As part of every table-write operation
DELETE), the storage engine is required to
update its internal index information.
The method used to update indexes will vary from storage engine to storage engine, depending on the method used to store the index.
In general, the storage engine will have to use row information
passed in methods such as
update_row()] in combination with index
information for the table to determine what index data needs to
be modified, and make the needed changes.
The method of associating an index with its row will depend on your storage approach. Current storage engines store the row offset.