Re: pl/python explicit subtransactions

Here's a patch implementing explicitly starting subtransactions mentioned in
http://archives.postgresql.org/pgsql-hackers/2010-12/msg01991.php. It's
an incremental patch on top of the spi-in-subxacts patch sent eariler.

Git branch for this patch:
https://github.com/wulczer/postgres/tree/explicit-subxacts.

The idea has been proposed in
http://archives.postgresql.org/pgsql-hackers/2010-11/msg00122.php

This patch provides a subtransaction context manager, in the vein of
http://www.python.org/dev/peps/pep-0343/.

When inside an explicit subtransaction, SPI calls do not start another
one, so you pay the subxact start overhead only once, and you get atomic
behaviour.

For instance this:

with plpy.subxact():
plpy.execute("insert into t values (1)")
plpy.execute("insert into t values (2)")
plpy.execute("ooops")

will not insert any rows into t. Just so you realise it, it *will* raise
the exception from the last execute, if you want to continue execution
you need to put your with block in a try/catch. If the code starts a
subtransaction but fails to close it, PL/Python will forcibly roll it
back to leave the backend in a clean state.

The patch lacks user-facing documentation, I'll add that later if it
gets accepted. For more usage examples refer to the unit tests that the
patch adds.

Cheers,
Jan

On 23/12/10 15:32, Jan Urbański wrote:
> Here's a patch implementing explicitly starting subtransactions mentioned in
> http://archives.postgresql.org/pgsql-hackers/2010-12/msg01991.php. It's
> an incremental patch on top of the spi-in-subxacts patch sent eariler.

Updated to the spi-in-subxacts version sent earlier.

On 11-01-27 05:11 PM, Jan Urbański wrote:
> On 23/12/10 15:32, Jan Urbański wrote:
>> Here's a patch implementing explicitly starting subtransactions mentioned in
>> http://archives.postgresql.org/pgsql-hackers/2010-12/msg01991.php. It's
>> an incremental patch on top of the spi-in-subxacts patch sent eariler.
>
> Updated to the spi-in-subxacts version sent earlier.
>
>
>

Submission Review
-----------------

The patch applies against master.
Test updates are included.

The patch doesn't included any documentation updates. The author did
mention that he'll do these if it looks like the patch is going to be
accepted.

The plpython_subxact regression test you addded is failing on both
python3 and 2.4 for me. It seems to be creating functions with the same
name twice and the second time is failing with "ERROR: function ....."
already exists. I think this is just an issue with your expect files.

Usability Review
---------------

The patch implements a python context manager that allows plpython
programs to control subtransactions with the python 'with' syntax.
The patch implements what it describes. Using the subtransaction
manager seems consistent with other examples of Python context managers.
This feature seems useful for pl/python developers.

The 'with' syntax was only officially added with python 2.6. I have
confirmed that the patch does not break plpython going as far back as
2.5 and 2.4. I have no reason to think that earlier versions will be
broken either, I just didn't test anything earlier than 2.4.

I think this feature is useful for developing more complicated functions
in pl/python and we don't have an existing way of managing savepoints
from pl/python. The context manager approach seems consistent with how
recent python versions deal with this type of thing in other areas.

Feature Test
------------
No issues found.

Code Review
------------

PLy_abort_open_subtransactions(...) line 1215:

ereport(WARNING,
(errmsg("Forcibly aborting a subtransaction "
"that has not been exited")));

"Forcibly" should be "forcibly" (lower case)

Similarly in PLy_subxact_enter and PLy_subxact_exit a few
PLy_exception_set calls start with an upper case character when I think
we want it to be lower case.

Other than that I don't see any issues. I am marking this as waiting
for author since the documentation is still outstanding.

On tor, 2010-12-23 at 15:32 +0100, Jan Urbański wrote:
> with plpy.subxact():
> plpy.execute("insert into t values (1)")
> plpy.execute("insert into t values (2)")
> plpy.execute("ooops")

Looks pretty cool, but maybe s/subxact/subtransaction/.

On 02/02/11 14:16, Steve Singer wrote:
> On 11-01-27 05:11 PM, Jan Urbański wrote:
>> On 23/12/10 15:32, Jan Urbański wrote:
>>> Here's a patch implementing explicitly starting subtransactions
>>> mentioned in
>>> http://archives.postgresql.org/pgsql-hackers/2010-12/msg01991.php. It's
>>> an incremental patch on top of the spi-in-subxacts patch sent eariler.
>>
>> Updated to the spi-in-subxacts version sent earlier.
>>
> [review]
>
> Other than that I don't see any issues. I am marking this as waiting
> for author since the documentation is still outstanding.

Thanks Steve, I'm writing docs and changing error messages to lowercase
as you suggested and will send an updated patch today.

Jan

PS: Oh, I just saw Peter's suggestion to rename the function from
plpy.subxact() to plpy.subtransaction(). Fine with me, will do that too.

J

On 02/02/11 14:16, Steve Singer wrote:
> On 11-01-27 05:11 PM, Jan Urbański wrote:
>> On 23/12/10 15:32, Jan Urbański wrote:
>>> Here's a patch implementing explicitly starting subtransactions
>>> mentioned in
>>> http://archives.postgresql.org/pgsql-hackers/2010-12/msg01991.php. It's
>>> an incremental patch on top of the spi-in-subxacts patch sent eariler.
>>
>> Updated to the spi-in-subxacts version sent earlier.
>>
>
> Submission Review
> -----------------
>
> The patch applies against master.
> Test updates are included.
>
> The patch doesn't included any documentation updates. The author did
> mention that he'll do these if it looks like the patch is going to be
> accepted.

PFA an updated patch with documentation.

> The plpython_subxact regression test you addded is failing on both
> python3 and 2.4 for me. It seems to be creating functions with the same
> name twice and the second time is failing with "ERROR: function ....."
> already exists. I think this is just an issue with your expect files.

The expect files for older Pythons were broken by the change to include
HINT and DETAIL messages when errors are reported from Python. I fixed
them and now the regression tests are passing for me on Python 2.4, 2.6
and 3.1.

> Code Review
> ------------
>
>
> PLy_abort_open_subtransactions(...) line 1215:
>
> ereport(WARNING,
> (errmsg("Forcibly aborting a subtransaction "
> "that has not been exited")));
>
> "Forcibly" should be "forcibly" (lower case)
>
> Similarly in PLy_subxact_enter and PLy_subxact_exit a few
> PLy_exception_set calls start with an upper case character when I think
> we want it to be lower case.

Yeah, changed them.

Thanks,
Jan

On 11-02-06 11:40 AM, Jan Urbański wrote:

> PFA an updated patch with documentation.

> Yeah, changed them.

Those changes look fine. The tests now pass.

I've attached a new version of the patch that fixes a few typos/wording
issues I saw in the documentation. I also changed the link to the
python reference manual section on context managers. I think it is
better to link to that versus the original PEP.

The documentation could probably still use more word-smithing but that
can happen later. I'm marking this as ready for a committer.

>
> Thanks,
> Jan

On tis, 2011-02-08 at 00:32 -0500, Steve Singer wrote:
> On 11-02-06 11:40 AM, Jan Urbański wrote:
>
> > PFA an updated patch with documentation.
>
> > Yeah, changed them.
>
> Those changes look fine. The tests now pass.
>
> I've attached a new version of the patch that fixes a few typos/wording
> issues I saw in the documentation. I also changed the link to the
> python reference manual section on context managers. I think it is
> better to link to that versus the original PEP.
>
> The documentation could probably still use more word-smithing but that
> can happen later. I'm marking this as ready for a committer.

Is it necessarily a good idea that an explicit subtransaction disables
the implicit sub-subtransactions? It might be conceivable that you'd
still want to do some try/catch within explicit subtransactions.

On 11-02-09 05:22 PM, Peter Eisentraut wrote:
> On tis, 2011-02-08 at 00:32 -0500, Steve Singer wrote:
>> On 11-02-06 11:40 AM, Jan Urbański wrote:
>>
>>> PFA an updated patch with documentation.
>>> Yeah, changed them.
>> Those changes look fine. The tests now pass.
>>
>> I've attached a new version of the patch that fixes a few typos/wording
>> issues I saw in the documentation. I also changed the link to the
>> python reference manual section on context managers. I think it is
>> better to link to that versus the original PEP.
>>
>> The documentation could probably still use more word-smithing but that
>> can happen later. I'm marking this as ready for a committer.
> Is it necessarily a good idea that an explicit subtransaction disables
> the implicit sub-subtransactions? It might be conceivable that you'd
> still want to do some try/catch within explicit subtransactions.
>
>

I had tested nested subtransactions but not a normal try/catch within a
subtransaction. That sounds reasonable to allow.

Unfortunately it leads to:

test=# create table foo(a int4 primary key);
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index
"foo_pkey" for table "foo"
CREATE TABLE
test=# DO $$
test$# try:
test$# with plpy.subtransaction():
test$# plpy.execute("insert into foo values(1)")
test$# try:
test$# plpy.execute("insert into foo values(1)")
test$# except:
test$# plpy.notice('inside exception')
test$# except plpy.SPIError:
test$# f=0
test$# $$ language plpythonu;
TRAP: FailedAssertion("!(afterTriggers->query_depth ==
afterTriggers->depth_stack[my_level])", File: "trigger.c", Line: 3846)
NOTICE: inside exception
CONTEXT: PL/Python anonymous code block
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.

On 10/02/11 01:26, Steve Singer wrote:
> On 11-02-09 05:22 PM, Peter Eisentraut wrote:
>> On tis, 2011-02-08 at 00:32 -0500, Steve Singer wrote:
>> Is it necessarily a good idea that an explicit subtransaction disables
>> the implicit sub-subtransactions? It might be conceivable that you'd
>> still want to do some try/catch within explicit subtransactions.
>>
>
> I had tested nested subtransactions but not a normal try/catch within a
> subtransaction. That sounds reasonable to allow.
>
> Unfortunately it leads to:
>
> [crash]

D'oh, I was thinking about whether it's safe to skip the internal
subxact if you're in an implicit one and somehow I always convinced
myself that since you eventually close the explicit one, it is.

Obviously my testing wasn't enough :( Attaching an updated patch with
improved docs incorporating Steve's fixes, and fixes & tests for not
statring the implicit subxact. That actually makes the patch a bit
smaller ;) OTOH I had to remove the section from the docs that claimed
performance improvement due to only starting the explicit subxact...

Cheers,
Jan

On 11-02-10 05:20 AM, Jan Urbański wrote:
>
> D'oh, I was thinking about whether it's safe to skip the internal
> subxact if you're in an implicit one and somehow I always convinced
> myself that since you eventually close the explicit one, it is.
>
> Obviously my testing wasn't enough :( Attaching an updated patch with
> improved docs incorporating Steve's fixes, and fixes& tests for not
> statring the implicit subxact. That actually makes the patch a bit
> smaller ;) OTOH I had to remove the section from the docs that claimed
> performance improvement due to only starting the explicit subxact...
>

This version of the patch looks fine to me and seems to work as expected.

> Cheers,
> Jan
>
>
>

On 11/02/11 17:22, Steve Singer wrote:
> On 11-02-10 05:20 AM, Jan Urbański wrote:
>>
>> D'oh, I was thinking about whether it's safe to skip the internal
>> subxact if you're in an implicit one and somehow I always convinced
>> myself that since you eventually close the explicit one, it is.
>>
>> Obviously my testing wasn't enough :( Attaching an updated patch with
>> improved docs incorporating Steve's fixes, and fixes & tests for not
>> statring the implicit subxact. That actually makes the patch a bit
>> smaller ;) OTOH I had to remove the section from the docs that claimed
>> performance improvement due to only starting the explicit subxact...
>>
>
> This version of the patch looks fine to me and seems to work as expected.

Thanks,

attached is a version merged with master.

Jan

On Wed, Feb 9, 2011 at 5:22 PM, Peter Eisentraut <peter_e(at)gmx(dot)net> wrote:
> On tis, 2011-02-08 at 00:32 -0500, Steve Singer wrote:
>> On 11-02-06 11:40 AM, Jan Urbański wrote:
>>
>> > PFA an updated patch with documentation.
>>
>> > Yeah, changed them.
>>
>> Those changes look fine.  The tests now pass.
>>
>> I've attached a new version of the patch that fixes a few typos/wording
>> issues I saw in the documentation.  I also changed the link to the
>> python reference manual section on context managers. I think it is
>> better to link to that versus the original PEP.
>>
>> The documentation could probably still use more word-smithing but that
>> can happen later.  I'm marking this as ready for a committer.
>
> Is it necessarily a good idea that an explicit subtransaction disables
> the implicit sub-subtransactions?  It might be conceivable that you'd
> still want to do some try/catch within explicit subtransactions.

Is this still an open question, or what is the remaining issue that
needs to be addressed with regards to this patch?

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

----- Original message -----
> On Wed, Feb 9, 2011 at 5:22 PM, Peter Eisentraut <peter_e(at)gmx(dot)net> wrote:
> > On tis, 2011-02-08 at 00:32 -0500, Steve Singer wrote:
> > > The documentation could probably still use more word-smithing but
> > > that can happen later.  I'm marking this as ready for a committer.
> >
> > Is it necessarily a good idea that an explicit subtransaction disables
> > the implicit sub-subtransactions?  It might be conceivable that you'd
> > still want to do some try/catch within explicit subtransactions.
>
> Is this still an open question, or what is the remaining issue that
> needs to be addressed with regards to this patch?

The docs are included in the latest patch, and it turned out that disabling implicit subxacts inside explicit subxacts is not a good idea, so it's been fixed in the last patch. There are no unresolved issues AFAICT.

Jan

2011/2/26 Jan Urbański <wulczer(at)wulczer(dot)org>:
> The docs are included in the latest patch, and it turned out that disabling implicit subxacts inside explicit subxacts is not a good idea, so it's been fixed in the last patch. There are no unresolved issues AFAICT.

OK. Peter, are you planning to commit this?

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

On lör, 2011-02-26 at 09:49 -0500, Robert Haas wrote:
> OK. Peter, are you planning to commit this?

Yes.

Committed.