[Pulp-dev] Concerns about bulk_create and PostgreSQL
Simon Baatz
gmbnomis at gmail.com
Thu Jan 3 19:28:31 UTC 2019
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)).
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
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> 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
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> 18. https://www.redhat.com/mailman/listinfo/pulp-dev
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--
Simon Baatz <gmbnomis at gmail.com>
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