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ops$tkyte%ORA11GR2> create table dept 2 as select * from scott.dept; Table created. ops$tkyte%ORA11GR2> create table emp 2 as select * from scott.emp; Table created. ops$tkyte%ORA11GR2> alter table dept 2 add constraint dept_pk 3 primary key(deptno); Table altered. ops$tkyte%ORA11GR2> alter table emp 2 add constraint emp_pk 3 primary key(empno); Table altered. ops$tkyte%ORA11GR2> alter table emp 2 add constraint emp_fk_dept 3 foreign key (deptno) 4 references dept(deptno); Table altered. ops$tkyte%ORA11GR2> create index emp_deptno_idx 2 on emp(deptno); Index created. Let s start a transaction now: ops$tkyte%ORA11GR2> update dept 2 set dname = initcap(dname); 4 rows updated. Now, let s look at the state of the system at this point. This example assumes a single-user system; otherwise, you may see many rows in V$TRANSACTION. Even in a single-user system, do not be surprised to see more than one row in V$TRANSACTION, as many of the background Oracle processes may be performing a transaction as well. ops$tkyte%ORA11GR2> 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;

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USERNAME SID RBS SLOT SEQ LMODE REQUEST --------- ---------- ---------- ---------- ---------- ---------- ---------OPS$TKYTE 703 2 26 8297 6 0 ops$tkyte%ORA11GR2> select XIDUSN, XIDSLOT, XIDSQN 2 from v$transaction; XIDUSN XIDSLOT XIDSQN ---------- ---------- ---------2 26 8297 The interesting points to note here are as follows: The LMODE is 6 in the V$LOCK table and the request is 0. If you refer to the definition of the V$LOCK table in the Oracle Server Reference manual, you will find that LMODE=6 is an exclusive lock. A value of 0 in the request means you are not making a request; you have the lock. There is only one row in this table. This V$LOCK table is more of a queuing table than a lock table. Many people expect four rows in V$LOCK since we have four rows locked. Remember, however, that Oracle does not store a master list of every row locked anywhere. To find out if a row is locked, we must go to that row. I took the ID1 and ID2 columns and performed some manipulation on them. Oracle needed to save three 16-bit numbers, but only had two columns in order to do it. So, the first column ID1 holds two of these numbers. By dividing by 2^16 with trunc(id1/power(2,16)) rbs, and by masking out the high bits with bitand(id1,to_number('ffff','xxxx'))+0 slot, I am able to get back the two numbers that are hiding in that one number. The RBS, SLOT, and SEQ values match the V$TRANSACTION information. This is my transaction ID.

It is important to note that lists are immutable: the cons :: and append @ operations do not modify the original lists; instead, they create new lists You can see this in the following interactive session: > let people = [ "Adam"; "Dominic"; "James" ];; val people : string list > people;; val it : string list = [ "Adam"; "Dominic"; "James" ] > "Chris" :: people;; val it : string list = [ "Chris"; "Adam"; "Dominic"; "James" ] > people;; val it : string list = [ "Adam"; "Dominic"; "James" ] Note that people has not been changed by the construction of a new list using the cons operator That is, lists and tuples are unchangeable, immutable values.

Now we ll start another session using the same username, update some rows in EMP, and then try to update DEPT: ops$tkyte%ORA11GR2> update emp set ename = upper(ename); 14 rows updated. ops$tkyte%ORA11GR2> update dept set deptno = deptno-10; We re now blocked in this session. If we run the V$ queries again, we see the following: ops$tkyte%ORA11GR2> select username, 2 v$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;