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Unique and non-unique indexes

An interesting questions is: what is in fact the difference between unique and non-unique indexes? For a long discussion, see Richard Foote's blog. Here, we look at the on-disk differences.

Let's start with environment setup and block dump creation:
connect system

create user itest identified by itest;

grant dba to itest;

create tablespace ITEST;

alter user itest default tablespace itest;

connect itest/itest

create table TDATA (pk varchar2(20));

begin
for i in 1..10000 loop
insert into TDATA values ('VAL'||i);
end loop;
commit;
end;

create index TIDX1 on TDATA(pk);

(I chose varchar2 type so that the actual characters are clearly seen in the dump. Also, I created a fresh new tablespace so the block numbers are small and probably consecutive.)
Now, find the extents involved:
select * from dba_extents where owner=user;And dump the blocks (take numbers from the query above - relative_fno, block_id):
alter system dump datafile 14 block min 33 block max 62;Save the trace, create a unique inde…

Who am I?

Quite often, a procedure needs to know some information about the session it is running in - at least for logging some information.

It seems to be easy to find this information - just look at SYS_CONTEXT documentation, and find the impressive list of attributes of USERENV context. And if something is missing, just grab the SESSIONID, SID or AUDSID and query v$session or other view.

Well, this works nicely, as long as you don't do it in a job. Unfortunately, in a job you always get 0.
You can try to battle with FG_JOB_ID and BG_JOB_ID, but I think this deserves a more elegant solution - and that's why I always query v$mystat to get the SID of my process. Just fetch the first line:
select * from v$session where sid = (select sid from v$mystat where rownum=1)You need a select grant on v_$mystat by SYS, but as this view does not expose anything important, it's not a security issue.

The next question that comes is: if I am run by a job (let's talk about dbms_job now), what'…

A list partitiong bug

Last week, I found a not-so-nice bug in 10gR2. Just try this short test case, creating a table with two list partitions, default and null:
alter session set nls_territory=America;
alter session set nls_language=American;

select version from product_component_version;

drop table th3;

prompt Creating TH3...
create table TH3
(
DF DATE,
DT DATE,
N NUMBER
)
partition by list (DT)
(
partition THTBL_CURRENT values (NULL)
tablespace USERS,
partition THTBL_OLD values (default)
tablespace USERS
)
;

prompt Loading TH3...
insert into TH3 (DF, DT, N)
values (to_date('01-01-2000', 'dd-mm-yyyy'), to_date('10-04-2008', 'dd-mm-yyyy'), 1);
insert into TH3 (DF, DT, N)
values (to_date('01-01-1000', 'dd-mm-yyyy'), to_date('01-03-3000', 'dd-mm-yyyy'), 2);
insert into TH3 (DF, DT, N)
values (to_date('10-04-2008', 'dd-mm-yyyy'), to_date('10-04-2008', 'dd-mm-yyyy'), 10);
insert into TH3 (DF, DT, N)
values (to_date('11-04-2008&…

10gR2 RAC on RHEL5.1 (x86_64)

Just a few issues you should be aware of when trying to install 10gR2 RAC on RHEL 5.1:

See Metalink 465001.1 for raw device configuration for RHEL 5. It's not explicitly said, but all disks used must be partitioned! Otherwise, if you don't partition the OCR disk, the root.sh from clusterware installation will fail (fail with “Failed to upgrade Oracle Cluster Registry configuration” error, with “Failed to call clsssinit” in log.)
The ASMLib will also refuse to stamp whole disk, a partition is required.

For OS configuration, see Metalink 421308.1. However, the sysctl parameters listed there do not exist on RHEL 5.1, you will have to use:
kernel.shmmni = 4096kernel.sem = 250 32000 100 128fs.file-max = 65536net.ipv4.ip_local_port_range = 1024 65000net.core.rmem_default = 262144net.core.rmem_max = 262144net.core.wmem_default = 262144net.core.wmem_max = 262144net.ipv4.tcp_rmem = 4194304 4194304 4194304
net.ipv4.tcp_wmem = 262144 262144 262144
You will also need to install one more rpm pa…

dbms_alert on RAC

Not long time ago, I came across a usage of dbms_alert to manage running jobs. As the solution implemented must work also for RAC, I wanted to know whether dbms_alert works on RAC across instances.

The documentation nor Metalink does not say anything (contrary to dbms_pipe, which does NOT work on RAC). So, if they don't warn, it should work... However, Julian Dyke says, that dbms_alert does not work and is the same as dbms_pipe (sources: http://juliandyke.com/Presentations/Presentations.html#ARoughGuideToRAC, page 17, or Pro Oracle Database 10g RAC on Linux, page 426).

You know, never trust anybody, so I conducted a test case (10.2.0.3 on Linux x86_64, VMware ESX server, 2-node RAC):

You will need two simultaneous sessions, I mark them with DWH1> and DWH2> here.
DWH1> select instance_name from v$instance;

INSTANCE_NAME
----------------
DWH1

DWH2> select instance_name from v$instance;

INSTANCE_NAME
----------------
DWH2

DWH2> exec dbms_alert.register('TST');

DWH2> set …