FOR UPDATE NOWAIT Fails to Detect Lock

Question

Locking a bitmap indexed row would cause other rows locked. I heard that, if FOR UPDATE NOWAIT is used on these accidentally locked rows (Oracle SQL High Performance Turning by Prentice hall), it may not be able to detect the lock. Is it true? I cannot find related documenation from Oracle's manual. And, what should we do to prevent an incorrect lock status returned by FOR UPDATE NOWAIT?

Answer 1

SELECT FOR UPDATE NOWAIT detects locks affected DATA blocks.
Look the example below:
SQL> create table t1 (id number, bit_col number);
Table created.
SQL> begin
  2  insert into t1 values(0,1);
  3  insert into t1 values(1,1);
  4  insert into t1 values(2,2);
  5  insert into t1 values(3,3);
  6  insert into t1 values(4,4);
  7  end;
  8  /
PL/SQL procedure successfully completed.
SQL> commit;
Commit complete.
SQL> create bitmap index t1_bit on t1(bit_col);
Index created.Now in session 1 we change the bitmap-indexed column and it affects
index node:
SQL> update t1
  2  set bit_col = 4
  3  where id = 2;
1 row updated.In accordance to bitmap index structure this operator locks the index section
the locked row pertains to:
2th session waits for the lock release even when it tries to lock another row -
two rows pertain to the same index section which is locked by the first session:
SQL> update t1
  2  set bit_col = 2
  3  where id = 3;After rollback in the first session the second one gets the resource:
SQL> update t1
  2  set bit_col = 2
  3  where id = 3;
1 row updated.Now lets do rollback in both and repeate the first UPDATE in the first session:
SQL> update t1
  2  set bit_col = 4
  3  where id = 2;
1 row updated.In the second session we can lock the row (not index section) using
SELECT FOR UPDATE:(in contrast with UPDATE statement which changes
indexed column):
SQL> select * from t1 where id=3 for update nowait;
        ID    BIT_COL
         3          3But certainly we detect row-level lock in the data block for ID = 2:
SQL> select * from t1 where id=2 for update nowait;
select * from t1 where id=2 for update nowait
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specifiedRgds.