As far as I know, CTE and subquery are equivalent when used only in a single place in the query.
CTE should be better when a single query is used several times. If we want to reuse the Subquery API, we should find a way to be able to reuse a subquery (hash its content and use that as a key to detect the duplication?). Le mardi 10 mai 2022 à 16:08:06 UTC+2, matthew.pava a écrit : > I will always suggest that we use the Subquery API to make CTEs. To make > them recursive, just add a keyword argument (recursive=True) and/or use a > union. > > > > It’s been a while since I looked at CTEs, so I might be missing something. > > > > I would hate to see us create an entirely separate API for CTEs. > > > > > > *From:* django-d...@googlegroups.com <django-d...@googlegroups.com> *On > Behalf Of *Gaga Ro > *Sent:* Tuesday, May 10, 2022 9:01 AM > *To:* Django developers (Contributions to Django itself) < > django-d...@googlegroups.com> > *Subject:* Re: Adding generated common table expressions > > > > Hello everyone, > > > > I'm often using django-cte and I'd be thrilled to have CTE in the core. > > > > If I'm not mistaken, the only DB currently supported by Django and not > having CTE support is MySQL 5.7 (with an end of life in October 2023). I > don't know if Django 4.2 will support it, but it should be dropped for > Django 5.0 as it will be released in 2023. So we should have all supported > DB supporting CTE when this feature would be over. > > > > The ticket (https://code.djangoproject.com/ticket/28919) has been stalled > for a few years now, this thread as well. I am willing to work on this but > I would like more information first. > > > > If I try to list all the requirements, we should have: > > > > * A way to add one or more CTE. > > * A way to reference the columns from the CTE. > > * A way to join them in the main query. > > * Setting a CTE as recursive? > > * Choosing if a CTE is materialized or not (Not all DB support that, and > I'm not sure if they all handle it the same way)? > > * Insert / delete CTE with returning data? > > > > Do we have a better idea now of what the API should look like? > > > > Thanks. > > Le jeudi 17 octobre 2019 à 23:43:49 UTC+2, buzzi....@gmail.com a écrit : > > 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 > <https://us-east-2.protection.sophos.com?d=author.id&u=aHR0cDovL2F1dGhvci5pZA==&i=NWVjN2YxNzUxNGEyNzMxNmMyMGRkZGU1&t=Z0JhMUVTd2l1QmxSMDlQeU9kaHNOQW03N21LM3UwTFlJTDB6aitGcE1URT0=&h=5e5d822fc76540f8bea7d6204744abc8>, > > author.name > <https://us-east-2.protection.sophos.com?d=author.name&u=aHR0cDovL2F1dGhvci5uYW1l&i=NWVjN2YxNzUxNGEyNzMxNmMyMGRkZGU1&t=WXM2NWFjb2s0ejAvN0ZWS3V2aXJDbVIrZnUzNmlhL0hnQTQyeWx2K1lEMD0=&h=5e5d822fc76540f8bea7d6204744abc8> > > FROM author > > LEFT JOIN q1 ON author.id > <https://us-east-2.protection.sophos.com?d=author.id&u=aHR0cDovL2F1dGhvci5pZA==&i=NWVjN2YxNzUxNGEyNzMxNmMyMGRkZGU1&t=Z0JhMUVTd2l1QmxSMDlQeU9kaHNOQW03N21LM3UwTFlJTDB6aitGcE1URT0=&h=5e5d822fc76540f8bea7d6204744abc8> > > = q1.author_id; > > > > Or, equivalently without the CTE: > > > SELECT author.id > <https://us-east-2.protection.sophos.com?d=author.id&u=aHR0cDovL2F1dGhvci5pZA==&i=NWVjN2YxNzUxNGEyNzMxNmMyMGRkZGU1&t=Z0JhMUVTd2l1QmxSMDlQeU9kaHNOQW03N21LM3UwTFlJTDB6aitGcE1URT0=&h=5e5d822fc76540f8bea7d6204744abc8>, > > author.name > <https://us-east-2.protection.sophos.com?d=author.name&u=aHR0cDovL2F1dGhvci5uYW1l&i=NWVjN2YxNzUxNGEyNzMxNmMyMGRkZGU1&t=WXM2NWFjb2s0ejAvN0ZWS3V2aXJDbVIrZnUzNmlhL0hnQTQyeWx2K1lEMD0=&h=5e5d822fc76540f8bea7d6204744abc8> > > FROM author > > LEFT JOIN ( SELECT author_id, avg(price) FROM book GROUP BY author_id) > ON author.id > <https://us-east-2.protection.sophos.com?d=author.id&u=aHR0cDovL2F1dGhvci5pZA==&i=NWVjN2YxNzUxNGEyNzMxNmMyMGRkZGU1&t=Z0JhMUVTd2l1QmxSMDlQeU9kaHNOQW03N21LM3UwTFlJTDB6aitGcE1URT0=&h=5e5d822fc76540f8bea7d6204744abc8> > > = 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 > > <https://us-east-2.protection.sophos.com?d=djangoproject.com&u=aHR0cHM6Ly9kb2NzLmRqYW5nb3Byb2plY3QuY29tL2VuLzEuMTEvcmVmL21vZGVscy9leHByZXNzaW9ucy8jc3VicXVlcnktZXhwcmVzc2lvbnM=&i=NWVjN2YxNzUxNGEyNzMxNmMyMGRkZGU1&t=MmdMampLV0lDLzhFamQyVzFmd0NWektaS3hDbUgzTkdaQ0J0Q0RmZXo5Zz0=&h=5e5d822fc76540f8bea7d6204744abc8> > > > > 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 > > <https://us-east-2.protection.sophos.com?d=github.com&u=aHR0cHM6Ly9naXRodWIuY29tL2RqYW5nby9kamFuZ28vY29tcGFyZS9tYXN0ZXIuLi5hc2hsZXl3YWl0ZTpjdGUtZGV2I2ZpbGVzX2J1Y2tldA==&i=NWVjN2YxNzUxNGEyNzMxNmMyMGRkZGU1&t=eVZVYlJFVzljMzBxbTU3Nm11MStCRFQ3eDVKR0dvT3pobFVkR1VCSHJyVT0=&h=5e5d822fc76540f8bea7d6204744abc8> > > 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. 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