Difference between redo and undo

Redo stay Oracle In the database , There is a kind of log file called redo log file redo log, Divided into two ： Online redo logs and archive logs .

         Online redo logs （online redo log） It is mainly used for ：Oracle The server where the database is located suddenly loses power 、 Restart or execute shutdown Wait for the command to restart ,Oracle The database cannot start the instance normally . here ,Oracle Will use online redo logs , Restore the database to the Jianyang moment before the server power failure , Make the database start normally .（ By default , The database will have at least two redo log groups , Recycling ）

        Archive log （archive log） Mainly used for hardware level errors ： The bad track of the disk makes it impossible to read and write 、 Failed to write 、 Disk damage leads to database data loss . At this time, the database archives log files , Restore the database to the time point where the archived log is located, and then restore the database to the current time point by online redoing the log file .

         For archive log files , It can be understood as online redo log file backup . That is, when a redo log file is filled , Archive the log file and keep a backup .（ Because it says , Online redo log files will be automatically overwritten ） therefore , Archive log files are backups of old online log files .

Undo  Undo data is reverse DML Information required for statement results , go by the name of “ Undo Data ”.

        undo data ： If the database has been converted, use the undo segment , And automatic revocation management , Then any existing rollback segment must be in offline mode , And cannot be set to online . according to UNDO_MANAGEMENT Different settings of parameters ,Oracle The database either uses undo segments , Or use rollback segments .

        undo Table space ： A database can have multiple Undo tablespaces , But only one undo table space can be used at any given time . Undo tablespaces must be created as persistent 、 Locally managed table spaces that can automatically expand the allocated space .

DML sentence （insert Insert 、update to update 、delete Delete ）, Operation produces redo and undo The most and the least ?

        answer ：undo： Rollback records the action of reverse operation , It is used to cover water after subsequent implementation . therefore delete Produce the most , The reverse operation of deletion is to insert , It means that the whole line of records is completely inserted back , That is to record all the field information of the table .insert least , The reverse operation is to delete , As long as the mark is positioned to the original position （ because insert Of undo The information is recorded in the inserted record rowid, The only mark , According to this rowid It is enough to locate the inserted record and delete , So as to achieve the purpose of fallback ）; and update In the middle , Just record the value of the field before updating and save it .

        redo Redo log ,delete Just record the relevant rowid You can delete it again , produce redo least , and insert If you want to finish it again , At least you need to know the values of all fields , Produced redo Definitely most ,update The situation remains the same , It's all in the middle .

undo There will be a corresponding redo?

         answer ： Meeting ,undo The information of is the front image information for preparing the user to retreat , It is used for the restoration action after everyone regrets their operation , These data also need to be protected , If we lose it, we can't restore it , and redo It is a very good recovery treasure .

        DML A feature of the statement ：DML Statements are different from query statements , Will change the data in the database . besides , There will also be for future recovery redo And for fallback undo. Another detail is , because undo It also needs protection , So there will be special protection undo Operation of the redo.