Undo Log

Undo Page

Page format

Normal Undo Page

First Page In Linked List

Specific Logs

insert

Speficic data

delete

Flow

Update

Not Update Primary Key

if it’s in-place update, it just update.

else, meaning that record will take more place and it’s not enough, it will delete original record, and insert a new record

Update Primary Key

• delete mask on original record
• create a new record

Second index

if it update second index, it will execute delete mask operation and change File Header’s PAGE_MAX_TRX_ID to the transaction’s id.

Transaction

for a transaction, mysql wiil create at most 2 kinds of undo page list for normal table, respectively insert undo list and update undo list. It’s a lazy load technology meaning that it will be create only it needed. for a temp table, Undo Page List do the same as normal table, but it doesn’t need to create redo page for temp table.

Rollback Segment

Architecture

Fifth Page

Rollback Segment Header Page

Reuse Undo List

If Undo List satisfies two conditions

• only one Undo Page
• used space in the page less than 3/4.

for different kinds of undo page list, has different strategy

• insert undo list: Just Use.
• update undo list : append write a new Undo Log Header and new logs.

Purge

When purge a undo list ?

if the minimum ReadVies’s transaction_no is bigger than this undo list’s transaction_no, it’s time to purge it, meaning that all the ReadViess can see this change.

How to purge a undo list

Insert Undo List

After a transaction commit, for insert undo list, it can be purged right now, because it’s designed to rallback and not useful anymore. But its trx_id and roll_pointer should be keeped, not been purged.

Update Undo List

after a transaction commit, write its transaction_no which means committing order to Undo Logo Header, and the whole list will be insert into history list. History list will range there Undo list order by their transaction_no.

Also, ReadView will be ranged an ordered list order by theirtransaction_no.

While purging, background thread will get current mimimum transaction_no of ReadView list, and check history list, if Uodo list’s transaction_no is less than ReadView’s, the Undo list could be purged.

if Undo list marked delete mask, it will delete record acctually.