Opened 7 years ago

Last modified 3 days ago

#28519 assigned New feature

Add filter(), exclude(), and other base QuerySet methods to combined QuerySets (union(), etc.)

Reported by: Stanislav Karpov Owned by: Damir Nafikov
Component: Database layer (models, ORM) Version: dev
Severity: Normal Keywords: union, intersection, difference
Cc: Florian Apolloner, harrim4n, Dave Halter, Jaap Roes, Skrattoune, Tom Carrick, Ülgen Sarıkavak Triage Stage: Accepted
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no

Description (last modified by Stanislav Karpov)

Hello.

I found a discussion about the filtering of combined QuerySet (union, intersection, difference): https://github.com/django/django/pull/7727#issuecomment-269283216
As I understood it, as a result of the discussion, it was suggested to filter the queries before union:

If I understood your code correctly (I still did not test it), you are still generating select * from (x union y) -- while this is more powerful it is also unnecessary in most cases. I'd also argue that it is less efficient, cause filtering on that result can (usually and probably should) be pushed down to the underlying queries (when possible) to limit the result set before joining them.

https://github.com/django/django/pull/7727#issuecomment-269461236

But such a decision blocks the possibility of re-use of QuerySet, which is very convenient when working with GenericForeignKey.

A similar example of the structure of a real project (only a structure, not an entities).

import typing

from django.db import models
from django.contrib.contenttypes.fields import GenericForeignKey, GenericRelation


class Manager(models.Model):
    first_name = models.CharField(max_length=255)
    last_name = models.CharField(max_length=255)


class Organization(models.Model):
    # pickup
    title = models.CharField(max_length=255)
    foundation_date = models.DateField()
    manager = models.ForeignKey(Manager, on_delete=models.CASCADE)


class Warehouse(models.Model):
    # pickup
    organization = models.ForeignKey(Organization, on_delete=models.CASCADE)
    address = models.TextField()


class City(models.Model):
    # bus
    title = models.CharField(max_length=255)
    code = models.CharField(max_length=10)
    manager = models.ForeignKey(Manager, on_delete=models.CASCADE)


class Depot(models.Model):
    # bus
    title = models.CharField(max_length=255)
    city = models.ForeignKey(City, on_delete=models.CASCADE)


class Garage(models.Model):
    # wagon
    address = models.TextField()
    manager = models.ForeignKey(Manager, on_delete=models.CASCADE)


class AirbagManager(models.Manager):
    def filter_by_manager__union_and_inner_join(
        self, manager: typing.Union[Manager, int],
    ) -> models.QuerySet:
        # NOT WORKING (only proposal)

        manager_id = self._clean_manager_id(manager)
        return (
            self
            # If I'm not mistaken, now we can't direct call `union` from `Manager`,
            # but this variant increases the readability
            .union(
                # INNER JOIN, INNER JOIN, INNER JOIN
                self.filter(pickup__warehouse__organization__manager_id=manager_id),
                # INNER JOIN, INNER JOIN, INNER JOIN
                self.filter(bus__depot__city__manager_id=manager_id),
                # INNER JOIN, INNER JOIN
                self.filter(wagon__garage__manager_id=manager_id),
            )
        )

    def filter_by_manager__left_join(
        self, manager: typing.Union[Manager, int],
    ) -> models.QuerySet:
        manager_id = self._clean_manager_id(manager)
        return self.filter(
            # LEFT JOIN, LEFT JOIN, LEFT JOIN
            models.Q(pickup__warehouse__organization__manager_id=manager_id)
            # LEFT JOIN, LEFT JOIN, LEFT JOIN
            | models.Q(bus__depot__city__manager_id=manager_id)
            # LEFT JOIN, LEFT JOIN
            | models.Q(wagon__garage__manager_id=manager_id),
        )

    def _clean_manager_id(self, manager: typing.Union[Manager, int]) -> int:
        if isinstance(manager, Manager):
            return manager.id
        elif isinstance(manager, int):
            return manager
        else:
            raise ValueError


class Airbag(models.Model):
    serial_number = models.CharField(max_length=255)
    state = models.IntegerField()

    vehicle_content_type = models.ForeignKey(
        'contenttypes.ContentType',
        on_delete=models.CASCADE,
    )
    vehicle_id = models.IntegerField(db_index=True)
    vehicle = GenericForeignKey('vehicle_content_type', 'vehicle_id')

    objects = AirbagManager()


class BaseVehicle(models.Model):
    production_date = models.DateField()
    airbags = GenericRelation(
        Airbag,
        object_id_field='vehicle_id',
        content_type_field='vehicle_content_type',
        related_query_name='%(class)s',
    )

    class Meta:
        abstract = True


class Pickup(BaseVehicle):
    carrying_kg = models.FloatField()
    warehouse = models.ForeignKey(Warehouse, on_delete=models.CASCADE)


class Bus(BaseVehicle):
    floors_number = models.IntegerField()
    depot = models.ForeignKey(Depot, on_delete=models.CASCADE)


class Wagon(BaseVehicle):
    garage = models.ForeignKey(Garage, on_delete=models.CASCADE)


Airbag.objects.filter_by_manager__union_and_inner_join(15).filter(state__gt=2)
# Expected SQL
"""
SELECT *
FROM (
  (
    SELECT
      "airbag"."id",
      "airbag"."serial_number",
      "airbag"."state",
      "airbag"."vehicle_content_type_id",
      "airbag"."vehicle_id"
    FROM "airbag"
      INNER JOIN "pickup" ON ("airbag"."vehicle_id" = "pickup"."id" AND ("airbag"."vehicle_content_type_id" = 46))
      INNER JOIN "warehouse" ON ("pickup"."warehouse_id" = "warehouse"."id")
      INNER JOIN "organization" ON ("warehouse"."organization_id" = "organization"."id")
    WHERE "organization"."manager_id" = 15
  )

  UNION

  (
    SELECT
      "airbag"."id",
      "airbag"."serial_number",
      "airbag"."state",
      "airbag"."vehicle_content_type_id",
      "airbag"."vehicle_id"
    FROM "airbag"
      INNER JOIN "bus" ON ("airbag"."vehicle_id" = "bus"."id" AND ("airbag"."vehicle_content_type_id" = 49))
      INNER JOIN "depot" ON ("bus"."depot_id" = "depot"."id")
      INNER JOIN "city" ON ("depot"."city_id" = "city"."id")
    WHERE "city"."manager_id" = 15
  )

  UNION
  (
    SELECT
      "airbag"."id",
      "airbag"."serial_number",
      "airbag"."state",
      "airbag"."vehicle_content_type_id",
      "airbag"."vehicle_id"
    FROM "airbag"
      INNER JOIN "wagon" ON ("airbag"."vehicle_id" = "wagon"."id" AND ("airbag"."vehicle_content_type_id" = 43))
      INNER JOIN "garage" ON ("wagon"."garage_id" = "garage"."id")
    WHERE "garage"."manager_id" = 15
  )
) AS U
WHERE U.state > 2;
"""

# VS

Airbag.objects.filter_by_manager__left_join(15).filter(state__gt=2)
# Real SQL
"""
SELECT
  "airbag"."id",
  "airbag"."serial_number",
  "airbag"."state",
  "airbag"."vehicle_content_type_id",
  "airbag"."vehicle_id"
FROM "airbag"
  LEFT OUTER JOIN "pickup" ON ("airbag"."vehicle_id" = "pickup"."id" AND ("airbag"."vehicle_content_type_id" = 46))
  LEFT OUTER JOIN "warehouse" ON ("pickup"."warehouse_id" = "warehouse"."id")
  LEFT OUTER JOIN "organization" ON ("warehouse"."organization_id" = "organization"."id")

  LEFT OUTER JOIN "bus" ON ("airbag"."vehicle_id" = "bus"."id" AND ("airbag"."vehicle_content_type_id" = 49))
  LEFT OUTER JOIN "depot" ON ("bus"."depot_id" = "depot"."id")
  LEFT OUTER JOIN "city" ON ("depot"."city_id" = "city"."id")

  LEFT OUTER JOIN "wagon" ON ("airbag"."vehicle_id" = "wagon"."id" AND ("airbag"."vehicle_content_type_id" = 43))
  LEFT OUTER JOIN "garage" ON ("wagon"."garage_id" = "garage"."id")
WHERE (
  (
      "organization"."manager_id" = 15
      OR "city"."manager_id" = 15
      OR "garage"."manager_id" = 15
  )
  AND "airbag"."state" > 2
);
"""

Select from a test database containing 30 million airbags with using UNION and INNER JOIN takes 1.1 seconds, and using LEFT JOIN - 10.0 seconds (as the database grows, the access time will increase). PostgreSQL 9.6 (default Docker image).

Change History (15)

comment:1 by Stanislav Karpov, 7 years ago

Description: modified (diff)

comment:2 by Tim Graham, 7 years ago

Cc: Florian Apolloner added
Summary: Add `filter`, `exclude` and other base `QuerySet` methods to combined `QuerySet`Add filter(), exclude(), and other base QuerySet methods to combined QuerySets (union(), etc.)

Florian, what do you think?

comment:3 by Florian Apolloner, 7 years ago

Triage Stage: UnreviewedAccepted
Version: 1.11master

I'd like to see it, but it will be a much much more invasive change and is not something I will have the time nor motivation for in the near future.

comment:4 by harrim4n, 5 years ago

Until (if) this is implemented, I suggest adding a check when .get(), .filter() etc. are called on such a QS. Currently the error messages (if there are any) do not explain the issue, the only way to find out about this is in the documentation of the QS API, section union().

Example:

qs1 = User.objects.all()
qs2 = User.objects.filter(username="nonexistant")
qs3 = qs1.difference(qs2)
print(qs1)
print(qs2)
print(qs3)
print(qs1.get(id=1))
print(qs3.get(id=1))

This fails with (could be any number instead of 4, of course)

MultipleObjectsReturned: get() returned more than one User -- it returned 4!

Even worse, if the difference of qs1 and qs2 only contains one element, this element is returned when calling qs3.get(id=1) without any error message, even though the objects ID is not 1.

comment:5 by harrim4n, 5 years ago

Cc: harrim4n added

comment:6 by Dave Halter, 4 years ago

Cc: Dave Halter added

comment:7 by Mariusz Felisiak, 4 years ago

I closed #32032 as a duplicate -- the case of trying to use aggregate() after union(). It should be enough to take into account combined queries in Query.rewrite_cols().

Last edited 4 years ago by Mariusz Felisiak (previous) (diff)

comment:8 by Jaap Roes, 4 years ago

Cc: Jaap Roes added

comment:9 by Skrattoune, 3 years ago

Cc: Skrattoune added

comment:10 by Mariusz Felisiak, 2 years ago

I closed #30209 as a duplicate -- the case of trying to use annotate() after union().

comment:11 by Tom Carrick, 5 months ago

Cc: Tom Carrick added

comment:12 by Ülgen Sarıkavak, 5 weeks ago

Cc: Ülgen Sarıkavak added

comment:13 by Damir Nafikov, 4 weeks ago

Owner: changed from nobody to Damir Nafikov
Status: newassigned

comment:15 by Simon Charette, 3 days ago

Closed #35369 for the aggregate method.

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