Skip navigation links
13.6.10.2 Physical Structure of an Index »
Section Navigation      [Toggle]

13.6.10.1. Clustered and Secondary Indexes

Every InnoDB table has a special index called the clustered index where the data for the rows is stored:

  • If you define a PRIMARY KEY on your table, InnoDB uses it as the clustered index.

  • If you do not define a PRIMARY KEY for your table, MySQL picks the first UNIQUE index that has only NOT NULL columns as the primary key 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.

Accessing a row through the clustered index is fast because the row data is on the same page where the index search leads. If a table is large, the clustered index architecture often saves a disk I/O operation when compared to storage organizations that store row data using a different page from the index record. (For example, MyISAM uses one file for data rows and another for index records.)

In InnoDB, the records in nonclustered indexes (also called secondary indexes) contain the primary key columns for the row that are not in the secondary index. InnoDB uses this primary key value to search for the row in the clustered index. If the primary key is long, the secondary indexes use more space, so it is advantageous to have a short primary key.