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 @@ def nested(self, format_callback=None):
             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 @@ def __init__(self, using):
         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 @@ def add_field_update(self, field, value, objs):
             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 @@ def collect(self, objs, source=None, nullable=False, collect_related=True,
         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 @@ def collect(self, objs, source=None, nullable=False, collect_related=True,
         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 @@ def collect(self, objs, source=None, nullable=False, collect_related=True,
             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 @@ def delete(self):
                     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 @@ def delete(self):
         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 @@ def as_sql(self):
         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 @@
 """
 
 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 @@ def delete_batch(self, pk_list, using, field=None):
                     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)