Fixed #18676 -- Allow fast-path deletion of objects

Objects can be fast-path deleted if there are no signals, and there are
no further cascades. If fast-path is taken, the objects do not need to
be loaded into memory before deletion.

Thanks to Jeremy Dunck, Simon Charette and Alex Gaynor for reviewing
the patch.

django/contrib/admin/util.py

@@ -191,6 +191,13 @@ class NestedObjects(Collector):
             roots.extend(self._nested(root, seen, format_callback))
         return roots
 
+    def can_fast_delete(self, *args, **kwargs):
+        """
+        We always want to load the objects into memory so that we can display
+        them to the user in confirm page.
+        """
+        return False
+
 
 def model_format_dict(obj):
     """

django/db/models/deletion.py

@@ -77,6 +77,9 @@ class Collector(object):
         self.data = {}
         self.batches = {} # {model: {field: set([instances])}}
         self.field_updates = {} # {model: {(field, value): set([instances])}}
+        # fast_deletes is a list of queryset-likes that can be deleted without
+        # fetching the objects into memory.
+        self.fast_deletes = [] 
 
         # Tracks deletion-order dependency for databases without transactions
         # or ability to defer constraint checks. Only concrete model classes
@@ -131,6 +134,43 @@ class Collector(object):
             model, {}).setdefault(
             (field, value), set()).update(objs)
 
+    def can_fast_delete(self, objs, from_field=None):
+        """
+        Determines if the objects in the given queryset-like can be
+        fast-deleted. This can be done if there are no cascades, no
+        parents and no signal listeners for the object class.
+
+        The 'from_field' tells where we are coming from - we need this to
+        determine if the objects are in fact to be deleted. Allows also
+        skipping parent -> child -> parent chain preventing fast delete of
+        the child.
+        """
+        if from_field and from_field.rel.on_delete is not CASCADE:
+            return False
+        if not (hasattr(objs, 'model') and hasattr(objs, '_raw_delete')):
+            return False
+        model = objs.model
+        if (signals.pre_delete.has_listeners(model)
+                or signals.post_delete.has_listeners(model)
+                or signals.m2m_changed.has_listeners(model)):
+            return False
+        # The use of from_field comes from the need to avoid cascade back to
+        # parent when parent delete is cascading to child.
+        opts = model._meta
+        if any(link != from_field for link in opts.concrete_model._meta.parents.values()):
+            return False
+        # Foreign keys pointing to this model, both from m2m and other
+        # models.
+        for related in opts.get_all_related_objects(
+            include_hidden=True, include_proxy_eq=True):
+            if related.field.rel.on_delete is not DO_NOTHING:
+                return False
+        # GFK deletes
+        for relation in opts.many_to_many:
+            if not relation.rel.through:
+                return False
+        return True
+
     def collect(self, objs, source=None, nullable=False, collect_related=True,
         source_attr=None, reverse_dependency=False):
         """
@@ -148,6 +188,9 @@ class Collector(object):
         models, the one case in which the cascade follows the forwards
         direction of an FK rather than the reverse direction.)
         """
+        if self.can_fast_delete(objs):
+            self.fast_deletes.append(objs)
+            return
         new_objs = self.add(objs, source, nullable,
                             reverse_dependency=reverse_dependency)
         if not new_objs:
@@ -160,6 +203,10 @@ class Collector(object):
         concrete_model = model._meta.concrete_model
         for ptr in six.itervalues(concrete_model._meta.parents):
             if ptr:
+                # FIXME: This seems to be buggy and execute a query for each
+                # parent object fetch. We have the parent data in the obj,
+                # but we don't have a nice way to turn that data into parent
+                # object instance.
                 parent_objs = [getattr(obj, ptr.name) for obj in new_objs]
                 self.collect(parent_objs, source=model,
                              source_attr=ptr.rel.related_name,
@@ -170,12 +217,12 @@ class Collector(object):
             for related in model._meta.get_all_related_objects(
                     include_hidden=True, include_proxy_eq=True):
                 field = related.field
-                if related.model._meta.auto_created:
-                    self.add_batch(related.model, field, new_objs)
-                else:
-                    sub_objs = self.related_objects(related, new_objs)
-                    if not sub_objs:
-                        continue
+                if field.rel.on_delete == DO_NOTHING:
+                    continue
+                sub_objs = self.related_objects(related, new_objs)
+                if self.can_fast_delete(sub_objs, from_field=field):
+                    self.fast_deletes.append(sub_objs)
+                elif sub_objs:
                     field.rel.on_delete(self, field, sub_objs, self.using)
 
             # TODO This entire block is only needed as a special case to
@@ -241,6 +288,10 @@ class Collector(object):
                     sender=model, instance=obj, using=self.using
                 )
 
+        # fast deletes
+        for qs in self.fast_deletes:
+            qs._raw_delete(using=self.using)
+
         # update fields
         for model, instances_for_fieldvalues in six.iteritems(self.field_updates):
             query = sql.UpdateQuery(model)

django/db/models/query.py

@@ -529,6 +529,14 @@ class QuerySet(object):
         self._result_cache = None
     delete.alters_data = True
 
+    def _raw_delete(self, using):
+        """
+        Deletes objects found from the given queryset in single direct SQL
+        query. No signals are sent, and there is no protection for cascades.
+        """
+        sql.DeleteQuery(self.model).delete_qs(self, using)
+    _raw_delete.alters_data = True
+
     def update(self, **kwargs):
         """
         Updates all elements in the current QuerySet, setting all the given

django/db/models/sql/compiler.py

@@ -934,7 +934,8 @@ class SQLDeleteCompiler(SQLCompiler):
         qn = self.quote_name_unless_alias
         result = ['DELETE FROM %s' % qn(self.query.tables[0])]
         where, params = self.query.where.as_sql(qn=qn, connection=self.connection)
-        result.append('WHERE %s' % where)
+        if where:
+            result.append('WHERE %s' % where)
         return ' '.join(result), tuple(params)
 
 class SQLUpdateCompiler(SQLCompiler):

django/db/models/sql/subqueries.py

@@ -3,6 +3,7 @@ Query subclasses which provide extra functionality beyond simple data retrieval.
 """
 
 from django.core.exceptions import FieldError
+from django.db import connections
 from django.db.models.constants import LOOKUP_SEP
 from django.db.models.fields import DateField, FieldDoesNotExist
 from django.db.models.sql.constants import *
@@ -46,6 +47,37 @@ class DeleteQuery(Query):
                     pk_list[offset:offset + GET_ITERATOR_CHUNK_SIZE]), AND)
             self.do_query(self.model._meta.db_table, where, using=using)
 
+    def delete_qs(self, query, using):
+        innerq = query.query
+        # Make sure the inner query has at least one table in use.
+        innerq.get_initial_alias()
+        # The same for our new query.
+        self.get_initial_alias()
+        innerq_used_tables = [t for t in innerq.tables
+                              if innerq.alias_refcount[t]]
+        if ((not innerq_used_tables or innerq_used_tables == self.tables)
+            and not len(innerq.having)):
+            # There is only the base table in use in the query, and there are
+            # no aggregate filtering going on.
+            self.where = innerq.where
+        else:
+            pk = query.model._meta.pk
+            if not connections[using].features.update_can_self_select:
+                # We can't do the delete using subquery.
+                values = list(query.values_list('pk', flat=True))
+                if not values:
+                    return
+                self.delete_batch(values, using)
+                return
+            else:
+                values = innerq
+                innerq.select = [(self.get_initial_alias(), pk.column)]
+            where = self.where_class()
+            where.add((Constraint(None, pk.column, pk), 'in', values), AND)
+            self.where = where
+        self.get_compiler(using).execute_sql(None)
+
+
 class UpdateQuery(Query):
     """
     Represents an "update" SQL query.

docs/ref/models/querysets.txt

@@ -1667,6 +1667,21 @@ methods on your models. It does, however, emit the
 :data:`~django.db.models.signals.post_delete` signals for all deleted objects
 (including cascaded deletions).
 
+.. versionadded:: 1.5
+    Allow fast-path deletion of objects
+
+Django needs to fetch objects into memory to send signals and handle cascades.
+However, if there are no cascades and no signals, then Django may take a
+fast-path and delete objects without fetching into memory. For large
+deletes this can result in significantly reduced memory usage. The amount of
+executed queries can be reduced, too.
+
+ForeignKeys which are set to :attr:`~django.db.models.ForeignKey.on_delete`
+DO_NOTHING do not prevent taking the fast-path in deletion.
+
+Note that the queries generated in object deletion is an implementation
+detail subject to change.
+
 .. _field-lookups:
 
 Field lookups

docs/releases/1.5.txt

@@ -149,6 +149,12 @@ Django 1.5 also includes several smaller improvements worth noting:
 * Django now provides a mod_wsgi :doc:`auth handler
   </howto/deployment/wsgi/apache-auth>`
 
+* The :meth:`QuerySet.delete() <django.db.models.query.QuerySet.delete>`
+  and :meth:`Model.delete() <django.db.models.Model.delete()>` can now take
+  fast-path in some cases. The fast-path allows for less queries and less
+  objects fetched into memory. See :meth:`QuerySet.delete()
+  <django.db.models.query.QuerySet.delete>` for details.
+
 Backwards incompatible changes in 1.5
 =====================================
 

tests/modeltests/delete/models.py

@@ -95,7 +95,7 @@ class MRNull(models.Model):
 
 
 class Avatar(models.Model):
-    pass
+    desc = models.TextField(null=True)
 
 
 class User(models.Model):
@@ -108,3 +108,21 @@ class HiddenUser(models.Model):
 
 class HiddenUserProfile(models.Model):
     user = models.ForeignKey(HiddenUser)
+
+class M2MTo(models.Model):
+    pass
+
+class M2MFrom(models.Model):
+    m2m = models.ManyToManyField(M2MTo)
+
+class Parent(models.Model):
+    pass
+
+class Child(Parent):
+    pass
+
+class Base(models.Model):
+    pass
+
+class RelToBase(models.Model):
+    base = models.ForeignKey(Base, on_delete=models.DO_NOTHING)

tests/modeltests/delete/tests.py

@@ -1,11 +1,12 @@
 from __future__ import absolute_import
 
-from django.db import models, IntegrityError
+from django.db import models, IntegrityError, connection
 from django.test import TestCase, skipUnlessDBFeature, skipIfDBFeature
 from django.utils.six.moves import xrange
 
 from .models import (R, RChild, S, T, U, A, M, MR, MRNull,
-    create_a, get_default_r, User, Avatar, HiddenUser, HiddenUserProfile)
+    create_a, get_default_r, User, Avatar, HiddenUser, HiddenUserProfile,
+    M2MTo, M2MFrom, Parent, Child, Base)
 
 
 class OnDeleteTests(TestCase):
@@ -74,6 +75,16 @@ class OnDeleteTests(TestCase):
         self.assertEqual(replacement_r, a.donothing)
         models.signals.pre_delete.disconnect(check_do_nothing)
 
+    def test_do_nothing_qscount(self):
+        """
+        Test that a models.DO_NOTHING relation doesn't trigger a query.
+        """
+        b = Base.objects.create()
+        with self.assertNumQueries(1):
+            # RelToBase should not be queried.
+            b.delete()
+        self.assertEqual(Base.objects.count(), 0)
+
     def test_inheritance_cascade_up(self):
         child = RChild.objects.create()
         child.delete()
@@ -229,16 +240,34 @@ class DeletionTests(TestCase):
         # 1 query to delete the avatar
         # The important thing is that when we can defer constraint checks there
         # is no need to do an UPDATE on User.avatar to null it out.
+
+        # Attach a signal to make sure we will not do fast_deletes.
+        calls = []
+        def noop(*args, **kwargs):
+            calls.append('')
+        models.signals.post_delete.connect(noop, sender=User)
+
         self.assertNumQueries(3, a.delete)
         self.assertFalse(User.objects.exists())
         self.assertFalse(Avatar.objects.exists())
+        self.assertEquals(len(calls), 1)
+        models.signals.post_delete.disconnect(noop, sender=User)
 
     @skipIfDBFeature("can_defer_constraint_checks")
     def test_cannot_defer_constraint_checks(self):
         u = User.objects.create(
             avatar=Avatar.objects.create()
         )
+        # Attach a signal to make sure we will not do fast_deletes.
+        calls = []
+        def noop(*args, **kwargs):
+            calls.append('')
+        models.signals.post_delete.connect(noop, sender=User)
+
         a = Avatar.objects.get(pk=u.avatar_id)
+        # The below doesn't make sense... Why do we need to null out
+        # user.avatar if we are going to delete the user immediately after it,
+        # and there are no more cascades.
         # 1 query to find the users for the avatar.
         # 1 query to delete the user
         # 1 query to null out user.avatar, because we can't defer the constraint
@@ -246,6 +275,8 @@ class DeletionTests(TestCase):
         self.assertNumQueries(4, a.delete)
         self.assertFalse(User.objects.exists())
         self.assertFalse(Avatar.objects.exists())
+        self.assertEquals(len(calls), 1)
+        models.signals.post_delete.disconnect(noop, sender=User)
 
     def test_hidden_related(self):
         r = R.objects.create()
@@ -254,3 +285,69 @@ class DeletionTests(TestCase):
 
         r.delete()
         self.assertEqual(HiddenUserProfile.objects.count(), 0)
+
+class FastDeleteTests(TestCase):
+
+    def test_fast_delete_fk(self):
+        u = User.objects.create(
+            avatar=Avatar.objects.create()
+        )
+        a = Avatar.objects.get(pk=u.avatar_id)
+        # 1 query to fast-delete the user
+        # 1 query to delete the avatar
+        self.assertNumQueries(2, a.delete)
+        self.assertFalse(User.objects.exists())
+        self.assertFalse(Avatar.objects.exists())
+
+    def test_fast_delete_m2m(self):
+        t = M2MTo.objects.create()
+        f = M2MFrom.objects.create()
+        f.m2m.add(t)
+        # 1 to delete f, 1 to fast-delete m2m for f
+        self.assertNumQueries(2, f.delete)
+
+    def test_fast_delete_revm2m(self):
+        t = M2MTo.objects.create()
+        f = M2MFrom.objects.create()
+        f.m2m.add(t)
+        # 1 to delete t, 1 to fast-delete t's m_set
+        self.assertNumQueries(2, f.delete)
+
+    def test_fast_delete_qs(self):
+        u1 = User.objects.create()
+        u2 = User.objects.create()
+        self.assertNumQueries(1, User.objects.filter(pk=u1.pk).delete)
+        self.assertEquals(User.objects.count(), 1)
+        self.assertTrue(User.objects.filter(pk=u2.pk).exists())
+
+    def test_fast_delete_joined_qs(self):
+        a = Avatar.objects.create(desc='a')
+        User.objects.create(avatar=a)
+        u2 = User.objects.create()
+        expected_queries = 1 if connection.features.update_can_self_select else 2
+        self.assertNumQueries(expected_queries,
+                              User.objects.filter(avatar__desc='a').delete)
+        self.assertEquals(User.objects.count(), 1)
+        self.assertTrue(User.objects.filter(pk=u2.pk).exists())
+
+    def test_fast_delete_inheritance(self):
+        c = Child.objects.create()
+        p = Parent.objects.create()
+        # 1 for self, 1 for parent
+        # However, this doesn't work as child.parent access creates a query,
+        # and this means we will be generating extra queries (a lot for large
+        # querysets). This is not a fast-delete problem.
+        # self.assertNumQueries(2, c.delete)
+        c.delete()
+        self.assertFalse(Child.objects.exists())
+        self.assertEquals(Parent.objects.count(), 1)
+        self.assertEquals(Parent.objects.filter(pk=p.pk).count(), 1)
+        # 1 for self delete, 1 for fast delete of empty "child" qs.
+        self.assertNumQueries(2, p.delete)
+        self.assertFalse(Parent.objects.exists())
+        # 1 for self delete, 1 for fast delete of empty "child" qs.
+        c = Child.objects.create()
+        p = c.parent_ptr
+        self.assertNumQueries(2, p.delete)
+        self.assertFalse(Parent.objects.exists())
+        self.assertFalse(Child.objects.exists())

tests/regressiontests/admin_util/models.py

@@ -39,3 +39,6 @@ class Guest(models.Model):
 
     class Meta:
         verbose_name = "awesome guest"
+
+class EventGuide(models.Model):
+    event = models.ForeignKey(Event, on_delete=models.DO_NOTHING)

tests/regressiontests/admin_util/tests.py

@@ -17,7 +17,7 @@ from django.utils.formats import localize
 from django.utils.safestring import mark_safe
 from django.utils import six
 
-from .models import Article, Count, Event, Location
+from .models import Article, Count, Event, Location, EventGuide
 
 
 class NestedObjectsTests(TestCase):
@@ -71,6 +71,17 @@ class NestedObjectsTests(TestCase):
         # Should not require additional queries to populate the nested graph.
         self.assertNumQueries(2, self._collect, 0)
 
+    def test_on_delete_do_nothing(self):
+        """
+        Check that the nested collector doesn't query for DO_NOTHING objects.
+        """
+        n = NestedObjects(using=DEFAULT_DB_ALIAS)
+        objs = [Event.objects.create()]
+        EventGuide.objects.create(event=objs[0])
+        with self.assertNumQueries(2):
+            # One for Location, one for Guest, and no query for EventGuide
+            n.collect(objs)
+
 class UtilTests(unittest.TestCase):
     def test_values_from_lookup_field(self):
         """

tests/regressiontests/delete_regress/tests.py

@@ -3,7 +3,7 @@ from __future__ import absolute_import
 import datetime
 
 from django.conf import settings
-from django.db import backend, transaction, DEFAULT_DB_ALIAS
+from django.db import backend, transaction, DEFAULT_DB_ALIAS, models
 from django.test import TestCase, TransactionTestCase, skipUnlessDBFeature
 
 from .models import (Book, Award, AwardNote, Person, Child, Toy, PlayedWith,
@@ -139,17 +139,24 @@ class DeleteCascadeTransactionTests(TransactionTestCase):
         eaten = Eaten.objects.create(food=apple, meal="lunch")
 
         apple.delete()
+        self.assertFalse(Food.objects.exists())
+        self.assertFalse(Eaten.objects.exists())
+
 
 class LargeDeleteTests(TestCase):
     def test_large_deletes(self):
         "Regression for #13309 -- if the number of objects > chunk size, deletion still occurs"
         for x in range(300):
             track = Book.objects.create(pagecount=x+100)
+        # attach a signal to make sure we will not fast-delete
+        def noop(*args, **kwargs):
+            pass
+        models.signals.post_delete.connect(noop, sender=Book)
         Book.objects.all().delete()
+        models.signals.post_delete.disconnect(noop, sender=Book)
         self.assertEqual(Book.objects.count(), 0)
 
 
-
 class ProxyDeleteTest(TestCase):
     """
     Tests on_delete behavior for proxy models.

tests/regressiontests/dispatch/tests/test_dispatcher.py

@@ -127,15 +127,15 @@ class DispatcherTests(unittest.TestCase):
         self._testIsClean(a_signal)
 
     def test_has_listeners(self):
-        self.assertIs(a_signal.has_listeners(), False)
-        self.assertIs(a_signal.has_listeners(sender=object()), False)
+        self.assertFalse(a_signal.has_listeners())
+        self.assertFalse(a_signal.has_listeners(sender=object()))
         receiver_1 = Callable()
         a_signal.connect(receiver_1)
-        self.assertIs(a_signal.has_listeners(), True)
-        self.assertIs(a_signal.has_listeners(sender=object()), True)
+        self.assertTrue(a_signal.has_listeners())
+        self.assertTrue(a_signal.has_listeners(sender=object()))
         a_signal.disconnect(receiver_1)
-        self.assertIs(a_signal.has_listeners(), False)
-        self.assertIs(a_signal.has_listeners(sender=object()), False)
+        self.assertFalse(a_signal.has_listeners())
+        self.assertFalse(a_signal.has_listeners(sender=object()))
 
 
 class ReceiverTestCase(unittest.TestCase):