Delete,insert,update與undo的關係[轉載TOM文章]

Delete,insert,update 與undo的關係

What is relationship (delete,insert,update) and undo?

during an insert - we have to generate undo equivalent to "delete+rowid"  
during an update - we have to generate undo equivalent to "update this rowid to have these before image values" (undo = rowid + column before images)  
during a delete - we have to generate undo equivalent to "insert at this rowid these values"  
so, considering an unindexed table - IN GENERAL, a delete and update will generate the most and an insert the least. IN GENERAL this is true (but not always).  
IN GENERAL=>delete>update>insert

if you add indexes - everything can change. the insert might generate gobs of undo for the index maintenance - the update might not update any indexed columns and hence only generate undo for the table - and the delete might have to maintain all of the indexes generating tons of undo again.  
so, it depends on the nature of the data, what you are inserting/updating/deleting..  
same for redo. It depends on what is modified...  

Add indexes

IN GENERAL=>delete>insert>update

 

 

總結：

通常情況下

Undo 產生量排序：delete>update>insert

Undo(table of indexes) 產生量排序：delete>insert>update

一些手機傳送業務上，往往會使用update去更新業務的狀態！

 

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