NOTE
InnoDB vs. MyISAM Indexes
How InnoDB clustered indexes differ from MyISAM's separate data and index storage.
This is a historical learning note and may contain outdated or incomplete understanding.
InnoDB organizes table rows around a clustered B+tree, normally keyed by the primary key. Secondary-index leaf entries contain the indexed columns plus the primary-key value, so a secondary lookup may require another traversal of the clustered index.
MyISAM stores table data separately from B-tree indexes whose leaf entries identify row locations. This architectural difference affects lookup paths, update cost, row organization, and recovery semantics. In modern MySQL, InnoDB is the normal transactional engine; MyISAM is mainly relevant for legacy cases.