Delete DML statment tales more time than Update or Insert.

Question

i want to know whether a delete statement takes more time than an update or insert DML command. Please help in solving the doubt.
Regards.

Answer 1

I agree: the amount of ROLLBACK (called UNDO) and ROLLFORWARD (called REDO) information written by the various statement has a crucial impact on the speed.
I did some simple benchmarks for INSERT, UPDATE and DELETE using a 1 million row simple table. As an alternative to the long UPDATEs and DELETEs, I tested also the usual workarounds (which have only partial applicability).
Here are the conclusions (quite important in my opinion, but not to be taken as universal truth):
1. Duration of DML statements for 1 million rows operations (with the size of redo generated):
--- INSERT: 3.5 sec (redo: 3.8 MB)
--- UPDATE: 24.8 sec (redo: 240 MB)
--- DELETE: 26.1 sec (redo: 228 MB)
2. Replacement of DELETE with TRUNCATE
--- DELETE: 26.1 sec (rollback: 228 MB)
--- TRUNCATE: 0.1 sec (rollback: 0.1 MB)
3. Replacement of UPDATE with CREATE new TABLE AS SELECT (followed by DROP ols and RENAME new AS old)
--- UPDATE: 24.8 sec (redo_size: 240 MB)
--- replacement: 3.5 sec (rollback: 0.3 MB)
-- * Preparation *
CREATE TABLE ao AS
    SELECT rownum AS id,
          'N' || rownum AS name
     FROM all_objects, all_objects
    WHERE rownum <= 1000000;
CREATE OR REPLACE PROCEDURE print_my_stat(p_name IN v$statname.NAME%TYPE) IS
    v_value v$mystat.VALUE%TYPE;
BEGIN
    SELECT b.VALUE
      INTO v_value
      FROM v$statname a,
           v$mystat   b
     WHERE a.statistic# = b.statistic# AND lower(a.NAME) LIKE lower(p_name);
    dbms_output.put_line('*' || p_name || ': ' || v_value);
END print_my_stat;
-- * Test 1: Comparison of INSERT, UPDATE and DELETE *
CREATE TABLE ao1 AS
    SELECT * FROM ao WHERE 1 = 2;
exec print_my_stat('redo_size')
*redo_size= 277,220,544
INSERT INTO ao1 SELECT * FROM ao;
1000000 rows inserted
executed in 3.465 seconds
exec print_my_stat('redo_size')
*redo_size= 301,058,852
commit;
UPDATE ao1 SET name = 'M' || SUBSTR(name, 2);
1000000 rows updated
executed in 24.786 seconds
exec print_my_stat('redo_size')
*redo_size= 545,996,280
commit;
DELETE FROM ao1;
1000000 rows deleted
executed in 26.128 seconds
exec print_my_stat('redo_size')
*redo_size= 783,655,196
commit;
-- * Test 2:  Replace DELETE with TRUNCATE *
DROP TABLE ao1;
CREATE TABLE ao1 AS
    SELECT * FROM ao;
exec print_my_stat('redo_size')
*redo_size= 807,554,512
TRUNCATE TABLE ao1;
executed in 0.08 seconds
exec print_my_stat('redo_size')
*redo_size= 807,616,528
-- * Test 3:  Replace UPDATE with CREATE TABLE AS SELECT *
INSERT INTO ao1 SELECT * FROM ao;
commit;
exec print_my_stat('redo_size')
*redo_size= 831,525,556
CREATE TABLE ao2 AS
    SELECT id, 'M' || SUBSTR(name, 2) name FROM ao1;
executed in 3.125 seconds
DROP TABLE ao1;
executed in 0.32 seconds
RENAME ao2 TO ao1;
executed in 0.01 seconds
exec print_my_stat('redo_size')
*redo_size= 831,797,608