What do you think of this syntax instead?
q1 = Book.objects.values('author_id').annotate(avg_price=Avg('price'))
q2 = Author.objects.attach('book_prices', q1, id=F('book_prices__author_id'
))
def attach(name, queryset, **params):
# Would look something like this.
...
Same sql output.
On Thursday, April 6, 2017 at 9:14:01 AM UTC-4, Anssi Kääriäinen wrote:
>
> On Thursday, April 6, 2017 at 11:53:32 AM UTC+3, Marc Tamlyn wrote:
>>
>> Regarding Anssi's comments about SubQuery, we do now have that in core as
>> of 1.11 [0]. It does look like an .attach() approach might actually have
>> been a nicer version of this, but on the other hand it's currently
>> implementable solely with the Expressions API. It seems like the OuterRef
>> is very similar to your queryset.ref(). An even nicer approach using attach
>> could be to say qs.attach(q1=some_qs).filter(a=F('q1__b'))?
>>
>
> Hmmh, we have one form of SubQuery, but that's actually for SELECT clause,
> not for FROM clause. I believe the same class won't work for the CTE or
> subquery in FROM clause case.
>
> As for the attach(), seems like a really nice syntax. We do need something
> for generating the join clause for the JOIN. If you look at an example:
> q1 = Book.objects.values('author_id').annotate(avg_price=Avg('price'))
> q2 = Author.objects.attach(q1=q1)
> it needs to create something like:
> WITH q1 AS (
> SELECT author_id, avg(price) FROM book GROUP BY author_id
> )
> SELECT author.id, author.name
> FROM author
> LEFT JOIN q1 ON author.id = q1.author_id;
>
> Or, equivalently without the CTE:
>
> SELECT author.id, author.name
> FROM author
> LEFT JOIN ( SELECT author_id, avg(price) FROM book GROUP BY author_id)
> ON author.id = q1.author_id;
>
> Now, the main points are:
> 1. There is no need to design this to be about CTEs. That just limits
> the feature from backends that don't have CTEs without any real benefit.
> From Django's perspective the two above queries are the same.
> 2. We do need something for the JOIN ON condition. In some cases Django
> could guess this, but there needs to be an explicit way to express the join
> condition.
>
> If we allow usage of expressions from the attached queryset, but don't try
> to go for cases where model instance are created from the attached
> queryset, this will be both possible to implement without having to write a
> change-everything patch, and this will also be a really nice feature.
>
> For recursive CTEs, I'd leave that strictly as a later step. The only
> thing we need to check right now is that we don't do something that
> prevents a good recursive CTEs implementation later on.
>
> - Anssi
>
>>
>> Looking forwards to seeing a DEP!
>>
>> [0]
>> https://docs.djangoproject.com/en/1.11/ref/models/expressions/#subquery-expressions
>>
>> On 22 March 2017 at 01:32, Ashley Waite <ashley....@gmail.com> wrote:
>>
>>> Here's the code changes I've made, noting that some of them were to
>>> shove in a generalised VALUES clause that mocks being a queryset, so that
>>> it plays with the same interface.
>>>
>>>
>>> https://github.com/django/django/compare/master...ashleywaite:cte-dev#files_bucket
>>>
>>> I've had a glance at cte-trees/cte-forest and once general CTEs are
>>> worked out expanding that to include recursive CTEs wouldn't be too
>>> difficult, and that would greatly simplify the implementation of cte-forest
>>> to the extent that it might be viable as a django data/reference type.
>>>
>>> - Ashley
>>>
>>>
>>> On Saturday, March 18, 2017 at 8:28:53 PM UTC+11, Josh Smeaton wrote:
>>>>
>>>> Thanks for bringing this up Ashley, and for all of the detail you
>>>> provided. I'd certainly like to see CTEs make their way into Django,
>>>> provided we could come up with a nice enough API. From the look of it,
>>>> you've already got something that works with an okay API so I'm hopeful.
>>>>
>>>> I'd be very interested in seeing your POC too if you're able to share.
>>>>
>>>> From looking very briefly at django-cte-trees it doesn't aim to support
>>>> user defined CTEs for anything other than recursive queries. I'd be
>>>> interested in seeing, as part of a DEP, how CTE inclusion in django core
>>>> could support the cte-trees project from an API perspective.
>>>>
>>>> On Friday, 17 March 2017 22:28:17 UTC+11, Ashley Waite wrote:
>>>>>
>>>>> Hey all,
>>>>>
>>>>>
>>>>> I'd like to suggest adding Common Table Expression (CTE) query
>>>>> generation as a feature to Django.
>>>>>
>>>>> I've been working on a project that required manipulation of many
>>>>> records at once, and as with many ORMs found that this wasn't an ideal
>>>>> use-case in Django. As the rest of our code base and related projects are
>>>>> in Django, there was a strong preference to find a way to do it and keep
>>>>> to
>>>>> the same model-is-the-truth design.
>>>>>
>>>>> I first did this by writing some hackish functions using raw querysets
>>>>> and generating my own CTE based queries, but it lacked ideal flexibility
>>>>> and maintainability. So I've now written some modifications into my
>>>>> Django
>>>>> to do this in a more Django-esque way and think that this functionality
>>>>> would be beneficial within the project itself, but am unsure exactly
>>>>> where
>>>>> to start the conversation about that.
>>>>>
>>>>>
>>>>> *Why generate CTE based queries from querysets?*
>>>>>
>>>>> By allowing querysets to be attached to each other, and setting
>>>>> appropriate WHERE clauses, arbitrary and nested SQL queries can be
>>>>> generated. Where the results of the queries are only necessary for the
>>>>> execution of following queries this saves a very substantial amount of
>>>>> time
>>>>> and database work. Once these features exist, other functionality can
>>>>> also
>>>>> transparently use these to generate more efficient queries (such as large
>>>>> IN clauses).
>>>>>
>>>>> This allows several powerful use cases I think Django would benefit
>>>>> from:
>>>>>
>>>>>
>>>>> *Large 'IN' clauses*, can be implemented as CTEs reducing expensive
>>>>> lookups to a single CTE INNER JOIN. For sets of thousands to match from
>>>>> tables of millions of records this can be a very substantial gain.
>>>>>
>>>>>
>>>>> *Composite 'IN' conditions,* where multiple fields must match and
>>>>> you're matching against a large set of condition rows. In my usage this
>>>>> was
>>>>> "where the md5/sha hashes match one of the million md5/sha tuples in my
>>>>> match set". This is simply a CTE JOIN with two clauses in the WHERE.
>>>>>
>>>>>
>>>>> *Nested data creation*, where the parent doesn't yet exist. Django
>>>>> doesn't currently do this as the primary keys are needed, and this makes
>>>>> normalised data structures unappealing. Using INSERTs as CTEs that supply
>>>>> those keys to following statements means that entire nested data
>>>>> structures
>>>>> of new information can be recreated in the database at once, efficiently
>>>>> and atomically.
>>>>>
>>>>>
>>>>> *Non-uniform UPDATE*s, such that a modified set of objects can all be
>>>>> updated with different data at the same time by utilising a CTE values
>>>>> statement JOINed to the UPDATE statement. As there's currently no way to
>>>>> do
>>>>> this kind of bulk update the alternative is to update each instance
>>>>> individually, and this doesn't scale well.
>>>>>
>>>>> These could also be used with aggregations and other calculated fields
>>>>> to create complex queries that aren't possible at the moment.
>>>>>
>>>>>
>>>>> *What my PoC looks like*
>>>>>
>>>>> With another mildly hackish PoC that creates a VALUEs set from a
>>>>> dict/namedtuple which can be used to provide large input data, my present
>>>>> modified version syntax looks a bit like this (not perfect queries):
>>>>>
>>>>> class Hashes(models.Model):
>>>>> md5 = models.UUIDField(verbose_name="MD5 hash (base16)",
>>>>> db_index=True)
>>>>> sha2 = models.CharField(max_length=44, null=True,
>>>>> verbose_name="SHA256 hash (base64)")
>>>>>
>>>>> # Mock QuerySet of values
>>>>> q_mo = Hashes.as_literal(input_hashes).values("md5", "sha2")
>>>>> # A big IN query
>>>>> q_in = Hashes.objects.attach(q_mo).filter(md5=q_mo.ref("md5"))
>>>>>
>>>>> # Matched existing values with composite 'IN' (where md5 and sha2 match,
>>>>> or md5 matches and existing record lacks sha2)
>>>>> q_ex =
>>>>> Hashes.objects.attach(q_mo).filter(md5=q_mo.ref("md5")).filter(Q(sha160=q_mo.ref("sha160"))
>>>>> | Q(sha160=None))
>>>>>
>>>>> # Create new records that don't exist
>>>>> q_cr = Hashes.objects.attach(q_mo,
>>>>> q_ex).filter(md5=q_mo.ref("md5")).exclude(md5=q_ex.ref("md5")).values("md5",
>>>>> "sha2").as_insert()
>>>>>
>>>>> Returning the newly created records.
>>>>>
>>>>> SQL can be generated that looks something like this:
>>>>>
>>>>> WITH cte_1_0 (md5, sha2) AS (
>>>>> VALUES ('00002d30243bfe9d06673765c432c2bd'::uuid,
>>>>> 'fsA8okuCuq9KybxqcAzNdjlIyAx1QJjTPdf1ZFK/hDI='::varchar(44)),
>>>>> ('0000f20a46e4e60338697948a0917423',
>>>>> '6bVZgpYZtit1E32BlANWXoKnFFFDNierDSIi0SraND4=')),
>>>>> cte_1 AS (
>>>>> SELECT "hashes"."id", "hashes"."md5", "hashes"."sha2"
>>>>> FROM "hashes" , "cte_1_0"
>>>>> WHERE ("hashes"."md5" = (cte_1_0.md5) AND ("hashes"."sha2" =
>>>>> (cte_1_0.sha2) OR "hashes"."sha2" IS NULL) ))
>>>>> SELECT "hashes"."md5"
>>>>> FROM "hashes" , "cte_1_0" , "cte_1"
>>>>> WHERE ("hashes"."md5" = (cte_1_0.md5) AND NOT ("hashes"."md5" =
>>>>> (cte_1.md5)))
>>>>>
>>>>> That is:
>>>>>
>>>>> - A qs.as_insert() and qs.as_update() on queryset to create *lazy*
>>>>> insert and update queries.
>>>>> - A qs.attach() that allows querysets to be attached to other
>>>>> querysets, and will generate them as CTE statements.
>>>>> - A qs.ref() that returns an expression that when the query is
>>>>> compiled will be a field reference on the CTE that represents that
>>>>> queryset.
>>>>> - Additional compilers on the QuerySet subclasses that these
>>>>> return (so no changes to base compilers meaning no functionality
>>>>> impact to
>>>>> existing usage)
>>>>> - Generation of WITH clauses for attached querysets, and RETURN
>>>>> clauses for lazy UPDATE and INSERT querysets with fields requested
>>>>> (via
>>>>> values() in this case)
>>>>>
>>>>> As these can be attached to querysets that are attached to querysets,
>>>>> that are... etc, many statements can be chained allowing substantial
>>>>> changes to be performed without needing Django to have to receive,
>>>>> process,
>>>>> and resend at every step.
>>>>>
>>>>> I've had a read through the enhancement proposal docs etc, and I'm
>>>>> willing to do what's needed to make this functionality solid, and put
>>>>> forth
>>>>> a proposal to add it. But am first seeking feedback on it, and whether
>>>>> this
>>>>> is a feature that will be considered.
>>>>>
>>>>>
>>>>> Thanks,
>>>>> - Ashley
>>>>>
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