Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 6 ■ LOCKING AND LATCHING
TM (DML Enqueue) Locks
TM locks are used to ensure that the structure of a table is not altered while you are modifying
its contents. For example, if you have updated a table, you will acquire a TM lock on that table.
This will prevent another user from executing DROP or ALTER commands on that table. If
another user attempts to perform DDL on the table while you have a TM lock on it, he’ll
receive the following error message:
drop table dept
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified
This is a confusing message at first, since there is no method to specify NOWAIT or WAIT
on a DROP TABLE at all. It is just the generic message you get when you attempt to perform
an operation that would be blocked, but the operation does not permit blocking. As you’ve
seen before, it’s the same message you get if you issue a SELECT FOR UPDATE NOWAIT against a
locked row.
The following shows how these locks would appear in the V$LOCK table:
ops$tkyte@ORA10G> create table t1 ( x int );
Table created.
ops$tkyte@ORA10G> create table t2 ( x int );
Table created.
ops$tkyte@ORA10G> insert into t1 values ( 1 );
1 row created.
ops$tkyte@ORA10G> insert into t2 values ( 1 );
1 row created.
ops$tkyte@ORA10G> select (select username
2 from v$session
3 where sid = v$lock.sid) username,
4 sid,
5 id1,
6 id2,
7 lmode,
8 request, block, v$lock.type
9 from v$lock
10 where sid = (select sid
11 from v$mystat
12 where rownum=1)
13 /