Update is not atomic
Following problem is encountered in Postgersql 7.1.2 on Linux
(I haven't tested on Solaris, but it seems that it is
platform-independent)
If we are trying to update field with unique constraint on it
(for instance, primary key) update might fail depending on order
how individual rows are updated. This is clear violation of SQL standard,
which states that individual sql operators are atomic, and constraints
should be valid either before beginning or after completion of operator.
Problem can be demonstrated by following example
create table a (a numeric primary key);
insert into a values (1);
insert into a values (2);
insert into a values (3);
insert into a values (4);
update a set a=a+1 where a>2;
ERROR: Cannot insert a duplicate key into unique index a_pkey
Same statement sequence in Oracle (8.1.5.1) produces expected result:
2 rows updated
SQL> select * from a;
A
----------
1
2
4
5
--
Victor Wagner vitus@ice.ru
Chief Technical Officer Office:7-(095)-748-53-88
Communiware.Net Home: 7-(095)-135-46-61
http://www.communiware.net http://www.ice.ru/~vitus
Victor Wagner writes:
create table a (a numeric primary key);
insert into a values (1);
insert into a values (2);
insert into a values (3);
insert into a values (4);
update a set a=a+1 where a>2;
ERROR: Cannot insert a duplicate key into unique index a_pkey
This is a known problem with unique contraints, but it's not easy to fix
it.
--
Peter Eisentraut peter_e@gmx.net http://funkturm.homeip.net/~peter
On Wed, 20 Jun 2001, Peter Eisentraut wrote:
From: Peter Eisentraut <peter_e@gmx.net>
Subject: Re: [BUGS] Update is not atomicVictor Wagner writes:
create table a (a numeric primary key);
insert into a values (1);
insert into a values (2);
insert into a values (3);
insert into a values (4);
update a set a=a+1 where a>2;
ERROR: Cannot insert a duplicate key into unique index a_pkey
This is a known problem with unique contraints, but it's not easy to fix
it.
Is there any known workaround?
Like specifying order of updates?
--
Victor Wagner vitus@ice.ru
Chief Technical Officer Office:7-(095)-748-53-88
Communiware.Net Home: 7-(095)-135-46-61
http://www.communiware.net http://www.ice.ru/~vitus