[Pulp-dev] Concerns about bulk_create and PostgreSQL
Jeff Ortel
jortel at redhat.com
Tue Jan 8 16:16:12 UTC 2019
On 1/3/19 1:28 PM, Simon Baatz wrote:
> On Thu, Jan 03, 2019 at 01:02:57PM -0500, David Davis wrote:
>> I don't think that using integer ids with bulk_create and supporting
>> mysql/mariadb are necessarily mutually exclusive. I think there might
>> be a way to find the records created using bulk_create if we know the
>> natural key. It might be more performant than using UUIDs as well.
> This assumes that there is a natural key. For content types with no
> digest information in the meta data, there may be a natural key
> for content within a repo version only, but no natural key for the
> overall content. (If we want to support non-immediate modes for such
> content. In immediate mode, a digest can be computed from the
> associated artifact(s)).
Can you give some examples of Content without a natural key?
>
> Of course, there are ways around that (use a UUID as the "natural" key,
> or add a UUID to the repo version key fields), but I would like to
> avoid that.
>
>> On Thu, Jan 3, 2019 at 11:04 AM Dennis Kliban <[1]dkliban at redhat.com>
>> wrote:
>>
>> Thank you Daniel for the explanation and for filing an issue[0] to do
>> performance analysis of UUIDs.
>> I really hope that we can switch back to using UUIDs so we can bring
>> back MariaDB support for Pulp 3.
>> [0] [2]https://pulp.plan.io/issues/4290
>>
>> On Wed, Dec 5, 2018 at 1:35 PM Daniel Alley <[3]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 <[4]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 <[5]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]:
>> [6]https://docs.djangoproject.com/en/2.1/ref/models/querysets/#bulk-
>> create
>> > [1]:
>> [7]https://github.com/pulp/pulp/pull/3764#discussion_r234780702
>> > [2]: [8]https://pulp.plan.io/issues/3848
>> > _______________________________________________
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>> References
>>
>> 1. mailto:dkliban at redhat.com
>> 2. https://pulp.plan.io/issues/4290
>> 3. mailto:dalley at redhat.com
>> 4. mailto:bbouters at redhat.com
>> 5. mailto:pcreech at redhat.com
>> 6. https://docs.djangoproject.com/en/2.1/ref/models/querysets/#bulk-create
>> 7. https://github.com/pulp/pulp/pull/3764#discussion_r234780702
>> 8. https://pulp.plan.io/issues/3848
>> 9. mailto:Pulp-dev at redhat.com
>> 10. https://www.redhat.com/mailman/listinfo/pulp-dev
>> 11. mailto:Pulp-dev at redhat.com
>> 12. https://www.redhat.com/mailman/listinfo/pulp-dev
>> 13. mailto:Pulp-dev at redhat.com
>> 14. https://www.redhat.com/mailman/listinfo/pulp-dev
>> 15. mailto:Pulp-dev at redhat.com
>> 16. https://www.redhat.com/mailman/listinfo/pulp-dev
>> 17. mailto:Pulp-dev at redhat.com
>> 18. https://www.redhat.com/mailman/listinfo/pulp-dev
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