1.30 InnoDB MVCC

1. What Is MVCC

• Multi-Version Concurrency Control.
• To allow read-write operations from different transactions to execute concurrently, data is maintained in multiple versions, and transaction visibility determines which data version the transaction should see.

2. MVCC Principle

• Where to get data from: version chain.
• Which version of the data to get: ReadView.

2.1. Version Chain

• InnoDB undo log.md
• Each record has two hidden fields: trx_id and roll_pointer.
  • trx_id: the transaction ID of the transaction that modified the record.
  • roll_pointer: after a record is modified, the old record is placed in the undo log, and roll_pointer points to the old record.
• Multiple undo logs for the same record are linked together into a linked list. This is the version chain.
  • 

2.2. ReadView

2.2.1. Purpose of ReadView

• Determine which version in the version chain is visible to the current transaction.

2.2.2. What Is ReadView

• m_ids: the list of active transaction IDs in the system when the ReadView is generated.
• min_trx_id: the minimum value in m_ids.
• max_trx_id: the ID value that should be assigned to the next transaction in the system when the ReadView is generated.
• creator_trx_id: the transaction ID of the transaction that generated the ReadView.

2.2.3. ReadView Decision Process

• The accessed version’s trx_id = creator_trx_id in the ReadView: the current transaction is accessing a record it modified itself, so this version can be accessed by the current transaction.
• The accessed version’s trx_id < min_trx_id in the ReadView: the transaction that generated this version committed before the current transaction generated the ReadView, so this version can be accessed by the current transaction.
• The accessed version’s trx_id >= max_trx_id in the ReadView: the transaction that generated this version started only after the current transaction generated the ReadView, so this version cannot be accessed by the current transaction.
• min_trx_id in the ReadView <= the accessed version’s trx_id < max_trx_id in the ReadView:
  • if trx_id is in the m_ids list, the transaction that generated this version was still active when the ReadView was created, so this version cannot be accessed;
  • if trx_id is not in the m_ids list, the transaction that generated this version had already committed when the ReadView was created, so this version can be accessed.