[Pulp-dev] Concerns about bulk_create and PostgreSQL

[Brian Bouterse]

@dalley that is an excellent writeup of what is going on.

So since we could be benefitting from PostgreSQL-only Django features, I
think we should add MySQL to the test matrix to ensure that at least
pulp/pulp and pulp/pulpcore-plugin are SQL agnostic. I made this issue here
to track that work. https://pulp.plan.io/issues/4270

Please raise any concerns or ideas on what we could do better for this.

On Wed, Dec 5, 2018 at 1:34 PM Daniel Alley <dalley at redhat.com> wrote:

> To rephrase the problem a little bit:
>
> We need to bulk_create() a bunch of objects, and then after we do that we
> want to immediately be able to relate them with other objects, which means
> we need their PKs of the objects that were just created.
>
> In the case of auto-increment integer PKs, we can't know that PK value
> before it gets saved into the database.  Luckily, PostgreSQL (and Oracle)
> support a "RETURNING" keyword that does provides this information.  The raw
> SQL would look something like this:
>
>> INSERT INTO items (name) values ('bear') RETURNING id;
>>
>> Django uses this feature to set the PK field on the model objects it
> returns when you call bulk_create() on a list of unsaved model objects.
>
> Unfortunately, MySQL doesn't support this, so there's no way to figure out
> what the PKs of the objects you just saved were, so the ORM can't set that
> information on the returned model objects.
>
> UUID PKs circumvent this because the PK gets created outside of the
> database, prior to being saved in the database, and so Django *can* know
> what the PK will be when it gets saved.
>
> On Wed, Dec 5, 2018 at 12:11 PM Brian Bouterse <bbouters at redhat.com>
> wrote:
>
>> +1 to experimentation and also making sure that we understand the
>> performance implications of the decision. I'm replying to this earlier note
>> to restate my observations of the problem a bit more.
>>
>> More ideas and thoughts are welcome. This is a decision with a lot of
>> aspects to consider.
>>
>>
>> On Tue, Nov 20, 2018 at 10:00 AM Patrick Creech <pcreech at redhat.com>
>> wrote:
>>
>>> On Mon, 2018-11-19 at 17:08 -0500, Brian Bouterse wrote:
>>> > When we switched from UUID to integers for the PK
>>> > with databases other than PostgreSQL [0].
>>> >
>>> > With a goal of database agnosticism for Pulp3, if plugin writers plan
>>> to use bulk_create with any object inherited
>>> > from one of ours, they can't will get different behaviors on different
>>> databases and they won't have PKs that they may
>>> > require. bulk_create is a normal django thing, so plugin writers
>>> making a django plugin should be able to use it. This
>>> > concerned me already, but today it was also brought up by non-RH
>>> plugin writers also [1] in a PR.
>>> >
>>> > The tradeoffs bteween UUIDs versus PKs are pretty well summed up in
>>> our ticket where we discussed that change [2].
>>> > Note, we did not consider this bulk_create downside at that time,
>>> which I think is the most significant downside to
>>> > consider.
>>> >
>>> > Having bulk_create effectively not available for plugin writers (since
>>> we can't rely on its pks being returned) I
>>> > think is a non-starter for me. I love how short the UUIDs made our
>>> URLs so that's the tradeoff mainly in my mind.
>>> > Those balanced against each other, I think we should switch back.
>>> >
>>> > Another option is to become PostgreSQL only which (though I love psql)
>>> I think would be the wrong choice for Pulp from
>>> > what I've heard from its users.
>>> >
>>> > What do you think? What should we do?
>>>
>>> So, my mind immediately goes to this question, which might be usefull
>>> for others to help make decisions, so I'll ask:
>>>
>>> When you say:
>>>
>>> "we lost the ability to have the primary key set during bulk_create"
>>>
>>> Can you clarify what you mean by this?
>>>
>>> My mind immediately goes to this chain of events:
>>>
>>>         When you use bulk_create, the existing in-memory model objects
>>> representing the data to create do not get
>>> updated with the primary key values that are created in the database.
>>>
>>>         Upon a subsequent query of the database, for the exact same set
>>> of objects just added, those objects _will_ have
>>> the primary key populated.
>>>
>>> In other words,
>>>
>>>         The database records themselves get the auto-increment IDs
>>> added, they just don't get reported back in that
>>> query to the ORM layer, therefore it takes a subsequent query to get
>>> those ids out.
>>>
>>> Does that about sum it up?
>>>
>>
>> Yes this describes the situation, but there is a bit more to tell. Since
>> PostgreSQL does return the ids the subsequent query that could be done to
>> get the ids isn't written in code today. We didn't need to because we
>> developed it against PostgreSQL. I'm pretty sure that if you configure Pulp
>> against MySQL Pulp won't work, which I think is a problem. So I'm observing
>> two things here. 1) This is a hazard that causes code to unexpectedly be
>> only compliant with PostgreSQL. 2) Pulp itself fell into this hazard and we
>> need to fix that too
>>
>> Do you also see these two issues? What should be done about these?
>>
>>
>>>
>>> >
>>> > [0]:
>>> https://docs.djangoproject.com/en/2.1/ref/models/querysets/#bulk-create
>>> > [1]: https://github.com/pulp/pulp/pull/3764#discussion_r234780702
>>> > [2]: https://pulp.plan.io/issues/3848
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