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

Search Discussions

  • Jan Urbański at Jan 27, 2011 at 10:10 pm

    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.
  • Steve Singer at Feb 2, 2011 at 1:31 pm

    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.
  • Jan Urbański at Feb 6, 2011 at 9:51 am

    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
  • Jan Urbański at Feb 6, 2011 at 4:39 pm

    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
  • Steve Singer at Feb 8, 2011 at 5:37 am

    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
  • Peter Eisentraut at Feb 9, 2011 at 10:22 pm

    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.
  • Steve Singer at Feb 10, 2011 at 12:26 am

    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.
  • Jan Urbański at Feb 10, 2011 at 10:18 am

    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
  • Steve Singer at Feb 11, 2011 at 4:22 pm

    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

  • Jan Urbański at Feb 12, 2011 at 10:57 am

    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
  • Peter Eisentraut at Feb 27, 2011 at 7:20 pm
    Committed.
  • Robert Haas at Feb 26, 2011 at 5:12 am

    On Wed, Feb 9, 2011 at 5:22 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.
    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
  • Jan Urbański at Feb 26, 2011 at 8:38 am
    ----- Original message -----
    On Wed, Feb 9, 2011 at 5:22 PM, Peter Eisentraut 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
  • Robert Haas at Feb 26, 2011 at 2:49 pm

    2011/2/26 Jan Urbański <wulczer@wulczer.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
  • Peter Eisentraut at Feb 26, 2011 at 3:08 pm

    On lör, 2011-02-26 at 09:49 -0500, Robert Haas wrote:
    OK. Peter, are you planning to commit this?
    Yes.
  • Peter Eisentraut at Feb 2, 2011 at 9:25 pm

    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/.

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