Exhibit.
Which statement causes more undo generation?
A.
SELECT * FROM emp;
B.
DELETE FROM emp WHERE empno=7934;
C.
UPDATE emp SET comm=400 WHERE empno=7844;
D.
INSERT INTO emp VALUES(7999,’JHON’,’CLERK’,7782,’10-MAY-83′,1500,NULL,10);
Which statement causes more undo generation?
Exhibit.
Which statement causes more undo generation?
Exhibit not showing
yea it is
In general, INSERT generated undo the least, because Oracle for this to be recorded only to “delete” a rowid (row ID).
UPDATE usually generate more than INSERT. For UPDATE, simply modify the byte records. You may only update (UPDATE) a small part of the entire data row, the most common case. Therefore, you must record a small portion of the line in the undo.
Generally, DELETE generated undo the most. For DELETE Oracle must be recorded before image the entire row to undo segment.
因為DELETE 的全部刪掉的再找回來