Alter Table + Default Value + Serializable
Hi,
I've got a problem with a query run on production system. We've got some
data export in a serializable transaction, and, 2 days ago, someone ran
a DDL ( alter table foo add column ba test default 'blabla'), and then,
the data export is empty. I try to reproduce the scenario below :
begin ;
drop table if exists test ;
create table test ( id serial primary key, t text ) ;
insert into test ( t ) values ( 'test1') ;
insert into test ( t ) values ( 'test2') ;
insert into test ( t ) values ( 'test3') ;
commit ;
-- session 1 |-- session 2
begin ; |
alter table test |
add column toto int |
default 1 ; |
|begin ;
|set transaction isolation level serializable ;
|select * from test ;
|
|
commit ; |
| id | t | toto
|----+---+------
|(0 rows)
|
|commit ;
|
|select * from test ;
| id | t | toto
|----+-------+------
| 1 | test1 | 1
| 2 | test2 | 1
| 3 | test3 | 1
|(3 rows)
I can't understand why, in the 2nd session, my serialisable transaction
see 0 rows ? It's not true, there is rows.
If the DDL in the first transaction doesn't have 'default 1', the
transaction see the 3 rows. If my transaction in the 2nd session is
'read committed', the same.
What's happen with the the serializable transaction and the default ?
Cheers,
--
S�bastien
On 11/05/2010 04:28 PM, S�bastien Lardi�re wrote:
Hi,
I've got a problem with a query run on production system. We've got some
data export in a serializable transaction, and, 2 days ago, someone ran
a DDL ( alter table foo add column ba test default 'blabla'), and then,
the data export is empty. I try to reproduce the scenario below :
I forgot to mention that this scenario works with 8.3 and 9.0.
--
S�bastien
=?ISO-8859-1?Q?S=E9bastien_Lardi=E8re?= <slardiere@hi-media.com> writes:
I've got a problem with a query run on production system. We've got some
data export in a serializable transaction, and, 2 days ago, someone ran
a DDL ( alter table foo add column ba test default 'blabla'), and then,
the data export is empty. I try to reproduce the scenario below :
[ serializable transaction reading from recently-rewritten table ]
Yeah, that's going to be a problem. By the time the serializable
transaction gets to read the altered table, it's a new table all of
whose rows were inserted by the ALTERing transaction. So none of them
are visible to the serializable transaction's snapshot. I don't think
there's a lot that can be done about that. There are some people
working on a reimplementation of serializable mode, but I'm not sure
that it addresses this particular issue; and even if it does, the
likely behavior would be that the serializable transaction would fail
outright rather than give you a surprising view of the table.
It's possible to defend against this type of scenario in the
serializable transaction: lock all the tables you want to touch
before starting the first SELECT. For instance
begin;
set transaction isolation level serializable ;
lock table test in access share mode;
select * from test;
...
This ensures you don't take your snapshot until any concurrent ALTERs
have committed. This might not be too practical for everyday work,
of course, but if you have to have a fix that's what to do.
regards, tom lane
On 11/05/2010 05:19 PM, Tom Lane wrote:
and even if it does, the
likely behavior would be that the serializable transaction would fail
outright rather than give you a surprising view of the table.
thanks for your answer,
I have to say that I would prefer an error in the serializable
transaction, instead of the actual behavior
Nevertheless, thank you, we will lock our tables
regards,
--
S�bastien