Re: Savepoints in PL/pgSQL

I understand that due to a lack of nested transaction support, it is
not possible to use START TRANSACTION within a PL/PgSQL function. What
I, however, do not understand is why I can't use SAVEPOINT either. I'm
writing long test functions wherein, at the start of the function, I'd
like to define all test data followed by a "SAVEPOINT
fresh_test_data;". Will this become possible in the (near) future? I
mean, savepoints are of limited use to me if they imply that I can't
stick my tests in stored procedures.

On a side note: I've seen a comment on this list that the error message
of trying to use a SAVEPOINT in PL/pgSQL isn't too clear (compared with
the error message for using SAVEPOINT in an SQL function). I can second
this. However, I noticed that the message is much clearer when you
EXECUTE the SAVEPOINT command.

On 19 Dec 2006 07:05:10 -0800, "BigSmoke" <bigsmoke(at)gmail(dot)com> wrote:
> I understand that due to a lack of nested transaction support, it is
> not possible to use START TRANSACTION within a PL/PgSQL function. What
> I, however, do not understand is why I can't use SAVEPOINT either. I'm
> writing long test functions wherein, at the start of the function, I'd
> like to define all test data followed by a "SAVEPOINT
> fresh_test_data;". Will this become possible in the (near) future? I
> mean, savepoints are of limited use to me if they imply that I can't
> stick my tests in stored procedures.
>

Use

BEGIN

...

EXCEPTION

...

END;

Blocks instead. The pl/pgsql exception handling is implemented on top
of PostgreSQL's SAVEPOINT infrastructure. We are lacking user defined
exception support, but you can raise generic errors with RAISE EXCEPTION.

See

http://www.postgresql.org/docs/8.2/interactive/plpgsql-control-structures.html#PLPGSQL-ERROR-TRAPPING

for details.

Bernd

On Dec 19, 4:16 pm, maili(dot)(dot)(dot)(at)oopsware(dot)de (Bernd Helmle) wrote:
> On 19 Dec 2006 07:05:10 -0800, "BigSmoke" <bigsm(dot)(dot)(dot)(at)gmail(dot)com> wrote:
>
> > I understand that due to a lack of nested transaction support, it is
> > not possible to use START TRANSACTION within a PL/PgSQL function. What
> > I, however, do not understand is why I can't use SAVEPOINT either. I'm
> > writing long test functions wherein, at the start of the function, I'd
> > like to define all test data followed by a "SAVEPOINT
> > fresh_test_data;". Will this become possible in the (near) future? I
> > mean, savepoints are of limited use to me if they imply that I can't
> > stick my tests in stored procedures.Use
>
> BEGIN
>
> ...
>
> EXCEPTION
>
> ...
>
> END;
>
> Blocks instead. The pl/pgsql exception handling is implemented on top
> of PostgreSQL's SAVEPOINT infrastructure. We are lacking user defined
> exception support, but you can raise generic errors with RAISE EXCEPTION.

I can't solve my problem with a BEGIN EXCEPTION END block because of
what I do in these functions. Here's an example function.

CREATE OR REPLACE FUNCTION test_something() RETURNS VOID AS $$
BEGIN
-- Define some test data

-- SAVEPOINT fresh_test_data; -- If only I could ...

IF some_test_assertion_fails THEN
RAISE EXCEPTION 'Some informative message';
END IF;

-- ROLLBACK TO SAVEPOINT fresh_test_data;
END;
$$ LANGUAGE plpgsql;

In these functions, I raise an exception whenever a test fails. Now, If
I want to create an implicit savepoint using BEGIN/END blocks around
individual tests, I don't see how I can still sanely preserve this
behavior without the most horrid of hacks. The following code is what I
think I would need to do to emulate savepoints without direct access to
them. :-( (I hope that I'm missing something.)

CREATE OR REPLACE FUNCTION test_something() RETURNS VOID AS $$
BEGIN
-- Define some test data

BEGIN
-- This is a useful test ;-)
IF TRUE THEN
RAISE EXCEPTION 'Aaargh! The test failed!';
END IF;

RAISE EXCEPTION '__dummy_restore_state__';

EXCEPTION WHEN raise_exception THEN
IF SQLERRM != '__dummy_restore_state__' THEN
RAISE EXCEPTION '%', SQLERRM;
END IF;
END;
END;
$$ LANGUAGE plpgsql;

On Dec 19, 5:00 pm, "BigSmoke" <bigsm(dot)(dot)(dot)(at)gmail(dot)com> wrote:
> On Dec 19, 4:16 pm, maili(dot)(dot)(dot)(at)oopsware(dot)de (Bernd Helmle) wrote:
>
>
>
> > On 19 Dec 2006 07:05:10 -0800, "BigSmoke" <bigsm(dot)(dot)(dot)(at)gmail(dot)com> wrote:
>
> > > I understand that due to a lack of nested transaction support, it is
> > > not possible to use START TRANSACTION within a PL/PgSQL function. What
> > > I, however, do not understand is why I can't use SAVEPOINT either. I'm
> > > writing long test functions wherein, at the start of the function, I'd
> > > like to define all test data followed by a "SAVEPOINT
> > > fresh_test_data;". Will this become possible in the (near) future? I
> > > mean, savepoints are of limited use to me if they imply that I can't
> > > stick my tests in stored procedures.Use
>
> > BEGIN
>
> > ...
>
> > EXCEPTION
>
> > ...
>
> > END;
>
> > Blocks instead. The pl/pgsql exception handling is implemented on top
> > of PostgreSQL's SAVEPOINT infrastructure. We are lacking user defined
> > exception support, but you can raise generic errors with RAISE EXCEPTION.I can't solve my problem with a BEGIN EXCEPTION END block because of
> what I do in these functions. Here's an example function.
>
> CREATE OR REPLACE FUNCTION test_something() RETURNS VOID AS $$
> BEGIN
> -- Define some test data
>
> -- SAVEPOINT fresh_test_data; -- If only I could ...
>
> IF some_test_assertion_fails THEN
> RAISE EXCEPTION 'Some informative message';
> END IF;
>
> -- ROLLBACK TO SAVEPOINT fresh_test_data;
> END;
> $$ LANGUAGE plpgsql;
>
> In these functions, I raise an exception whenever a test fails. Now, If
> I want to create an implicit savepoint using BEGIN/END blocks around
> individual tests, I don't see how I can still sanely preserve this
> behavior without the most horrid of hacks. The following code is what I
> think I would need to do to emulate savepoints without direct access to
> them. :-( (I hope that I'm missing something.)
>
> CREATE OR REPLACE FUNCTION test_something() RETURNS VOID AS $$
> BEGIN
> -- Define some test data
>
> BEGIN
> -- This is a useful test ;-)
> IF TRUE THEN
> RAISE EXCEPTION 'Aaargh! The test failed!';
> END IF;
>
> RAISE EXCEPTION '__dummy_restore_state__';
>
> EXCEPTION WHEN raise_exception THEN
> IF SQLERRM != '__dummy_restore_state__' THEN
> RAISE EXCEPTION '%', SQLERRM;
> END IF;
> END;
> END;
> $$ LANGUAGE plpgsql;

What would solve my problem is if there was a method to, at the end of
a begin/end block, I could rollback the changes made in that block
without having to raise an exception. Is it somehow possible to
explicitly rollback to one of these savepoints which are created by
begin/end blocks?

On Dec 19, 5:32 pm, "BigSmoke" <bigsm(dot)(dot)(dot)(at)gmail(dot)com> wrote:
> On Dec 19, 5:00 pm, "BigSmoke" <bigsm(dot)(dot)(dot)(at)gmail(dot)com> wrote:
>
>
>
> > On Dec 19, 4:16 pm, maili(dot)(dot)(dot)(at)oopsware(dot)de (Bernd Helmle) wrote:
>
> > > On 19 Dec 2006 07:05:10 -0800, "BigSmoke" <bigsm(dot)(dot)(dot)(at)gmail(dot)com> wrote:
>
> > > > I understand that due to a lack of nested transaction support, it is
> > > > not possible to use START TRANSACTION within a PL/PgSQL function. What
> > > > I, however, do not understand is why I can't use SAVEPOINT either. I'm
> > > > writing long test functions wherein, at the start of the function, I'd
> > > > like to define all test data followed by a "SAVEPOINT
> > > > fresh_test_data;". Will this become possible in the (near) future? I
> > > > mean, savepoints are of limited use to me if they imply that I can't
> > > > stick my tests in stored procedures.Use
>
> > > BEGIN
>
> > > ...
>
> > > EXCEPTION
>
> > > ...
>
> > > END;
>
> > > Blocks instead. The pl/pgsql exception handling is implemented on top
> > > of PostgreSQL's SAVEPOINT infrastructure. We are lacking user defined
> > > exception support, but you can raise generic errors with RAISE EXCEPTION.I can't solve my problem with a BEGIN EXCEPTION END block because of
> > what I do in these functions. Here's an example function.
>
> > CREATE OR REPLACE FUNCTION test_something() RETURNS VOID AS $$
> > BEGIN
> > -- Define some test data
>
> > -- SAVEPOINT fresh_test_data; -- If only I could ...
>
> > IF some_test_assertion_fails THEN
> > RAISE EXCEPTION 'Some informative message';
> > END IF;
>
> > -- ROLLBACK TO SAVEPOINT fresh_test_data;
> > END;
> > $$ LANGUAGE plpgsql;
>
> > In these functions, I raise an exception whenever a test fails. Now, If
> > I want to create an implicit savepoint using BEGIN/END blocks around
> > individual tests, I don't see how I can still sanely preserve this
> > behavior without the most horrid of hacks. The following code is what I
> > think I would need to do to emulate savepoints without direct access to
> > them. :-( (I hope that I'm missing something.)
>
> > CREATE OR REPLACE FUNCTION test_something() RETURNS VOID AS $$
> > BEGIN
> > -- Define some test data
>
> > BEGIN
> > -- This is a useful test ;-)
> > IF TRUE THEN
> > RAISE EXCEPTION 'Aaargh! The test failed!';
> > END IF;
>
> > RAISE EXCEPTION '__dummy_restore_state__';
>
> > EXCEPTION WHEN raise_exception THEN
> > IF SQLERRM != '__dummy_restore_state__' THEN
> > RAISE EXCEPTION '%', SQLERRM;
> > END IF;
> > END;
> > END;
> > $$ LANGUAGE plpgsql;What would solve my problem is if there was a method to, at the end of
> a begin/end block, I could rollback the changes made in that block
> without having to raise an exception. Is it somehow possible to
> explicitly rollback to one of these savepoints which are created by
> begin/end blocks?

I'm sorry for the sloppy English. Of course I meant to say "at the end
of a begin/end block, rollback" instead of "at the end of a begin/end
block, I could rollback". Hopefully, this didn't add in the confusion.