8. MVCC

• MVCC provides a consistent read view, which achieves RC (read committed) and RR (repeatable read)
• InnoDB assigns an auto-increment transation ID to every transaction when it starts.
  • When the transaction modifies a certain row, a new version of that row is generated and its trx_id column is assigned the transaction ID of this current transaction.
  • Physically, InnoDB does not store multiple versions of the same row. Instead, it only stores the latest version and uses undo log to represent its older versions logically.
• With MVCC, a consistent read view makes sure:
  • A value is visible if its trx_id is smaller than or equal to the transaction ID of the current transaction
  • A value is invisible if its trx_id is greater than the transaction ID of the current transaction (thus need to follow undo log to find its older versions)