary index may be larger than expected , Because the leaf node of the secondary index contains the primary key column of the reference row ;
7. Secondary index access requires two index lookups , Not once ;

Each leaf node of the cluster index contains the primary key value 、 Business ID、 For business and MVCC Rollback pointer and all remaining columns . If the primary key is a column prefix index ,InnoDB It will also contain the complete primary key column and other remaining columns .

Use InnoDB You should insert data in primary key order as much as possible , And try to insert new rows by monotonically increasing the value of the cluster key .

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