FOR UPDATE OF table.field

Question

Why does the following not work
create table division (code varchar2(2) primary key, div_desc varchar2(20));
insert into division values ('01', 'Ninja assassins');
insert into division values ('02', 'Working for the man');
create table employees (tk number, first_name varchar2(10), last_name varchar2(10), code varchar2(2) references division(code));
insert into employees values (1, 'Chuck', 'Smith', '01');
insert into employees values (2, 'John',  'Smith', '02');
DECLARE
  CURSOR my_csr IS
    Select e.tk, e.first_name, e.last_name
    From employees e, division d
    Where e.code = d.code
      and e.code = '01'
      and e.last_name = 'Smith'
    For update;
    cnt_updated NUMBER;
BEGIN
  cnt_updated := 0;
  FOR my_row IN my_csr
  LOOP
    Update employees
    Set last_name = 'Forbes'
    Where current of my_csr;
    cnt_updated := cnt_updated + SQL%ROWCOUNT;
    dbms_output.put_line('You updated '||cnt_updated||' records');
  END LOOP;
END;but just adding to the "for update" clause allows for an update?
DECLARE
  CURSOR my_csr IS
    Select e.tk, e.first_name, e.last_name
    From employees e, division d
    Where e.code = d.code
      and e.code = '01'
      and e.last_name = 'Smith'
    For update *of e.tk*;We found this out recently, and fellow Developers are asking "Why?". Is there a documented reason, or is that just how it is?
Thanks,
--=Chuck

Answer 1

It is not about locking it is how oracle translate the "where current". it is a simple rowid.
did a little trace, and look what I got from 10046 trace file:
For the update without the "update of", look at the 0 at the cr in the plan,
Oracle already knows that this update will not do anything:
PARSING IN CURSOR #6 len=62 dep=1 uid=82 oct=6 lid=82 tim=88904356722 hv=1877266405 ad='1a9b0404' sqlid='5a5zpz5ry9mz5'
UPDATE EMPLOYEES_T SET LAST_NAME = 'Forbes' WHERE ROWID = :B1
END OF STMT
PARSE #6:c=0,e=258,p=0,cr=0,cu=0,mis=1,r=0,dep=1,og=1,tim=88904356717
BINDS #6:
Bind#0
  oacdty=208 mxl=3950(3950) mxlc=00 mal=00 scl=00 pre=00
  oacflg=00 fl2=206001 frm=00 csi=00 siz=3952 off=0
  kxsbbbfp=0c2ef090  bln=3950  avl=00  flg=05
EXEC #6:c=0,e=1108,p=0,cr=0,cu=0,mis=1,r=0,dep=1,og=1,tim=88904357955
STAT #6 id=1 cnt=0 pid=0 pos=1 obj=0 op='UPDATE  EMPLOYEES_T (cr=0 pr=0 pw=0 time=0 us)'
STAT #6 id=2 cnt=0 pid=1 pos=1 obj=74422 op='TABLE ACCESS BY USER ROWID EMPLOYEES_T (cr=0 pr=0 pw=0 time=0 us cost=1 size=29 card=1)'for the update with the "update of", look at the 1 at the cr in the plan:
PARSING IN CURSOR #6 len=62 dep=1 uid=82 oct=6 lid=82 tim=88913977309 hv=1877266405 ad='1a9b0404' sqlid='5a5zpz5ry9mz5'
UPDATE EMPLOYEES_T SET LAST_NAME = 'Forbes' WHERE ROWID = :B1
END OF STMT
PARSE #6:c=0,e=87,p=0,cr=0,cu=0,mis=0,r=0,dep=1,og=1,tim=88913977304
BINDS #6:
Bind#0
  oacdty=208 mxl=3950(3950) mxlc=00 mal=00 scl=00 pre=00
  oacflg=00 fl2=206001 frm=00 csi=00 siz=3952 off=0
  kxsbbbfp=09f3abfc  bln=3950  avl=13  flg=05
  value=
Dump of memory from 0x09F3ABFC to 0x09F3AC09
9F3ABF0                            22010001              [..."]
9F3AC00 000400B6 00780401 00000000           [......x.....]   
EXEC #6:c=0,e=379,p=0,cr=1,cu=1,mis=0,r=1,dep=1,og=1,tim=88913977802
STAT #6 id=1 cnt=1 pid=0 pos=1 obj=0 op='UPDATE  EMPLOYEES_T (cr=1 pr=0 pw=0 time=0 us)'
STAT #6 id=2 cnt=1 pid=1 pos=1 obj=74422 op='TABLE ACCESS BY USER ROWID EMPLOYEES_T (cr=1 pr=0 pw=0 time=0 us cost=1 size=29 card=1)'Also notice that in the update version with the "update of" we can see dump of memory.
There are some binds type that oracle won't show directly in the dump (e.g. timestamp, rowid, etc)
For future, more hex oriented readers here is the rowid from table:
SQL> select rowid from employees_t;
ROWID
AAASK2AAEAAAQR4AAAIf, somebody out there, can show me a way to read the memory dump back into the real rowid,
I will be very happy.
Amiel