Re: [HACKERS] UPDATE of partition key - Mailing list pgsql-hackers

On Thu, May 11, 2017 at 5:41 PM, Amit Khandekar <amitdkhan.pg@gmail.com> wrote:
> On 11 May 2017 at 17:23, Amit Kapila <amit.kapila16@gmail.com> wrote:
>> On Fri, Mar 17, 2017 at 4:07 PM, Amit Khandekar <amitdkhan.pg@gmail.com> wrote:
>>> On 4 March 2017 at 12:49, Robert Haas <robertmhaas@gmail.com> wrote:
>>>> On Thu, Mar 2, 2017 at 11:53 AM, Amit Khandekar <amitdkhan.pg@gmail.com> wrote:
>>>>> I think it does not make sense running after row triggers in case of
>>>>> row-movement. There is no update happened on that leaf partition. This
>>>>> reasoning can also apply to BR update triggers. But the reasons for
>>>>> having a BR trigger and AR triggers are quite different. Generally, a
>>>>> user needs to do some modifications to the row before getting the
>>>>> final NEW row into the database, and hence [s]he defines a BR trigger
>>>>> for that. And we can't just silently skip this step only because the
>>>>> final row went into some other partition; in fact the row-movement
>>>>> itself might depend on what the BR trigger did with the row. Whereas,
>>>>> AR triggers are typically written for doing some other operation once
>>>>> it is made sure the row is actually updated. In case of row-movement,
>>>>> it is not actually updated.
>>>>
>>>> How about running the BR update triggers for the old partition and the
>>>> AR update triggers for the new partition?  It seems weird to run BR
>>>> update triggers but not AR update triggers.  Another option would be
>>>> to run BR and AR delete triggers and then BR and AR insert triggers,
>>>> emphasizing the choice to treat this update as a delete + insert, but
>>>> (as Amit Kh. pointed out to me when we were in a room together this
>>>> week) that precludes using the BEFORE trigger to modify the row.
>>>>
>>
>> I also find the current behavior with respect to triggers quite odd.
>> The two points that appears odd are (a) Executing both before row
>> update and delete triggers on original partition sounds quite odd.
> Note that *before* trigger gets fired *before* the update happens. The
> actual update may not even happen, depending upon what the trigger
> does. And then in our case, the update does not happen; not just that,
> it is transformed into delete-insert. So then we should fire
> before-delete trigger.
>

Sure, I am aware of that point, but it doesn't seem obvious that both
update and delete BR triggers get fired for original partition.  As a
developer, it might be obvious to you that as you have used delete and
insert interface, it is okay that corresponding BR/AR triggers get
fired, however, it is not so obvious for others, rather it appears
quite odd.  If we try to compare it with the non-partitioned update,
there also it is internally a delete and insert operation, but we
don't fire triggers for those.

>> (b) It seems inconsistent to consider behavior for row and statement
>> triggers differently
>
> I am not sure whether we should compare row and statement triggers.
> Statement triggers are anyway fired only per-statement, depending upon
> whether it is update or insert or delete. It has nothing to do with
> how the rows are modified.
>

Okay.  The broader point I was trying to convey was that the way this
patch defines the behavior of triggers doesn't sound good to me.  It
appears to me that in this thread multiple people have raised points
around trigger behavior and you should try to consider those.   Apart
from the options, Robert has suggested, another option could be that
we allow firing BR-AR update triggers for original partition and BR-AR
insert triggers for the new partition.  In this case, one can argue
that we have not actually updated the row in the original partition,
so there is no need to fire AR update triggers, but I feel that is
what we do for non-partitioned table update and it should be okay here
as well.

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com