MVCC

To solve non-repeatable read:
1. lock
2. mvcc

As long as there is an update, then generate a new record with current tx_id. undo log points to previous one.

When read, it should have:

ReadView {
    m_ids_list,
    min_trx_id,
    max_trx_id,
    creator_trx_id
}

When read or doing a transaction, it should have current tx_id and its ReadView. Go to the record, check if the top record’s tx_id is smaller than the tx_id in ReadView. If so, then this is the value for it. If not, check the next record in undo log…

Good article for MVCC.

https://blog.csdn.net/flying_hengfei/article/details/106965517