Originally posted by abhaysk
coz, [with update] old image has to be written in Rollback which almost equals DELETE, then new image of data has to be written/modified to the blocks in question which almost equals INSERT
Well, only if you are updating all of the columns in a row. But if you update only a small fraction of the entire row's data (for example only one column), then only the changed bytes of that row are stored in the undo. Also, only a small change needs to be recorded in the redo. But with DELETE, the whole record needs to be stored in the undo and almost no information in redo, and then with INSERT almost no information is stored in undo but the whole row must be stored in redo. So if you change only a few columns in a row, the UPDATE wins over DELETE+INSERT both on the amount of generated undo and redo.

That's the reason why I asked how many columns of the row he is usualy updating.