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The following bug has been logged online:

Bug reference: 2103
Logged by: wangshj
Email address: wangshj@sduept.com
PostgreSQL version: 8.1.0
Operating system: CentOS 4
Description: System column oid is not unique
Details:

The following is my test.

oidtest=# CREATE TABLE oidtest( name text ) WITH OIDS;
CREATE TABLE
oidtest=# INSERT INTO oidtest ( name ) VALUES( 'name1' ) ;
INSERT 54512 1
oidtest=# COPY oidtest (name) WITH OIDS FROM stdin;
Enter data to be copied followed by a newline.
End with a backslash and a period on a line by itself.

>>>> 54513 copyname
>>>> \.

oidtest=# INSERT INTO oidtest ( name ) VALUES( 'name2' ) ;
INSERT 54513 1
oidtest=# SELECT oid,name from oidtest ;
oid | name
-------+----------
54512 | name1
54513 | copyname
54513 | name2
(3 rows)

oidtest=#
Then I got two records with same oid(54513).

In the second test, I created two tables and added primary key (oid) to
them. As below:

oidtest=# CREATE TABLE oidtestpk (name text, PRIMARY KEY (oid) ) WITH OIDS
;
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index
"oidtestpk_pkey" for table "oidtestpk"
CREATE TABLE
oidtest=# CREATE TABLE oidtestpk_another (name text, PRIMARY KEY (oid) )
WITH OIDS ;
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index
"oidtestpk_another_pkey" for table "oidtestpk_another"
CREATE TABLE
oidtest=# INSERT INTO oidtestpk(name) VALUES ('name0');
INSERT 54562 1
oidtest=# COPY oidtestpk(name) WITH OIDS FROM stdin;
Enter data to be copied followed by a newline.
End with a backslash and a period on a line by itself.

>>>> 54564 copyname64
>>>> \.

oidtest=# INSERT INTO oidtestpk_another(name) VALUES ('name63');
INSERT 54563 1
oidtest=# INSERT INTO oidtestpk_another(name) VALUES ('name64');
INSERT 54564 1
oidtest=# SELECT oid,name FROM oidtestpk where oid=54564;
oid | name
-------+------------
54564 | copyname64
(1 row)

oidtest=# SELECT oid,name FROM oidtestpk_another where oid=54564;
oid | name
-------+--------
54564 | name64
(1 row)

And then I got records with same oid(54564).
Is it a bug?

---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match

"wangshj" <wangshj@sduept.com> writes:
> Description: System column oid is not unique

There isn't, and never has been nor will be, any promise that it is,
except to the extent that uniqueness is enforced by a unique index
on a particular table. OID is just a kind of serial sequence generator.

regards, tom lane

---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match