Обсуждение: An example needed for Serializable conflict...

Hi!

Durumdara wrote:
> Please send me an example (pseudo-code) for Serializable conflict.
> And I wanna know, if possible, that if more transactions only
> read the tables in Serializable mode, and one or others write
> to it, can I got some conflicts in read operation?

You get a serialization conflict if you try to modify a row
in a serializable transaction T1 that has been changed by a second
transaction T2 after T1 started.

Sample 1:

T1: START TRANSACTION ISOLATION LEVEL SERIALIZABLE;

T1: SELECT * FROM t;
 id | val
----+------
  1 | test
(1 row)

T2: DELETE FROM t WHERE id=1;

T1: UPDATE t SET val='new' WHERE id=1;
ERROR:  could not serialize access due to concurrent update

Sample 2:

T1: START TRANSACTION ISOLATION LEVEL SERIALIZABLE;

T1: SELECT * FROM t;
 id | val
----+------
  1 | test
(1 row)

T2: UPDATE t SET val=val WHERE id=1;

T1: DELETE FROM t;
ERROR:  could not serialize access due to concurrent update


Yours,
Laurenz Albe

Hi!

Thanks for your help!

Another question if I use only "SELECTS" can I get some Serialization Error?

For example:
I need a report tool that can show the actual state of the business.
Because of I use value-dependent logic, I MUST use consistent state to preserve the equality of many values (sums, counts, etc.).

So some (Read Committer) threads are update/delete/insert (sum modify) rows, but this report tool only READ the tables, and only works for temp tables.

Can I get some S. error from this transaction?
Or can I get some error from modifier threads if this (serializer report) thread actually read the rows that they are want to modify?

This is the main question about it.

Thanks for your read/answer!

    dd



2009.07.07. 11:36 keltezéssel, Albe Laurenz írta:

Durumdara wrote: 

Please send me an example (pseudo-code) for Serializable conflict.
And I wanna know, if possible, that if more transactions only 
read the tables in Serializable mode, and one or others write 
to it, can I got some conflicts in read operation?   

You get a serialization conflict if you try to modify a row
in a serializable transaction T1 that has been changed by a second
transaction T2 after T1 started.

Sample 1:

T1: START TRANSACTION ISOLATION LEVEL SERIALIZABLE;

T1: SELECT * FROM t;id | val  
----+------ 1 | test
(1 row)

T2: DELETE FROM t WHERE id=1;

T1: UPDATE t SET val='new' WHERE id=1;
ERROR:  could not serialize access due to concurrent update

Sample 2:

T1: START TRANSACTION ISOLATION LEVEL SERIALIZABLE;

T1: SELECT * FROM t;id | val  
----+------ 1 | test
(1 row)

T2: UPDATE t SET val=val WHERE id=1;

T1: DELETE FROM t;
ERROR:  could not serialize access due to concurrent update


Yours,
Laurenz Albe 

Hi durumdara,

On Tuesday 07 July 2009 12:10:52 durumdara wrote:
> Another question if I use only "SELECTS" can I get some Serialization
> Error?
No.

> For example:
> I need a report tool that can show the actual state of the business.
> Because of I use value-dependent logic, I MUST use consistent state to
> preserve the equality of many values (sums, counts, etc.).
> So some (Read Committer) threads are update/delete/insert (sum modify)
> rows, but this report tool only READ the tables, and only works for temp
> tables.
> Can I get some S. error from this transaction?
> Or can I get some error from modifier threads if this (serializer
> report) thread actually read the rows that they are want to modify?
You can get errors between the writers but not between a writer and a reader.

It probably would be a good idea to read the documentation about mvcc (
http://www.postgresql.org/docs/current/static/mvcc.html) to understand the
possibilities/constraints better.

Andres