MySQL: Clustered Index

Clustered Index is the InnoDB term for a primary key index. 

InnoDB table storage is organized based on the values of the primary key columns, to speed up queries and sorts involving the primary key columns. 

If you do not define a PRIMARY KEY for your table, MySQL locates the first UNIQUE index where all the key columns are NOT NULL and InnoDB uses it as the clustered index.

If the table has no PRIMARY KEY or suitable UNIQUE index, InnoDB internally generates a hidden clustered index on a synthetic column containing row ID values. 

The rows are ordered by the ID that InnoDB assigns to the rows in such a table. The row ID is a 6-byte field that increases monotonically as new rows are inserted.
Thus, the rows ordered by the row ID are physically in insertion order. 

All indexes other than the clustered index are known as secondary indexes.
In InnoDB, each record in a secondary index contains the primary key columns for the row, as well as the columns specified for the secondary index. 

InnoDB uses this primary key value to search for the row in the clustered index.

Physical Strcuture:
All InnoDB indexes are B-trees where the index records are stored in the leaf pages of the tree. 
The default size of an index page is 16KB. 
When new records are inserted, InnoDB tries to leave 1/16 of the page free for future insertions and updates of the index records.

The page size can be specified for all InnoDB tablespaces in a MySQL instance by setting the innodb_page_size configuration option before creating the instance.
 
Once the page size for a MySQL instance is set, you cannot change it.
Supported sizes are 16KB, 8KB, and 4KB, corresponding to the option values 16k, 8k, and 4k.

A MySQL instance using a particular InnoDB page size cannot use data files or log files from an instance that uses a different page size.