Обсуждение: FK's to refer to rows in inheritance child

Hello list,

FK's cannot refer to rows in inheritance childs. With some changes in
LockRows, together with removing the ONLY keyword in ri_trigger.c, it
was possible to refer to the rows in child relations. (WIP patch attached)

Though it passes simple tests, it is far from complete. To our knowledge
the at minimal the following is missing:

1) child relations must be in sync with their parents regarding PK's as
well. What 'in sync' means is probably also open for debate, i.e. what
to propagate or block on ALTER TABLE and how to deal with multiple
inheritance with no unique root parent.
2) uniqueness must be guaranteed not for each individual physical
relation, but for the key in the whole inheritance tree. (how to name
this? global uniqueness?)
3) support for CASCADE actions - currently actions on referenced rows in
child relations are not cascaded.

We're currently figuring if there is consensus on the design approach
for this topic. The wiki mentions these issues
(http://wiki.postgresql.org/wiki/Todo#Inheritance) but must issues
raised seem not to apply anymore.

Issue 1) will probably be straightforward to implement, once there is
consensus how to implement the invariant 'if X, Y are relations and Y is
child of X, then X.x is PK of X iff Y.x is PK of Y' (and there is
consensus that that invariant should hold)

Issue 2) might be less trivial. On our radar is currently
ExecInsertIndexTuples(). When inserting into a relation in an
inheritance chain, also query the other (or all) relations in the chain
for the inserted value, and throw an error when there is a non-empty
result. Instead of constructing an inheritance tree manually in C, it
might be an idea to use SPI in the style of the referential integrity
checks, where the query would be a SELECT on the parent root(s) without
the ONLY keyword, leaving figuring out which relations(indexes) to query
to the inheritance planner.

Any thoughts or ideas are welcomed.

regards,
Yeb Havinga
Willem Dijkstra

Attached is the WIP patch that allows

BEGIN;
CREATE TABLE parent (a int PRIMARY KEY);
CREATE TABLE child (a int PRIMARY KEY) INHERITS (parent);
CREATE TABLE fk (a int REFERENCES parent(a));
END;

INSERT INTO child VALUES (10);
INSERT INTO fk VALUES (10);     -- must succeed
INSERT INTO fk VALUES (11);     -- should fail

Yeb Havinga <yebhavinga@gmail.com> writes:
> FK's cannot refer to rows in inheritance childs. With some changes in 
> LockRows, together with removing the ONLY keyword in ri_trigger.c, it 
> was possible to refer to the rows in child relations. (WIP patch attached)

> Though it passes simple tests, it is far from complete.

Indeed.  This isn't even worth the time to review, unless you have a
proposal for fixing the unique-index-across-multiple-tables problem.
        regards, tom lane

On 2010-12-01 15:27, Tom Lane wrote:
> Yeb Havinga<yebhavinga@gmail.com>  writes:
>> FK's cannot refer to rows in inheritance childs. With some changes in
>> LockRows, together with removing the ONLY keyword in ri_trigger.c, it
>> was possible to refer to the rows in child relations. (WIP patch attached)
>> Though it passes simple tests, it is far from complete.
> Indeed.  This isn't even worth the time to review, unless you have a
> proposal for fixing the unique-index-across-multiple-tables problem.
That was in the part that you chose to not quote.

regards,
Yeb Havinga

Yeb Havinga <yebhavinga@gmail.com> writes:
> On 2010-12-01 15:27, Tom Lane wrote:
>> Indeed.  This isn't even worth the time to review, unless you have a
>> proposal for fixing the unique-index-across-multiple-tables problem.

> That was in the part that you chose to not quote.

Perhaps I should have said "possibly workable proposal".  What you wrote
doesn't even begin to cover the interesting part of the problem, namely
how to ensure uniqueness is preserved in the face of concurrent
insertions.

(My current feelings about this are that a general-purpose solution
would probably cost more than it's worth.  What people really care
about is FK to a partitioned table, which is a structure in which
we don't have to solve the general problem: if we know that the
partitioning prevents the same key from appearing in multiple
partitions, then we only have to look into one partition.  So this
is just another item that's pending introduction of real partitioning
infrastructure.)
        regards, tom lane

On Dec1, 2010, at 15:27 , Tom Lane wrote:
> Yeb Havinga <yebhavinga@gmail.com> writes:
>> FK's cannot refer to rows in inheritance childs. With some changes in
>> LockRows, together with removing the ONLY keyword in ri_trigger.c, it
>> was possible to refer to the rows in child relations. (WIP patch attached)
>
>> Though it passes simple tests, it is far from complete.
>
> Indeed.  This isn't even worth the time to review, unless you have a
> proposal for fixing the unique-index-across-multiple-tables problem.


I've wondered in the past if a unique index is really necessary on the columns referenced by a FK in all cases, though.

It certainly is the only sane thing to do for For FKs which are marked ON UPDATE/DELETE CASCADE, SET NULL or SET
DEFAULT.But couldn't we weaken that requirement for FKs marked ON UPDATE/DELETE NO ACTION or RESTRICT? It seems to me
thatthe current RI code already handles that case quite nicely once "ONLY" is removed from the queries 

The only flaw i can see is that INSERTs or UPDATEs on the child table would lock *all* matching parent table rows,
whichis more than is necessary in theory. But given that solving this would probably require full-fledged predicate
locking,I believe this flaw can be lived with. It might be missing something, though... 

Another idea might be to allow this if "tableoid" is part of the FK. In that case, unique indices on the remaining
columnsin the individual child table would be enough to guarantee global uniqueness. For efficiently, you'd probably
needto teach the constraint exclusion mechanism that every table has an implicit constraint of the form "tableoid =
<thetable's oid>". I haven't thought this through, though - so there might be other obstacles as well... 

BTW, my "serializable_lock_consisteny" patch would allow you to do this purely within pl/pgsql in a race-condition free
way.So if that patch should get applied you might want to consider this as a workaround. Whether it will get applied is
yetto be seen, though - currently there doesn't seem to be unanimous vote one way or the other. 

best regards,
Florian Pflug

Florian Pflug <fgp@phlo.org> wrote:
> BTW, my "serializable_lock_consisteny" patch would allow you to do
> this purely within pl/pgsql in a race-condition free way. So if
> that patch should get applied you might want to consider this as a
> workaround. Whether it will get applied is yet to be seen, though
> - currently there doesn't seem to be unanimous vote one way or the
> other.
I don't recall seeing any objections to your patch.  Do you have a
reference to a post that argues against it?
-Kevin

On Wed, 2010-12-01 at 15:07 +0100, Yeb Havinga wrote:

> FK's cannot refer to rows in inheritance childs. With some changes in 
> LockRows, together with removing the ONLY keyword in ri_trigger.c, it 
> was possible to refer to the rows in child relations. (WIP patch attached)

This has a userspace solution, which seems preferable technically.

Just have an Object table that is referred to by all parts of the
hierarchy, maintained by triggers. With PK.

CASCADE doesn't work, but then that was an open issue there anyway.

-- Simon Riggs           http://www.2ndQuadrant.com/books/PostgreSQL Development, 24x7 Support, Training and Services

On Dec1, 2010, at 17:11 , Kevin Grittner wrote:
> Florian Pflug <fgp@phlo.org> wrote:
>
>> BTW, my "serializable_lock_consisteny" patch would allow you to do
>> this purely within pl/pgsql in a race-condition free way. So if
>> that patch should get applied you might want to consider this as a
>> workaround. Whether it will get applied is yet to be seen, though
>> - currently there doesn't seem to be unanimous vote one way or the
>> other.
>
> I don't recall seeing any objections to your patch.  Do you have a
> reference to a post that argues against it?


Tom expressed his doubts in http://archives.postgresql.org/pgsql-hackers/2010-05/msg00699.php pretty clearly. He hasn't
weightedin since then AFAIK, so I don't know if his objection still stands. But I still figured a word of warning would
beindicated, to prevent the OP from relying on my proposed workaround as long as it might still fail to materialize. 

best regards,
Florian Pflug

On Dec 1, 2010, at 8:07 AM, Yeb Havinga wrote:
> FK's cannot refer to rows in inheritance childs.

We have partially solved this issue at work. In our scenario, we're not using inheritance for partitioning, we're using
itfor, well, inheriting. As part of that, we have a field in the parent table that tells you what "type" of object each
rowis, and constraints on the child tables that enforce that. We've created triggers that perform the same operations
thatthe built-in RI triggers do, namely grabbing a share lock on the target row. The difference is that our trigger
looksat the "type" field to determine exactly what table it needs to try and grab shared locks on (we need to do this
becausethe backend doesn't allow you to SELECT ... FROM parent FOR SHARE). 

Our solution is not complete though. Offhand, I know it doesn't support cascade, but I think there's more stuff it
doesn'tdo. AFAIK all of those shortcomings could be handled with whats available at a user level though, so someone
withenough motivation could produce an entire RI framework that worked with inheritance (though the framework would
needa way to work around the uniqueness issue). 
--
Jim C. Nasby, Database Architect                   jim@nasby.net
512.569.9461 (cell)                         http://jim.nasby.net

On 2010-12-01 16:05, Tom Lane wrote:
>
> Perhaps I should have said "possibly workable proposal".  What you wrote
> doesn't even begin to cover the interesting part of the problem, namely
> how to ensure uniqueness is preserved in the face of concurrent
> insertions.
The usual page locking done by nbtree doesn't work in this case, since a 
conflict can be two inserts into two different indexes. This means that 
one way or another, some extra kind of lock is needed. The question is 
what current lock mechanism is suitable for this purpose, the answer to 
that may depend on the way conflicts are checked for. Checking for 
conflicts can be done at row insert time or commit time.

With 'inheritance chain' I mean any set of more than one relation that 
is the transitive closure of the inheritance parent and child relation 
on a given relation.

- Check at row insert time:
transaction A
-- before insert of row into an inheritance chain, suppose with key value 10
-- lock X exclusively (this is to be a fined grained lock, X could be a 
name derived from the inserted key value 10)
-- query inheritance chain for existing values with SnapshotNow
-- on existing value, release 'X locks' taken by current backend and error
-- else normal processing continues (actual insert etc..)
-- after commit, release 'X locks' taken by current backend

If another transaction B wants to insert the same key value 10, it will 
block on the lock taken by A, which A releases after commit, then B 
continues and when it queries the inheritance chain, it sees the 
committed record of A and will then report an error.

The drawback here is lock proliferation: for each row a lock seems unwanted.

- Check at commit time
transaction A
-- inserts row into inheritance chain
-- record row and tuple for later check
-- at commit time, if any check was recorded
--- for every inheritance chain with inserts, ordered by lowest relation oid
---- lock Y exclusively (where Y is a name derived from the inheritance 
chain, e.g. lowest relation oid)
---- for every inserted value, query for duplicate key values in 
inheritance chain with SnapshotNow
---- on error, release Y and abort transaction (serialization error)
--- after commit / heap tuples marked committed, release all Y locks.

transaction B
May insert duplicate values into the inheritance chain, but at commit 
time will either get a lock or block on lock. It can continue after A 
releases locks, and then it will read the changes committed by B.

Though this solution has fewer locks as the per-row case, a deadlock can 
occur if the inheritance chains are not locked in order. This is avoided 
by ordering the chains on the lowest oid of its relations. Perhaps the 
biggest drawback is that a serialization error must be thrown at commit, 
even when the user did not set the transaction isolation level to 
SERIALIZABLE and that might violate POLA.
> (My current feelings about this are that a general-purpose solution
> would probably cost more than it's worth.  What people really care
> about is FK to a partitioned table, which is a structure in which
> we don't have to solve the general problem: if we know that the
> partitioning prevents the same key from appearing in multiple
> partitions, then we only have to look into one partition.  So this
> is just another item that's pending introduction of real partitioning
> infrastructure.)
While this is true for the partitioning use case, it will never be for 
'true' inheritance use cases, and therefore a real partitioning 
structure doesn't help the inheritance use cases. (Such as ours that is 
a implementation of HL7's reference implementation model. 
http://www.hl7.org/v3ballot/html/infrastructure/rim/rim.html) A lot can 
be said about object model implementations in relational databases. To 
keep it short: Codd stressed the value of the relational model in it's 
ACM Turing award lecture: a practical foundation for productivity. It is 
not productive, that users must implement foreign key checking in 
user-space, instead of using a database primitive.

regards,
Yeb Havinga

On 2010-12-02 11:27, Yeb Havinga wrote:
> With 'inheritance chain' I mean any set of more than one relation that 
> is the transitive closure of the inheritance parent and child relation 
> on a given relation.
s/transitive closure/transitive reflexive closure

On 2010-12-01 16:58, Florian Pflug wrote:
> On Dec1, 2010, at 15:27 , Tom Lane wrote:
>> Indeed.  This isn't even worth the time to review, unless you have a
>> proposal for fixing the unique-index-across-multiple-tables problem.
>
> I've wondered in the past if a unique index is really necessary on the columns referenced by a FK in all cases,
though.
Me too, but Tom wrote, in one of the archive threads linked to from the 
todo wiki, that it was in the SQL spec, so..
> Another idea might be to allow this if "tableoid" is part of the FK. In that case, unique indices on the remaining
columnsin the individual child table would be enough to guarantee global uniqueness.
 
Yes that would work, but if the database user must make the tableoid 
column, and include in the PK and FK's (i.e. not transparant for the 
user), then it would still be a workaround.

> BTW, my "serializable_lock_consisteny" patch would allow you to do this purely within pl/pgsql in a race-condition
freeway. So if that patch should get applied you might want to consider this as a workaround. Whether it will get
appliedis yet to be seen, though - currently there doesn't seem to be unanimous vote one way or the other.
 
Thanks for the reference, I will try and see how to use it for this purpose.

regards,
Yeb Havinga

On 2010-12-02 01:18, Jim Nasby wrote:
> On Dec 1, 2010, at 8:07 AM, Yeb Havinga wrote:
>> FK's cannot refer to rows in inheritance childs.
> We have partially solved this issue at work. In our scenario, we're not using inheritance for partitioning, we're
usingit for, well, inheriting. As part of that, we have a field in the parent table that tells you what "type" of
objecteach row is, and constraints on the child tables that enforce that. We've created triggers that perform the same
operationsthat the built-in RI triggers do, namely grabbing a share lock on the target row. The difference is that our
triggerlooks at the "type" field to determine exactly what table it needs to try and grab shared locks on (we need to
dothis because the backend doesn't allow you to SELECT ... FROM parent FOR SHARE).
 
That part is exactly what the current WIP patch takes care of: grabbing 
share locks on the right relation.
> Our solution is not complete though. Offhand, I know it doesn't support cascade, but I think there's more stuff it
doesn'tdo. AFAIK all of those shortcomings could be handled with whats available at a user level though, so someone
withenough motivation could produce an entire RI framework that worked with inheritance (though the framework would
needa way to work around the uniqueness issue).
 
But is 'it can be solved on the user level' enough reason to not 
implement it in the server code? Foreign key and unique constraint 
checking are features the server should provide.

regards,
Yeb Havinga

On Wed, Dec 1, 2010 at 3:05 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Perhaps I should have said "possibly workable proposal".  What you wrote
> doesn't even begin to cover the interesting part of the problem, namely
> how to ensure uniqueness is preserved in the face of concurrent
> insertions.

I think it wouldn't be very hard to implement "global indexes" which
are just regular indexes that index records from multiple tables. The
index tuple would have to have a relid as well as a ctid. Of course it
would be a lot of work to touch up all the places in the source where
indexes are assumed to reference only one table, and there may be some
tricky parts, but the index itself wouldn't be too hard to implement.

> (My current feelings about this are that a general-purpose solution
> would probably cost more than it's worth.  What people really care
> about is FK to a partitioned table, which is a structure in which
> we don't have to solve the general problem: if we know that the
> partitioning prevents the same key from appearing in multiple
> partitions, then we only have to look into one partition.  So this
> is just another item that's pending introduction of real partitioning
> infrastructure.)

I agree 100% here. Global indexes are kind of a check-list item for
Oracle that reassures people that partitioned tables are fully
functional but that people usually end up not wanting to actually use.
They remove a lot of the benefit of having partitioned tables at all.


--
greg

Greg Stark <gsstark@mit.edu> writes:
> On Wed, Dec 1, 2010 at 3:05 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Perhaps I should have said "possibly workable proposal". �What you wrote
>> doesn't even begin to cover the interesting part of the problem, namely
>> how to ensure uniqueness is preserved in the face of concurrent
>> insertions.

> I think it wouldn't be very hard to implement "global indexes" which
> are just regular indexes that index records from multiple tables. The
> index tuple would have to have a relid as well as a ctid. Of course it
> would be a lot of work to touch up all the places in the source where
> indexes are assumed to reference only one table, and there may be some
> tricky parts, but the index itself wouldn't be too hard to implement.

That's been proposed before, and shot down before, though I don't recall
all the reasons offhand.  One obvious problem is VACUUM, which assumes
that you can't have two processes trying to vacuum the same index
concurrently.  Another is what happens when you drop one of the tables
involved in the index.  Even the locking involved to make a uniqueness
check against a different table would be not-nice (locking a table after
you already have lock on its index risks deadlock against operations
going the other way).
        regards, tom lane

On 12/04/2010 11:56 AM, Tom Lane wrote:
> Greg Stark<gsstark@mit.edu>  writes:
>
>>   [ suggestion for cross-table indexes ]
> That's been proposed before, and shot down before, though I don't recall
> all the reasons offhand.  One obvious problem is VACUUM, which assumes
> that you can't have two processes trying to vacuum the same index
> concurrently.  Another is what happens when you drop one of the tables
> involved in the index.  Even the locking involved to make a uniqueness
> check against a different table would be not-nice (locking a table after
> you already have lock on its index risks deadlock against operations
> going the other way).
>
>             

Those are difficulties, certainly. Are they insurmountable obstacles, 
though? This is something that has been on the TODO list for ages and I 
think is very worth doing, if we can.

cheers

andrew

Andrew Dunstan <andrew@dunslane.net> writes:
> Those are difficulties, certainly. Are they insurmountable obstacles, 
> though? This is something that has been on the TODO list for ages and I 
> think is very worth doing, if we can.

They're possibly surmountable with enough effort, though I think in
reality what you'd end up with is annoying corner-case behaviors
(eg, deadlock failures in some cases).  Anyway Greg and I seem to
agree on the bottom line: such a feature is not actually interesting
for partitioned tables, as it destroys most of the benefits of
partitioning.  I have a feeling that the feature will languish on the
TODO list forever, unless someone comes up with a brilliant idea to
avoid the problems.  As is, the return on investment to do it just
isn't there.
        regards, tom lane

Tom Lane <tgl@sss.pgh.pa.us> writes:
> partitioning.  I have a feeling that the feature will languish on the
> TODO list forever, unless someone comes up with a brilliant idea to
> avoid the problems.  As is, the return on investment to do it just
> isn't there.

That's calling for a try :)

What about a new index type that follows the partitioning scheme in its
root pages in a way that allow the locking to occur in a subpart of it,
rather than for the whole index.

Well, maybe that needs to have an index OID per known partition all
handled into the same physical index for the locking system to work, but
that could mean that indexes would get their objsubid like relations
have their attributes now. It could even be that what we need is a
meta-index and a new kind if index page pointer that would lead to
another physical index.

Oh and that's yet another idea that depends on seeing a partition syntax
supporting current features in core. When will we sketch a plan on that
so that individual hackers interested into a subpart are able to work on
it? I think the last years are telling us that nobody will ever will to
handle it all by himself. Well, or you have to hire Simon :)

Given such an agenda, I'd argue for a feature branch being open for
partitioning so that incremental reviewed work can happen there until we
have enough pieces to consider merging into HEAD.

Regards,
--
Dimitri Fontaine
http://2ndQuadrant.fr     PostgreSQL : Expertise, Formation et Support

On 12/04/2010 01:22 PM, Dimitri Fontaine wrote:
>
> Given such an agenda, I'd argue for a feature branch being open for
> partitioning so that incremental reviewed work can happen there until we
> have enough pieces to consider merging into HEAD.
>
>

Anyone can create a branch and publish it. That's the democracy that git 
allows. So if you want it, don't argue for it, just do it.

cheers

andrew

On 2010-12-04 19:22, Dimitri Fontaine wrote:
>
> That's calling for a try :)
>
> What about a new index type that follows the partitioning scheme in its
> root pages in a way that allow the locking to occur in a subpart of it,
> rather than for the whole index.
Would that (global) index be totally ordered? That seems a prerequisite 
for unique constraint checking with _bt_doinsert.

regards,
Yeb Havinga

Andrew Dunstan <andrew@dunslane.net> writes:
> Anyone can create a branch and publish it. That's the democracy that git
> allows. So if you want it, don't argue for it, just do it.

The showstopper is having a unified plan for what to put in there, for
once, and having -core people (commiters) willing to either put giant
effort into review at merge time or to follow the development and take
care of the merge themselves as part as commit fests. In a non released
branch until the work is complete enough.

The idea I tried to express would be to allow pieces of the overall
scheme to happen within our usual process. See Itagaki syntax-only
patch, and any recent effort on improving subsystems to handle
partitioning better (from indexes to foreign keys to COPY to you name
it).

So having a bunch of people working on their own local branch then
publishing it (in the form of patches) is already what has been
happening here for some years, with *zero* success. I'm trying to
understand what we (as the community) intend to do about it, I'm *not*
trying to open yet another private-branch-then-rejected-patch.

Regards,
-- 
Dimitri Fontaine
http://2ndQuadrant.fr     PostgreSQL : Expertise, Formation et Support

On Sat, Dec 04, 2010 at 12:19:59PM -0500, Tom Lane wrote:
> Andrew Dunstan <andrew@dunslane.net> writes:
> > Those are difficulties, certainly. Are they insurmountable
> > obstacles, though? This is something that has been on the TODO
> > list for ages and I think is very worth doing, if we can.
> 
> They're possibly surmountable with enough effort, though I think in
> reality what you'd end up with is annoying corner-case behaviors
> (eg, deadlock failures in some cases).  Anyway Greg and I seem to
> agree on the bottom line: such a feature is not actually interesting
> for partitioned tables, as it destroys most of the benefits of
> partitioning.  I have a feeling that the feature will languish on
> the TODO list forever, unless someone comes up with a brilliant idea
> to avoid the problems.  As is, the return on investment to do it
> just isn't there.

If we can teach the database things like, "the foo and bar columns are
guaranteed never to overlap among the partitions," it should be
possible make uniqueness guarantees over sets of partitions without
using a common index.

Cheers,
David.
-- 
David Fetter <david@fetter.org> http://fetter.org/
Phone: +1 415 235 3778  AIM: dfetter666  Yahoo!: dfetter
Skype: davidfetter      XMPP: david.fetter@gmail.com
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On Dec 4, 2010, at 1:22 PM, Dimitri Fontaine <dimitri@2ndQuadrant.fr>
> That's calling for a try :)
>
> What about a new index type that follows the partitioning scheme in its
> root pages in a way that allow the locking to occur in a subpart of it,
> rather than for the whole index.
>
> Well, maybe that needs to have an index OID per known partition all
> handled into the same physical index for the locking system to work, but
> that could mean that indexes would get their objsubid like relations
> have their attributes now. It could even be that what we need is a
> meta-index and a new kind if index page pointer that would lead to
> another physical index.

Surely the top-level structure here needs to be organized by key, not partition; else it's no better than an index per
partition.

> Oh and that's yet another idea that depends on seeing a partition syntax
> supporting current features in core. When will we sketch a plan on that
> so that individual hackers interested into a subpart are able to work on
> it? I think the last years are telling us that nobody will ever will to
> handle it all by himself. Well, or you have to hire Simon :)
>
> Given such an agenda, I'd argue for a feature branch being open for
> partitioning so that incremental reviewed work can happen there until we
> have enough pieces to consider merging into HEAD.

I wouldn't necessarily be opposed to  official topic branches at some point in the future, but I think it's premature
tospeculate about whether it'd be useful here. What is needed right now is design work, not code. 

...Robert

On 12/04/2010 07:12 PM, Robert Haas wrote:
>
> I wouldn't necessarily be opposed to  official topic branches at some point in the future, but I think it's premature
tospeculate about whether it'd be useful here.
 

I'd need a lot of convincing if it imposed an extra burden on people 
like Tom. The only way I could see  working is if some committer took 
ownership of the topic branch and guaranteed to keep it pretty much in 
sync with the master branch.

>   What is needed right now is design work, not code.
>

Indeed. In this case I don't think we even have agreement on the 
features let alone how they might work.


cheers

andreew

Andrew Dunstan <andrew@dunslane.net> writes:
> On 12/04/2010 07:12 PM, Robert Haas wrote:
>> I wouldn't necessarily be opposed to  official topic branches at some point in the future, but I think it's
prematureto speculate about whether it'd be useful here.
 

> I'd need a lot of convincing if it imposed an extra burden on people 
> like Tom. The only way I could see  working is if some committer took 
> ownership of the topic branch and guaranteed to keep it pretty much in 
> sync with the master branch.

Well, allegedly this is one of the reasons we moved to git.  Anybody can
do that in their own repository, just as easily as a core committer
could.  AFAICS it's not necessary for the core repo to contain the
branch, up until the point where it's ready to merge into master.

>> What is needed right now is design work, not code.

> Indeed. In this case I don't think we even have agreement on the 
> features let alone how they might work.

Yeah.  But it's fair to look ahead to how development might proceed.
        regards, tom lane

On 12/05/2010 12:10 PM, Tom Lane wrote:
> Andrew Dunstan<andrew@dunslane.net>  writes:
>> On 12/04/2010 07:12 PM, Robert Haas wrote:
>>> I wouldn't necessarily be opposed to  official topic branches at some point in the future, but I think it's
prematureto speculate about whether it'd be useful here.
 
>> I'd need a lot of convincing if it imposed an extra burden on people
>> like Tom. The only way I could see  working is if some committer took
>> ownership of the topic branch and guaranteed to keep it pretty much in
>> sync with the master branch.
> Well, allegedly this is one of the reasons we moved to git.  Anybody can
> do that in their own repository, just as easily as a core committer
> could.  AFAICS it's not necessary for the core repo to contain the
> branch, up until the point where it's ready to merge into master.
>

Well, ISTM that amounts to not having "official topic branches" :-) I 
agree that this is supposed to be one of git's strengths (or more 
exactly a strength of distributed SCM's generally).  I don't really see 
any great value in sanctifying a particular topic branch with some 
official status.

What I would like to see is people publishing the location of 
development repos so that they can be pulled from or merged, especially 
for any large patch.

cheers

andrew

On Sun, Dec 5, 2010 at 12:41 PM, Andrew Dunstan <andrew@dunslane.net> wrote:
> Well, ISTM that amounts to not having "official topic branches" :-) I agree
> that this is supposed to be one of git's strengths (or more exactly a
> strength of distributed SCM's generally).  I don't really see any great
> value in sanctifying a particular topic branch with some official status.

I think the value in an official topic branch would be to allow formal
incremental commit of large patches.  In other words, we could decide
that a commit to the official topic branch must meet the same
standards of quality normally applied to a commit to the master
branch, and must go through the same process.  It would be understood
that the topic branch would eventually be merged (with or without
squash) back into the master branch, but that objections were to be
raised as pieces were committed to the topic branch, not at merge
time.  The merge itself would require consensus as to timing, but we'd
agree to take a dim view of "I haven't reviewed anything that's been
going on here for the last six months but now hate all of it".

I think that this sort of approach could be useful either for really
big projects, like perhaps SQL/MED; or possibly for certain categories
of changes (things we want to include the next time we do a libpq
version bump).  Although, the trouble with the latter is that we do
compatibility breaks so rarely that the effort involved in maintaining
the branch might well exceed the value we'd get out of having it.

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

Excerpts from Andrew Dunstan's message of dom dic 05 14:41:20 -0300 2010:

> What I would like to see is people publishing the location of 
> development repos so that they can be pulled from or merged, especially 
> for any large patch.

Yes, this is pretty useful.  Dimitri published his repos for the
extension stuff which I followed for a while, made some smallish changes
and sent them back, etc.  Very time-saving.

-- 
Álvaro Herrera <alvherre@commandprompt.com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

Robert Haas <robertmhaas@gmail.com> writes:
> On Sun, Dec 5, 2010 at 12:41 PM, Andrew Dunstan <andrew@dunslane.net> wrote:
>> Well, ISTM that amounts to not having "official topic branches" :-) I agree
>> that this is supposed to be one of git's strengths (or more exactly a
>> strength of distributed SCM's generally). �I don't really see any great
>> value in sanctifying a particular topic branch with some official status.

> I think the value in an official topic branch would be to allow formal
> incremental commit of large patches.  In other words, we could decide
> that a commit to the official topic branch must meet the same
> standards of quality normally applied to a commit to the master
> branch, and must go through the same process.  It would be understood
> that the topic branch would eventually be merged (with or without
> squash) back into the master branch, but that objections were to be
> raised as pieces were committed to the topic branch, not at merge
> time.  The merge itself would require consensus as to timing, but we'd
> agree to take a dim view of "I haven't reviewed anything that's been
> going on here for the last six months but now hate all of it".

Topic branches defined that way seem like a pretty bad idea from here.
They would save no effort at all for committers, because if you're not
allowed to object to something after it's gone into a topic branch, then
it's just like master in terms of having to keep up with changes as they
happen.  Moreover, we'd have to keep them in pretty close sync with
master --- otherwise what happens when you discover that some long-ago
change on master breaks the topic branch?  So AFAICS this would just
increase the amount of keeping-branches-in-sync dogwork without any
offsetting advantage.
        regards, tom lane

Tom Lane <tgl@sss.pgh.pa.us> writes:
> Topic branches defined that way seem like a pretty bad idea from here.
> They would save no effort at all for committers, because if you're not
> allowed to object to something after it's gone into a topic branch, then
> it's just like master in terms of having to keep up with changes as they
> happen.  Moreover, we'd have to keep them in pretty close sync with
> master --- otherwise what happens when you discover that some long-ago
> change on master breaks the topic branch?  So AFAICS this would just
> increase the amount of keeping-branches-in-sync dogwork without any
> offsetting advantage.

The only advantage I can think about is twofold:- publish what feature set you want to have complete to consider merge-
havemore than one person able to partake into making it happen
 

Maybe it's just me, but it seems like some people sent in patches to
solve a partial set of the partitioning-for-real work and those have
been rejected on the grounds that it's not complete.  This topic
branches idea is all about being able to review and apply the patches,
and say "we still need this and that stuff to get done someday before we
consider a merge".

Do you prefer this all to happen externally and without commiters
helping in the incremental effort? I can see the burden if we include
such topic branch in our existing processes, but also the advantages.
You call that a judgement call, right? It's surely not mine to make.

Regards,
-- 
Dimitri Fontaine
http://2ndQuadrant.fr