Re: [SQL] transaction processing after error in statement

[Holger Jakobs]

Hi Stephan,

On  9 Nov, Stephan Szabo wrote:
> On Fri, 7 Nov 2003 [EMAIL PROTECTED] wrote:
> 
>> Whenever an error occurs within the transaction, PostgreSQL puts the
>> whole transaction in an *ABORT* state, so that there is no difference
>> at all between COMMITing or ROLLBACKing it. Even commands
>> successfully carried out before the error ocurred are rolled back,
>> even if I COMMIT the transaction, where no error message whatsoever
>> is shown.
> 
> In PostgreSQL all errors are currently considered unrecoverable, and
> all statements in a transaction must commit or rollback together as a
> single unit. In the future an implementation of nested transactions or
> savepoints would presumably relax this limitation to only the
> successfully committed subtransactions or statements that were not
> separately rolled back to a previous savepoint.
 
What I meant was not subtransactions or savepoints, but the funny
behaviour that operations already successfully carried out never will be
committed, just because some other operation later within the same
transaction fails. This is far different from the behaviour of all other
DMBS I know. Why not:

   begin work;
   insert into x values (1, 'hi');
   --> success
   insert into x values (1, 'there');
   --> failure due to primary key violation
   insert into x values (2, 'foo');
   --> success
   commit work;

and have two new tuples in the table? Why do _all_ of these operations
have to be rolled back? I just don't get it that this has anything to do
with savepoints or so. I don't see any problem with an error being
recoverable, because the second insert failed and does not have to be
recovered while the first and the third worked fine and does not have to
be recovered either. When committing a transaction the effects of all
operations that did not fail will be made permanent. This is how
transaction processing is described in the literature.

If a programmer wants the whole transaction to fail because one part
failed, (s)he can always program a rollback in case of at least one
error. But there should always be a difference between a rollback and a
commit, after at least one statement changing data has reported a
success.

Hopefully this can be cleared and perhaps improved within PostgreSQL.
Otherwise, PostgreSQL always claims to be close to standards.

Sincerely,

Holger
-- 
[EMAIL PROTECTED], Bergisch Gladbach, Germany
Telefon (0 22 02) 5 99 91 oder (01 77) 7 92 24 66


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Re: [SQL] transaction processing after error in statement

[Holger Jakobs]

> 
> Why is that "funny behaviour" for you? By putting the statements into
> a transaction block you told the data management system "I want this
> group of statements to be atomic". Atomic means all or nothing. It
> might not be exactly what you intended to say, and you have a point
> if you conclude that PostgreSQL is limited because it doesn't let you
> say anything in between all or nothing. But after all, thus far it is
> only doing what you asked for.
> 

It is "funny behaviour", because I expect those operations of the
transaction, which executed successfully, to be performed in an atomic
way. It is obvious that I cannot expect an operation which reported an
error to have any effect.

"Atomic" means that all operations (whether successful or not) will be
carried out all together or none of them - but only the successful ones
will have had an effect.

Again: Why not make a difference between "commit" and "rollback" in a
transaction in this case? Why not let the user decide which parts should
be commited? The practical reason is that programming would become a lot
more convenient. (if there is a practical reason it does not necessarily
need a technical reason, I believe.)

Sincerely,

Holger

-- 
[EMAIL PROTECTED], Bergisch Gladbach, Germany
Telefon (0 22 02) 5 99 91 oder (01 77) 7 92 24 66


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Re: [SQL] transaction processing after error in statement

[Holger Jakobs]

On 11 Nov, Jan Wieck wrote:
> As long as we talk in an SQL context, can you please stick to SQL
> terms? I don't know exactly what you mean with "operation". If for
> example the statement
> 
>  DELETE FROM order_line WHERE ol_ordernum = 4711;
> 
> has 12 matching rows in order_line, is an operation the removal of one 
> single order line or do the actions performed by the triggers fired
> due to their removal count as separate operations for you? And if
> there is one that cannot be deleted because a row in another table
> with a foreign key references it, do you delete none of them or the
> remaining 11? And if you decide to delete none, how do you magically
> undo the work of the BEFORE triggers if you hit the foreign key after
> successfully processing 5 rows? Is there an SQL return code for
> "partial success"?
OK, let's say "statement" instead of "operation". 

No, there is no partial success. Either a statement delivers an "OK" or
it doesn't. Actually, you will have to undo anything the statement did
before the first error occurs. This may mean that you need some kind of
savepoint. If so, the necessity to implent this shows and should be
given a high priority. I don't know how the other RDMBS do it, but they
do. I am talking from the perspective of an RDBMS user, not as an
implementor.

Calling a procedure is a statement as well, and it includes all other
procedures called from this procedure. So the statement level is always
the statements that were carried out directly in the transaction. If
anything within one statement fails, the statement was not carried out
and must not have any effect. It is not important whether the procedure
was fired by a trigger or called by another procedure.


Are there any Open Source RDBMS which behave like Oracle, Access and
Allbase? If so, one might look into their code to find out how they have
implented it.


Coming back to Standards, here is a quote from the ANSI document:

   4.28 SQL-transactions

   The execution of a  may be initiated implicitly
   by an implementation when it detects unrecoverable errors. When
   such an error occurs, an exception condition is raised: transaction
   rollback with an implementation-defined subclass code.

This means that a rollback does not have to be initiated if an 
unrecoverable error occurs, it only _may_ happen. Since it is 
impractical, it should not.

AN EXAMPLE:

Let's have two tables, employees and doctors in a hospital.

create table emp (
  empno  integer primary key,
  name varchar(40)
);

create table doctor (
  empno integer primary key references emp,
  beepernumber integer unique
);

Now let a user enter the data of a doctor. First the data of the
employee part are sent to the database:
  insert into emp values (1, 'Fred');
  --> success
  
Second the doctor-special data are sent to the database:
  insert into doctor values (1, 34);
  -->error, beepernumber already present, unique key violation

Since there was an error, we let the user key in a different 
beeper number for the doctor and send the data to the
database:
  insert into doctor (1, 45);
  -->should be successful (if 45 is not already there) and it _is_ in
  Allbase, Oracle, Access
  -->Postgres tells you something about and *ABORT* state

We commit the transaction: 
  commit work;
  
Effect in all other databases: a successfully entered doctor
Effect in PostgreSQL: nothing!

To get the same effect in PostgreSQL, we would have to rollback
(or commit, no difference) the transaction after the attempt of
inserting the non-unique beeper number and then re-insert all
data into all tables. WHY? It only makes programming more
complex, thus error-prone.


I would appreciate if most of the difficult tasks could be done within
the database system - that's what it's there fore. The first reason is
that the implementors usually are far more advanced programmers than the
programmers of applications. The second reason is that this isolates the
tricky programming from the already complex logic of the application.

Additionally, whether one might personally think that this behaviour is
important or not, it is what most commercial RDBMS do. So it would make
porting of applications from e. g. Oracle to PostgreSQL dramatically
easier. Until now, this has been one of the pitfalls. That's why I
brought this up in the first place.


Sincerely,

Holger

-- 
[EMAIL PROTECTED], Bergisch Gladbach, Germany
Telefon (0 22 02) 5 99 91 oder (01 77) 7 92 24 66


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