MySQL

A holds row one and B holds row two; opposing right-angle requests form a cycle, with a whole-victim-transaction rollback label beneath.

InnoDB deadlocks roll back the whole victim transaction

An InnoDB deadlock occurs when transactions form a cycle of incompatible lock requests. With deadlock detection enabled, InnoDB chooses a victim and rolls back that entire transaction so another participant can proceed. The application receives error 1213 with SQLSTATE 40001. Retrying only the statement that reported the error omits earlier work that was rolled back with it. ...

A secondary-record outline encloses status, time and primary-key fields, while a separate payload block points outside the record.

InnoDB covering indexes carry the query's columns

A covering index contains the columns needed to evaluate a particular query and return its requested values. In InnoDB, secondary-index records also carry the primary-key columns, so a query can be covered without explicitly listing the primary key again in the secondary-index definition. Coverage is a relationship between one query and an index, not a permanent property that applies to every query against the table. ...