Q&A
Q&AHow does the InnoDB primary key affect secondary indexes?
InnoDB secondary-index entries carry the primary-key value used to find the clustered row.
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InnoDB organizes table rows by the clustered primary key. Secondary indexes store their indexed columns plus the primary-key value, so a query requesting columns absent from the secondary index may need a second lookup. Wide primary keys therefore increase the footprint of every secondary index.