Error on failed COMMIT

Vik Fearing <vik@postgresfriends.org> writes:

There is a current discussion off-list about what should happen when a
COMMIT is issued for a transaction that cannot be committed for whatever
reason. PostgreSQL returns ROLLBACK as command tag but otherwise succeeds.

It seems like [ trying to commit a failed transaction ]
should actually produce something like this:

postgres=!# commit;
ERROR: 40P00: transaction cannot be committed
DETAIL: First error was "42601: syntax error at or near "error""

So I assume you're imagining that that would leave us still in
transaction-aborted state, and the session is basically dead in
the water until the user thinks to issue ROLLBACK instead?

Is this reading correct?

Probably it is, according to the letter of the SQL spec, but I'm
afraid that changing this behavior now would provoke lots of hate
and few compliments. An application that's doing the spec-compliant
thing and issuing ROLLBACK isn't going to be affected, but apps that
are relying on the existing behavior are going to be badly broken.

A related problem is what happens if you're in a perfectly-fine
transaction and the commit itself fails, e.g.,

regression=# create table tt (f1 int primary key deferrable initially deferred);
CREATE TABLE
regression=# begin;
BEGIN
regression=# insert into tt values (1);
INSERT 0 1
regression=# insert into tt values (1);
INSERT 0 1
regression=# commit;
ERROR: duplicate key value violates unique constraint "tt_pkey"
DETAIL: Key (f1)=(1) already exists.

At this point PG considers that you're out of the transaction:

regression=# rollback;
WARNING: there is no transaction in progress
ROLLBACK

but I bet the spec doesn't. So if we change that, again we break
applications that work today. Meanwhile, an app that is doing it
the spec-compliant way will issue a ROLLBACK that we consider
useless, so currently that draws an ignorable WARNING and all is
well. So here also, the prospects for making more people happy
than we make unhappy seem pretty grim. (Maybe there's a case
for downgrading the WARNING to NOTICE, though?)

(Don't even *think* of suggesting that having a GUC to change
this behavior would be appropriate. The long-ago fiasco around
autocommit showed us the hazards of letting GUCs affect such
fundamental behavior.)

Speaking of autocommit, I wonder how that would interact with
this...

regards, tom lane

On 11/02/2020 23:35, Tom Lane wrote:

Vik Fearing <vik@postgresfriends.org> writes:

There is a current discussion off-list about what should happen when a
COMMIT is issued for a transaction that cannot be committed for whatever
reason. PostgreSQL returns ROLLBACK as command tag but otherwise succeeds.

It seems like [ trying to commit a failed transaction ]
should actually produce something like this:

postgres=!# commit;
ERROR: 40P00: transaction cannot be committed
DETAIL: First error was "42601: syntax error at or near "error""

So I assume you're imagining that that would leave us still in
transaction-aborted state, and the session is basically dead in
the water until the user thinks to issue ROLLBACK instead?

Actually, I was imagining that it would end the transaction as it does
today, just with an error code.

This is backed up by General Rule 9 which says "The current
SQL-transaction is terminated."

Is this reading correct?

Probably it is, according to the letter of the SQL spec, but I'm
afraid that changing this behavior now would provoke lots of hate
and few compliments. An application that's doing the spec-compliant
thing and issuing ROLLBACK isn't going to be affected, but apps that
are relying on the existing behavior are going to be badly broken.

I figured that was likely. I'm hoping to at least get a documentation
patch out of this thread, though.

A related problem is what happens if you're in a perfectly-fine
transaction and the commit itself fails, e.g.,

regression=# create table tt (f1 int primary key deferrable initially deferred);
CREATE TABLE
regression=# begin;
BEGIN
regression=# insert into tt values (1);
INSERT 0 1
regression=# insert into tt values (1);
INSERT 0 1
regression=# commit;
ERROR: duplicate key value violates unique constraint "tt_pkey"
DETAIL: Key (f1)=(1) already exists.

At this point PG considers that you're out of the transaction:

regression=# rollback;
WARNING: there is no transaction in progress
ROLLBACK

but I bet the spec doesn't. So if we change that, again we break
applications that work today.

I would argue that the this example is entirely compliant and consistent
with my original question (except that it gives a class 23 instead of a
class 40).

Meanwhile, an app that is doing it
the spec-compliant way will issue a ROLLBACK that we consider
useless, so currently that draws an ignorable WARNING and all is
well. So here also, the prospects for making more people happy
than we make unhappy seem pretty grim.

I'm not entirely sure what should happen with a free-range ROLLBACK. (I
*think* it says it should error with "2D000 invalid transaction
termination" but it's a little confusing to me.)

(Maybe there's a case for downgrading the WARNING to NOTICE, though?)

Maybe. But I think its match (a double START TRANSACTION) should remain
a warning if we do change this.

(Don't even *think* of suggesting that having a GUC to change
this behavior would be appropriate. The long-ago fiasco around
autocommit showed us the hazards of letting GUCs affect such
fundamental behavior.)

That thought never crossed my mind.

Speaking of autocommit, I wonder how that would interact with
this...

I don't see how it would be any different.
--
Vik Fearing

Vik Fearing <vik@postgresfriends.org> writes:

On 11/02/2020 23:35, Tom Lane wrote:

So I assume you're imagining that that would leave us still in
transaction-aborted state, and the session is basically dead in
the water until the user thinks to issue ROLLBACK instead?

Actually, I was imagining that it would end the transaction as it does
today, just with an error code.
This is backed up by General Rule 9 which says "The current
SQL-transaction is terminated."

Hm ... that would be sensible, but I'm not entirely convinced. There
are several preceding rules that say that an exception condition is
raised, and normally you can stop reading at that point; nothing else
is going to happen. If COMMIT acts specially in this respect, they
ought to say so.

In any case, while this interpretation might change the calculus a bit,
I think we still end up concluding that altering this behavior has more
downside than upside.

regards, tom lane

On Tue, 11 Feb 2020 at 17:35, Tom Lane <tgl@sss.pgh.pa.us> wrote:

Vik Fearing <vik@postgresfriends.org> writes:

There is a current discussion off-list about what should happen when a
COMMIT is issued for a transaction that cannot be committed for whatever
reason. PostgreSQL returns ROLLBACK as command tag but otherwise

succeeds.

It seems like [ trying to commit a failed transaction ]
should actually produce something like this:

postgres=!# commit;
ERROR: 40P00: transaction cannot be committed
DETAIL: First error was "42601: syntax error at or near "error""

So I assume you're imagining that that would leave us still in
transaction-aborted state, and the session is basically dead in
the water until the user thinks to issue ROLLBACK instead?

Is this reading correct?

Probably it is, according to the letter of the SQL spec, but I'm
afraid that changing this behavior now would provoke lots of hate
and few compliments. An application that's doing the spec-compliant
thing and issuing ROLLBACK isn't going to be affected, but apps that
are relying on the existing behavior are going to be badly broken.

A related problem is what happens if you're in a perfectly-fine
transaction and the commit itself fails, e.g.,

regression=# create table tt (f1 int primary key deferrable initially
deferred);
CREATE TABLE
regression=# begin;
BEGIN
regression=# insert into tt values (1);
INSERT 0 1
regression=# insert into tt values (1);
INSERT 0 1
regression=# commit;
ERROR: duplicate key value violates unique constraint "tt_pkey"
DETAIL: Key (f1)=(1) already exists.

At this point PG considers that you're out of the transaction:

regression=# rollback;
WARNING: there is no transaction in progress
ROLLBACK

interesting as if you do a commit after violating a not null it simply does
a rollback
with no warning whatsoever

begin;
BEGIN
test=# insert into hasnull(i) values (null);
ERROR: null value in column "i" violates not-null constraint
DETAIL: Failing row contains (null).
test=# commit;
ROLLBACK

but I bet the spec doesn't. So if we change that, again we break
applications that work today. Meanwhile, an app that is doing it
the spec-compliant way will issue a ROLLBACK that we consider
useless, so currently that draws an ignorable WARNING and all is
well. So here also, the prospects for making more people happy
than we make unhappy seem pretty grim. (Maybe there's a case
for downgrading the WARNING to NOTICE, though?)

Actually the bug reporter was looking for an upgrade from a warning to an

ERROR

I realize we are unlikely to change the behaviour however it would be
useful if we
did the same thing for all cases, and document this behaviour. We actually
have places where
we document where we don't adhere to the spec.

Dave

Am 12.02.2020 um 00:27 schrieb Tom Lane:

Vik Fearing <vik@postgresfriends.org> writes:

Actually, I was imagining that it would end the transaction as it does
today, just with an error code.
This is backed up by General Rule 9 which says "The current
SQL-transaction is terminated."

Hm ... that would be sensible, but I'm not entirely convinced. There
are several preceding rules that say that an exception condition is
raised, and normally you can stop reading at that point; nothing else
is going to happen. If COMMIT acts specially in this respect, they
ought to say so.

In any case, while this interpretation might change the calculus a bit,
I think we still end up concluding that altering this behavior has more
downside than upside.

Let me illustrate this issue from an application (framework) developer's
perspective:

When an application interacts with a database, it must be clearly
possible to determine, whether a commit actually succeeded (and made all
changes persistent), or the commit failed for any reason (and all of the
changes have been rolled back). If a commit succeeds, an application
must be allowed to assume that all changes it made in the preceeding
transaction are made persistent and it is valid to update its internal
state (e.g. caches) to the values updated in the transaction. This must
be possible, even if the transaction is constructed collaboratively by
multipe independent layers of the application (e.g. a framework and an
application layer). Unfortunately, this seems not to be possible with
the current implementation - at least not with default settings:

Assume the application is written in Java and sees Postgres through the
JDBC driver:

composeTransaction() {
   Connection con = getConnection(); // implicitly "begin"
   try {
      insertFrameworkLevelState(con);
      insertApplicationLevelState(con);
      con.commit();
      publishNewState();
   } catch (Throwable ex) {
      con.rollback();
   }
}

With the current implementation, it is possible, that the control flow
reaches "publishNewState()" without the changes done in
"insertFrameworkLevelState()" have been made persistent - without the
framework-level code (which is everything except
"insertApplicationLevelState()") being able to detect the problem (e.g.
if "insertApplicationLevelState()" tries add a null into a non-null
column catching the exception or any other application-level error that
is not properly handled through safepoints).

From a framework's perspective, this behavior is absolutely
unacceptable. Here, the framework-level code sees a database that
commits successfully but does not make its changes persistent.
Therefore, I don't think that altering this behavior has more downside
than upside.

Best regards

Bernhard

On Thu, Feb 13, 2020 at 2:38 AM Haumacher, Bernhard <haui@haumacher.de> wrote:

Assume the application is written in Java and sees Postgres through the
JDBC driver:

composeTransaction() {
Connection con = getConnection(); // implicitly "begin"
try {
insertFrameworkLevelState(con);
insertApplicationLevelState(con);
con.commit();
publishNewState();
} catch (Throwable ex) {
con.rollback();
}
}

With the current implementation, it is possible, that the control flow
reaches "publishNewState()" without the changes done in
"insertFrameworkLevelState()" have been made persistent - without the
framework-level code (which is everything except
"insertApplicationLevelState()") being able to detect the problem (e.g.
if "insertApplicationLevelState()" tries add a null into a non-null
column catching the exception or any other application-level error that
is not properly handled through safepoints).

From a framework's perspective, this behavior is absolutely
unacceptable. Here, the framework-level code sees a database that
commits successfully but does not make its changes persistent.
Therefore, I don't think that altering this behavior has more downside
than upside.

I am not sure that this example really proves anything. If
insertFrameworkLevelState(con), insertApplicationLevelState(con), and
con.commit() throw exceptions, or if they return a status code and you
check it and throw an exception if it's not what you expect, then it
will work. If database errors that occur during those operations are
ignored, then you've got a problem, but it does not seem necessary to
change the behavior of the database to fix that problem.

I think one of the larger issues in this area is that people have
script that go:

BEGIN;
-- do stuff
COMMIT;
BEGIN;
-- do more stuff
COMMIT;

...and they run these scripts by piping them into psql. Now, if the
COMMIT leaves the session in a transaction state, this is going to
have pretty random behavior. Like your example, this can be fixed by
having proper error checking in the application, but that does require
that your application is something more than a psql script.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On Fri, 14 Feb 2020 at 12:37, Robert Haas <robertmhaas@gmail.com> wrote:

On Thu, Feb 13, 2020 at 2:38 AM Haumacher, Bernhard <haui@haumacher.de>
wrote:

Assume the application is written in Java and sees Postgres through the
JDBC driver:

composeTransaction() {
Connection con = getConnection(); // implicitly "begin"
try {
insertFrameworkLevelState(con);
insertApplicationLevelState(con);
con.commit();
publishNewState();
} catch (Throwable ex) {
con.rollback();
}
}

With the current implementation, it is possible, that the control flow
reaches "publishNewState()" without the changes done in
"insertFrameworkLevelState()" have been made persistent - without the
framework-level code (which is everything except
"insertApplicationLevelState()") being able to detect the problem (e.g.
if "insertApplicationLevelState()" tries add a null into a non-null
column catching the exception or any other application-level error that
is not properly handled through safepoints).

From a framework's perspective, this behavior is absolutely
unacceptable. Here, the framework-level code sees a database that
commits successfully but does not make its changes persistent.
Therefore, I don't think that altering this behavior has more downside
than upside.

I am not sure that this example really proves anything. If
insertFrameworkLevelState(con), insertApplicationLevelState(con), and
con.commit() throw exceptions, or if they return a status code and you
check it and throw an exception if it's not what you expect, then it
will work.

Thing is that con.commit() DOESN'T return a status code, nor does it throw
an exception as we silently ROLLBACK here.

As noted up thread it's somewhat worse as depending on how the transaction
failed we seem to do different things

In Tom's example we do issue a warning and say there is no transaction
running. I would guess we silently rolled back earlier.
In my example we don't issue the warning we just roll back.

I do agree with Tom that changing this behaviour at this point would make
things worse for more people than it would help so I am not advocating
throwing an error here.

I would however advocate for consistently doing the same thing with failed
transactions

Dave Cramer

www.postgres.rocks

On Fri, Feb 14, 2020 at 1:04 PM Dave Cramer <davecramer@postgres.rocks> wrote:

Thing is that con.commit() DOESN'T return a status code, nor does it throw an exception as we silently ROLLBACK here.

Why not? There's nothing keeping the driver from doing either of those
things, is there? I mean, if using libpq, you can use PQcmdStatus() to
get the command tag, and find out whether it's COMMIT or ROLLBACK. If
you're implementing the wire protocol directly, you can do something
similar.

https://www.postgresql.org/docs/current/libpq-exec.html#LIBPQ-EXEC-NONSELECT

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On Fri, 14 Feb 2020 at 13:29, Robert Haas <robertmhaas@gmail.com> wrote:

On Fri, Feb 14, 2020 at 1:04 PM Dave Cramer <davecramer@postgres.rocks>
wrote:

Thing is that con.commit() DOESN'T return a status code, nor does it

throw an exception as we silently ROLLBACK here.

Why not? There's nothing keeping the driver from doing either of those
things, is there? I mean, if using libpq, you can use PQcmdStatus() to
get the command tag, and find out whether it's COMMIT or ROLLBACK. If
you're implementing the wire protocol directly, you can do something
similar.

https://www.postgresql.org/docs/current/libpq-exec.html#LIBPQ-EXEC-NONSELECT

Well now you are asking the driver to re-interpret the results in a
different way than the server which is not what we tend to do.

The server throws an error we throw an error. We really aren't in the
business of re-interpreting the servers responses.

Dave Cramer
www.postgres.rocks

On Fri, Feb 14, 2020 at 2:08 PM Dave Cramer <davecramer@postgres.rocks> wrote:

Well now you are asking the driver to re-interpret the results in a different way than the server which is not what we tend to do.

The server throws an error we throw an error. We really aren't in the business of re-interpreting the servers responses.

I don't really see a reason why the driver has to throw an exception
if and only if there is an ERROR on the PostgreSQL side. But even if
you want to make that rule for some reason, it doesn't preclude
correct behavior here. All you really need is to have con.commit()
return some indication of what the command tag was, just as, say, psql
would do. If the server provides you with status information and you
throw it out instead of passing it along to the application, that's
not ideal.

Another thing that kinda puzzles me about this situation is that, as
far as I know, the only time COMMIT returns ROLLBACK is if the
transaction has already previously reported an ERROR. But if an ERROR
gets turned into an exception, then, in the code snippet previously
provided, we'd never reach con.commit() in the first place.

I'm not trying to deny that you might find some other server behavior
more convenient. You might. And, to Vik's original point, it might be
more compliant with the spec, too. But since changing that would have
a pretty big blast radius at this stage, I think it's worth trying to
make things work as well as they can with the server behavior that we
already have. And I don't really see anything preventing the driver
from doing that technically. I don't understand the idea that the
driver is somehow not allowed to notice the command tag.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On Fri, 14 Feb 2020 at 14:37, Robert Haas <robertmhaas@gmail.com> wrote:

On Fri, Feb 14, 2020 at 2:08 PM Dave Cramer <davecramer@postgres.rocks>
wrote:

Well now you are asking the driver to re-interpret the results in a

different way than the server which is not what we tend to do.

The server throws an error we throw an error. We really aren't in the

business of re-interpreting the servers responses.

I don't really see a reason why the driver has to throw an exception
if and only if there is an ERROR on the PostgreSQL side. But even if
you want to make that rule for some reason, it doesn't preclude
correct behavior here. All you really need is to have con.commit()
return some indication of what the command tag was, just as, say, psql
would do. If the server provides you with status information and you
throw it out instead of passing it along to the application, that's
not ideal.

Well con.commit() returns void :(

Another thing that kinda puzzles me about this situation is that, as
far as I know, the only time COMMIT returns ROLLBACK is if the
transaction has already previously reported an ERROR. But if an ERROR
gets turned into an exception, then, in the code snippet previously
provided, we'd never reach con.commit() in the first place.

The OP is building a framework where it's possible for the exception to be
swallowed by the consumer of the framework.

I'm not trying to deny that you might find some other server behavior
more convenient. You might. And, to Vik's original point, it might be
more compliant with the spec, too. But since changing that would have
a pretty big blast radius at this stage, I think it's worth trying to
make things work as well as they can with the server behavior that we
already have. And I don't really see anything preventing the driver
from doing that technically. I don't understand the idea that the
driver is somehow not allowed to notice the command tag.

We have the same blast radius.
I have offered to make the behaviour requested dependent on a configuration
parameter but apparently this is not sufficient.

Dave Cramer
www.postgres.rocks

Dave Cramer <davecramer@postgres.rocks> writes:

On Fri, 14 Feb 2020 at 14:37, Robert Haas <robertmhaas@gmail.com> wrote:

I'm not trying to deny that you might find some other server behavior
more convenient. You might. And, to Vik's original point, it might be
more compliant with the spec, too. But since changing that would have
a pretty big blast radius at this stage, I think it's worth trying to
make things work as well as they can with the server behavior that we
already have. And I don't really see anything preventing the driver
from doing that technically. I don't understand the idea that the
driver is somehow not allowed to notice the command tag.

We have the same blast radius.
I have offered to make the behaviour requested dependent on a configuration
parameter but apparently this is not sufficient.

Nope, that is absolutely not happening. We learned very painfully, back
around 7.3 when we tried to put in autocommit on/off, that if server
behaviors like this are configurable then most client code has to be
prepared to work with every possible setting. The argument that "you can
just set it to work the way you expect" is a dangerous falsehood. I see
no reason to think that a change like this wouldn't suffer the same sort
of embarrassing and expensive failure that autocommit did.

regards, tom lane

On Fri, 14 Feb 2020 at 15:07, Tom Lane <tgl@sss.pgh.pa.us> wrote:

Dave Cramer <davecramer@postgres.rocks> writes:

On Fri, 14 Feb 2020 at 14:37, Robert Haas <robertmhaas@gmail.com> wrote:

I'm not trying to deny that you might find some other server behavior
more convenient. You might. And, to Vik's original point, it might be
more compliant with the spec, too. But since changing that would have
a pretty big blast radius at this stage, I think it's worth trying to
make things work as well as they can with the server behavior that we
already have. And I don't really see anything preventing the driver
from doing that technically. I don't understand the idea that the
driver is somehow not allowed to notice the command tag.

We have the same blast radius.
I have offered to make the behaviour requested dependent on a

configuration

parameter but apparently this is not sufficient.

Nope, that is absolutely not happening.

I should have been more specific.

I offered to make the behaviour in the JDBC driver dependent on a
configuration parameter

Dave Cramer
www.postgres.rocks

On 2020-Feb-14, Dave Cramer wrote:

I offered to make the behaviour in the JDBC driver dependent on a
configuration parameter

Do you mean "if con.commit() results in a rollback, then an exception is
thrown, unless the parameter XYZ is set to PQR"?

--
�lvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

On Fri, 14 Feb 2020 at 15:19, Alvaro Herrera <alvherre@2ndquadrant.com>
wrote:

On 2020-Feb-14, Dave Cramer wrote:

I offered to make the behaviour in the JDBC driver dependent on a
configuration parameter

Do you mean "if con.commit() results in a rollback, then an exception is
thrown, unless the parameter XYZ is set to PQR"?

No. JDBC has a number of connection parameters which change internal
semantics of various things.

I was proposing to have a connection parameter which would be
throwExceptionOnFailedCommit (or something) which would do what it says.

None of this would touch the server. It would just change the driver
semantics.

Dave Cramer
www.postgres.rocks

Am 14.02.2020 um 20:36 schrieb Robert Haas:

On Fri, Feb 14, 2020 at 2:08 PM Dave Cramer <davecramer@postgres.rocks> wrote:

Well now you are asking the driver to re-interpret the results in a different way than the server which is not what we tend to do.

The server throws an error we throw an error. We really aren't in the business of re-interpreting the servers responses.

I don't really see a reason why the driver has to throw an exception
if and only if there is an ERROR on the PostgreSQL side. But even if
you want to make that rule for some reason, it doesn't preclude
correct behavior here. All you really need is to have con.commit()
return some indication of what the command tag was, just as, say, psql
would do.

I think, this would be an appropriate solution. PostgreSQL reports the
"unsuccessful" commit through the "ROLLBACK" status code and the driver
translates this into a Java SQLException, because this is the only way
to communicate the "non-successfullness" from the void commit() method.
Since the commit() was not successful, from the API point of view this
is an error and it is fine to report this using an exception.

Am 14.02.2020 um 21:07 schrieb Tom Lane:

Dave Cramer <davecramer@postgres.rocks> writes:

We have the same blast radius.
I have offered to make the behaviour requested dependent on a configuration
parameter but apparently this is not sufficient.

Nope, that is absolutely not happening. We learned very painfully, back
around 7.3 when we tried to put in autocommit on/off, that if server
behaviors like this are configurable then most client code has to be
prepared to work with every possible setting. The argument that "you can
just set it to work the way you expect" is a dangerous falsehood. I see
no reason to think that a change like this wouldn't suffer the same sort
of embarrassing and expensive failure that autocommit did.

Doing this in a (non-default) driver setting is not ideal, because I
expect do be notified *by default* from a database (driver) if a commit
was not successful (and since the API is void, the only notification
path is an exception). We already have a non-default option named
"autosafe", which fixes the problem somehow.

If we really need both behaviors ("silently ignore failed commits" and
"notify about failed commits") I would prefer adding a
backwards-compatible option
"silently-ignore-failed-commit-due-to-auto-rollback" (since it is a
really aburd setting from my point of view, since consistency is at risk
if this happens - the worst thing to expect from a database).

Regards,  Bernhard

On Mon, 17 Feb 2020 at 13:02, Haumacher, Bernhard <haui@haumacher.de> wrote:

Am 14.02.2020 um 20:36 schrieb Robert Haas:

On Fri, Feb 14, 2020 at 2:08 PM Dave Cramer <davecramer@postgres.rocks>

wrote:

Well now you are asking the driver to re-interpret the results in a

different way than the server which is not what we tend to do.

The server throws an error we throw an error. We really aren't in the

business of re-interpreting the servers responses.

I don't really see a reason why the driver has to throw an exception
if and only if there is an ERROR on the PostgreSQL side. But even if
you want to make that rule for some reason, it doesn't preclude
correct behavior here. All you really need is to have con.commit()
return some indication of what the command tag was, just as, say, psql
would do.

I think, this would be an appropriate solution. PostgreSQL reports the
"unsuccessful" commit through the "ROLLBACK" status code and the driver
translates this into a Java SQLException, because this is the only way
to communicate the "non-successfullness" from the void commit() method.
Since the commit() was not successful, from the API point of view this
is an error and it is fine to report this using an exception.

Well it doesn't always report the unsuccessful commit as a rollback
sometimes it says
"there is no transaction" depending on what happened in the transaction

Also when there is an error there is also a status provided by the backend.
Since this is not an error to the backend there is no status that the
exception can provide.

Am 14.02.2020 um 21:07 schrieb Tom Lane:

Dave Cramer <davecramer@postgres.rocks> writes:

We have the same blast radius.
I have offered to make the behaviour requested dependent on a

configuration

parameter but apparently this is not sufficient.

Nope, that is absolutely not happening. We learned very painfully, back
around 7.3 when we tried to put in autocommit on/off, that if server
behaviors like this are configurable then most client code has to be
prepared to work with every possible setting. The argument that "you can
just set it to work the way you expect" is a dangerous falsehood. I see
no reason to think that a change like this wouldn't suffer the same sort
of embarrassing and expensive failure that autocommit did.

Doing this in a (non-default) driver setting is not ideal, because I
expect do be notified *by default* from a database (driver) if a commit
was not successful (and since the API is void, the only notification
path is an exception). We already have a non-default option named
"autosafe", which fixes the problem somehow.

The challenge with making this the default, is as Tom noted, many other
people don't expect this.

I think the notion that every JDBC driver works exactly the same way for
every API call is a challenge.
Take for instance SERIALIZABLE transaction isolation.
Only PostgreSQL actually implements it correctly. AFAIK Oracle SERIALIZABLE
is actually REPEATABLE READ

What many other frameworks do is have vendor specific behaviour.
Perhaps writing a proxying driver might solve the problem?

If we really need both behaviors ("silently ignore failed commits" and
"notify about failed commits") I would prefer adding a
backwards-compatible option
"silently-ignore-failed-commit-due-to-auto-rollback" (since it is a
really aburd setting from my point of view, since consistency is at risk
if this happens - the worst thing to expect from a database).

The error has been reported to the client. At this point the client is
expected to do a rollback.

Regards,
Dave

On 2020-Feb-14, Dave Cramer wrote:

On Fri, 14 Feb 2020 at 15:19, Alvaro Herrera <alvherre@2ndquadrant.com>
wrote:

Do you mean "if con.commit() results in a rollback, then an exception is
thrown, unless the parameter XYZ is set to PQR"?

No. JDBC has a number of connection parameters which change internal
semantics of various things.

I was proposing to have a connection parameter which would be
throwExceptionOnFailedCommit (or something) which would do what it says.

None of this would touch the server. It would just change the driver
semantics.

That's exactly what I was saying.

--
�lvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Am 17.02.2020 um 23:12 schrieb Dave Cramer:

On Mon, 17 Feb 2020 at 13:02, Haumacher, Bernhard <haui@haumacher.de
<mailto:haui@haumacher.de>> wrote:

... would be an appropriate solution. PostgreSQL reports the
"unsuccessful" commit through the "ROLLBACK" status code and the
driver
translates this into a Java SQLException, because this is the only
way
to communicate the "non-successfullness" from the void commit()
method.
Since the commit() was not successful, from the API point of view
this
is an error and it is fine to report this using an exception.

Well it doesn't always report the unsuccessful commit as a rollback
sometimes it says
"there is no transaction" depending on what happened in the transaction

even worse...

Also when there is an error there is also a status provided by the
backend.
Since this is not an error to the backend there is no status that the
exception can provide.

be free to choose/define one...

Doing this in a (non-default) driver setting is not ideal, because I
expect do be notified *by default* from a database (driver) if a
commit
was not successful (and since the API is void, the only notification
path is an exception). We already have a non-default option named
"autosafe", which fixes the problem somehow.

The challenge with making this the default, is as Tom noted, many
other people don't expect this.

Nobody expects a database reporting a successful commit, while it
internally rolled back.

If there is code out there depending on this bug, it is fair to provide
a backwards-compatible option to re-activate this unexpected behavior.

What many other frameworks do is have vendor specific behaviour.
Perhaps writing a proxying driver might solve the problem?

That's exactly what we do - extending our database abstraction layer to
work around database-specific interpretations of the JDBC API.

But of cause, the abstraction layer is not able to reconstruct an error
from a commit() call, that has been dropped by the driver. Of cause, I
could try to insert another dummy entry into a dummy table immediately
before each commit to get again the exception reporting that the
transaction is in rollback-only-mode... but this does not sound
reasonable to me.

If we really need both behaviors ("silently ignore failed commits"
and
"notify about failed commits") I would prefer adding a
backwards-compatible option
"silently-ignore-failed-commit-due-to-auto-rollback" (since it is a
really aburd setting from my point of view, since consistency is
at risk
if this happens - the worst thing to expect from a database).

The error has been reported to the client. At this point the client is
expected to do a rollback.

As I explained, there is not "the client" but there are several software
layers - and the error only has been reported to some of these layers
that may decide not to communicate the problem down the road. Therefore,
the final commit() must report the problem again.

Best regard, Bernhard