MySQL :: MySQL 8.4 Reference Manual :: 17.6.2.1 Clustered and Secondary Indexes

Enabling Automatic InnoDB Configuration for a Dedicated MySQL Server

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the records in nonclustered indexes contain the primary key columns ... 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.