http://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:839412906735 ---- LOCKING
Here is a small example showing how this happens. We will use three V$ tables in order
to see how this works; V$TRANSACTION, V$SESSION, and V$LOCK. V$TRANSACTION contains an
entry for every active transaction. V$LOCK contains an entry for all locks held as well
as locks being waited on. V$SESSION shows us the sessions logged in. We'll start by
starting a transaction in one session and looking at the state of the system at that
point:
tkyte@TKYTE816> update dept set deptno = deptno+10;
4 rows updated.
tkyte@TKYTE816> select username,
2 v$lock.sid,
3 trunc(id1/power(2,16)) rbs,
4 bitand(id1,to_number('ffff','xxxx'))+0 slot,
5 id2 seq,
6 lmode,
7 request
8 from v$lock, v$session
9 where v$lock.type = 'TX'
10 and v$lock.sid = v$session.sid
11 and v$session.username = USER
12 /
USERNAME SID RBS SLOT SEQ LMODE REQUEST
-------- ---------- ---------- ---------- ---------- ---------- ----------
TKYTE 8 2 46 160 6 0
tkyte@TKYTE816> select XIDUSN, XIDSLOT, XIDSQN
2 from v$transaction
3 /
XIDUSN XIDSLOT XIDSQN
---------- ---------- ----------
2 46 160
The interesting things to note here are:
?á The LMODE is 6 in the V$LOCK table and the request is 0. If you refer to the
definition of the V$LOCK table in the Oracle Server Reference, you will find that LMODE=6
is an exclusive lock. A value of 0 in the request means we are not making a request ¡V we
have the lock.
?á There is only one row in this table. This V$LOCK table is more of a queuing table than
a lock table. Many people expect there would be four rows in V$LOCK since we have four
rows locked. What you must remember however is that Oracle does not store a master list
of every row locked anywhere. To find out if a row is locked, we must go to that row.
?á I took the ID1 and ID2 columns, and performed a bit of bit manipulation on them.
Oracle needed to save three 16bit numbers, but only had two columns in order to do it.
So, the first column ID1 holds two of these numbers. By dividing by 2^16 with
trunc(id1/power(2,16)) rbs and by masking out the high bits with
bitand(id1,to_number('ffff','xxxx'))+0 slot, I am able to get the two numbers that are
hiding in that one number back out.
?á The RBS, SLOT, and SEQ values match the V$TRANSACTION information. This is my
transaction ID.
Now I'll start another session using the same user name, update some rows in EMP, and
then try to update DEPT:
tkyte@TKYTE816> update emp set ename = upper(ename);
14 rows updated.
tkyte@TKYTE816> update dept set deptno = deptno-10;
I am now blocked in this session. If we run the V$ queries again, we see:
tkyte@TKYTE816> select username,
2 v$lock.sid,
3 trunc(id1/power(2,16)) rbs,
4 bitand(id1,to_number('ffff','xxxx'))+0 slot,
5 id2 seq,
6 lmode,
7 request
8 from v$lock, v$session
9 where v$lock.type = 'TX'
10 and v$lock.sid = v$session.sid
11 and v$session.username = USER
12 /
USERNAME SID RBS SLOT SEQ LMODE REQUEST
-------- ---------- ---------- ---------- ---------- ---------- ----------
TKYTE 8 2 46 160 6 0
TKYTE 9 2 46 160 0 6
TKYTE 9 3 82 163 6 0
tkyte@TKYTE816> select XIDUSN, XIDSLOT, XIDSQN
2 from v$transaction
3 /
XIDUSN XIDSLOT XIDSQN
---------- ---------- ----------
3 82 163
2 46 160
What we see here is that a new transaction has begun ¡V (3,82,163) is the transaction ID.
It has two rows in V$LOCK this time. One row represents the locks that it owns (where
LMODE=6). It also has a row in there that shows a REQUEST with a value of 6. This is a
request for an exclusive lock. The interesting thing to note here is that the
RBS/SLOT/SEQ values of this request row are the transaction ID of the holder of the lock.
We can easily see that the transaction with SID=8 is blocking the transaction with SID=9.
Now, if we commit in SID = 8 the above changes:
tkyte@TKYTE816> select username,
2 v$lock.sid,
3 trunc(id1/power(2,16)) rbs,
4 bitand(id1,to_number('ffff','xxxx'))+0 slot,
5 id2 seq,
6 lmode,
7 request, block
8 from v$lock, v$session
9 where v$lock.type = 'TX'
10 and v$lock.sid = v$session.sid
11 and v$session.username = USER
12 /
USERNAME SID RBS SLOT SEQ LMODE REQUEST
-------- ---------- ---------- ---------- ---------- ---------- ----------
TKYTE 9 3 82 163 6 0
tkyte@TKYTE816> select XIDUSN, XIDSLOT, XIDSQN
2 from v$transaction
3 /
XIDUSN XIDSLOT XIDSQN
---------- ---------- ----------
3 82 163
that request row has gone ¡V it disappeared the instant the other session gave up its
lock. That request row was the queuing mechanism. The database is able to wake up the
blocked sessions the instant the transaction is completed. Note that the above gives us a
very easy way to see blockers and waiters:
tkyte@TKYTE816> select
(select username from v$session where sid=a.sid) blocker,
2 a.sid,
3 ' is blocking ',
4 (select username from v$session where sid=b.sid) blockee,
5 b.sid
6 from v$lock a, v$lock b
7 where a.block = 1
8 and b.request > 0
9 and a.id1 = b.id1
10 and a.id2 = b.id2
11 /
BLOCKER SID 'ISBLOCKING' BLOCKEE SID
-------- ---------- ------------- -------- ----------
TKYTE 8 is blocking TKYTE 9
simply by doing a self-join of V$LOCK with itself (I ran this query before committing the
session with SID=8).
2) exclusive lock -- I updated a row. no one else can update it until I commit. I have
an X lock on that row and only one person at a time can have an X lock. an X lock
provides serialization to a resource.
A shared lock -- when I update a table, I take a shared lock on the DEFINITION of the
table. Everyone else can do that as well (more then one session can get a shared lock on
the table definition). So, more than one person at a time can update the table. If you
wanted to ALTER the table, you would need an X lock on the defintion. You cannot get an
X lock when there are shared locks so you wait until there are no shared locks.
3) mystat has the statistics (cpu use, blocks read, cursors opened, etc) for your session
only. v$locked_object shows you all of the locks in the system.
4) no, never.
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