Refs #26430 -- Re-introduced empty aggregation optimization.

The introduction of the Expression.empty_aggregate_value interface
allows the compilation stage to enable the EmptyResultSet optimization
if all the aggregates expressions implement it.

This also removes unnecessary RegrCount/Count.convert_value() methods.
Disabling the empty result set aggregation optimization when it wasn't
appropriate prevented None returned for a Count aggregation value.

Thanks Nick Pope for the review.

8 files changed, 101 insertions, 34 deletions

diff --git a/django/contrib/postgres/aggregates/statistics.py b/django/contrib/postgres/aggregates/statistics.py
index c3254e3677..f3e1450d17 100644
--- a/django/contrib/postgres/aggregates/statistics.py
+++ b/django/contrib/postgres/aggregates/statistics.py
@@ -36,9 +36,7 @@ class RegrAvgY(StatAggregate):
 class RegrCount(StatAggregate):
     function = 'REGR_COUNT'
     output_field = IntegerField()
-
-    def convert_value(self, value, expression, connection):
-        return 0 if value is None else value
+    empty_aggregate_value = 0
 
 
 class RegrIntercept(StatAggregate):
diff --git a/django/db/models/aggregates.py b/django/db/models/aggregates.py
index 8b10829eb8..8598ba9178 100644
--- a/django/db/models/aggregates.py
+++ b/django/db/models/aggregates.py
@@ -20,6 +20,7 @@ class Aggregate(Func):
     filter_template = '%s FILTER (WHERE %%(filter)s)'
     window_compatible = True
     allow_distinct = False
+    empty_aggregate_value = None
 
     def __init__(self, *expressions, distinct=False, filter=None, **extra):
         if distinct and not self.allow_distinct:
@@ -107,6 +108,7 @@ class Count(Aggregate):
     name = 'Count'
     output_field = IntegerField()
     allow_distinct = True
+    empty_aggregate_value = 0
 
     def __init__(self, expression, filter=None, **extra):
         if expression == '*':
@@ -115,9 +117,6 @@ class Count(Aggregate):
             raise ValueError('Star cannot be used with filter. Please specify a field.')
         super().__init__(expression, filter=filter, **extra)
 
-    def convert_value(self, value, expression, connection):
-        return 0 if value is None else value
-
 
 class Max(Aggregate):
     function = 'MAX'
diff --git a/django/db/models/expressions.py b/django/db/models/expressions.py
index feb04d4585..b30d1f959b 100644
--- a/django/db/models/expressions.py
+++ b/django/db/models/expressions.py
@@ -153,6 +153,7 @@ class BaseExpression:
     # aggregate specific fields
     is_summary = False
     _output_field_resolved_to_none = False
+    empty_aggregate_value = NotImplemented
     # Can the expression be used in a WHERE clause?
     filterable = True
     # Can the expression can be used as a source expression in Window?
@@ -795,6 +796,10 @@ class Value(SQLiteNumericMixin, Expression):
         if isinstance(self.value, UUID):
             return fields.UUIDField()
 
+    @property
+    def empty_aggregate_value(self):
+        return self.value
+
 
 class RawSQL(Expression):
     def __init__(self, sql, params, output_field=None):
diff --git a/django/db/models/functions/comparison.py b/django/db/models/functions/comparison.py
index 8a1c34430b..2dac47a0d2 100644
--- a/django/db/models/functions/comparison.py
+++ b/django/db/models/functions/comparison.py
@@ -65,6 +65,14 @@ class Coalesce(Func):
             raise ValueError('Coalesce must take at least two expressions')
         super().__init__(*expressions, **extra)
 
+    @property
+    def empty_aggregate_value(self):
+        for expression in self.get_source_expressions():
+            result = expression.empty_aggregate_value
+            if result is NotImplemented or result is not None:
+                return result
+        return None
+
     def as_oracle(self, compiler, connection, **extra_context):
         # Oracle prohibits mixing TextField (NCLOB) and CharField (NVARCHAR2),
         # so convert all fields to NCLOB when that type is expected.
diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index fabb346418..87be8ea9f0 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -490,12 +490,19 @@ class Query(BaseExpression):
             self.default_cols = False
             self.extra = {}
 
+        empty_aggregate_result = [
+            expression.empty_aggregate_value
+            for expression in outer_query.annotation_select.values()
+        ]
+        elide_empty = not any(result is NotImplemented for result in empty_aggregate_result)
         outer_query.clear_ordering(force=True)
         outer_query.clear_limits()
         outer_query.select_for_update = False
         outer_query.select_related = False
-        compiler = outer_query.get_compiler(using, elide_empty=False)
+        compiler = outer_query.get_compiler(using, elide_empty=elide_empty)
         result = compiler.execute_sql(SINGLE)
+        if result is None:
+            result = empty_aggregate_result
 
         converters = compiler.get_converters(outer_query.annotation_select.values())
         result = next(compiler.apply_converters((result,), converters))
diff --git a/docs/ref/models/expressions.txt b/docs/ref/models/expressions.txt
index 9ab502d244..d45cfbe00a 100644
--- a/docs/ref/models/expressions.txt
+++ b/docs/ref/models/expressions.txt
@@ -410,6 +410,14 @@ The ``Aggregate`` API is as follows:
         allows passing a ``distinct`` keyword argument. If set to ``False``
         (default), ``TypeError`` is raised if ``distinct=True`` is passed.
 
+    .. attribute:: empty_aggregate_value
+
+        .. versionadded:: 4.0
+
+        Override :attr:`~django.db.models.Expression.empty_aggregate_value` to
+        ``None`` since most aggregate functions result in ``NULL`` when applied
+        to an empty result set.
+
 The ``expressions`` positional arguments can include expressions, transforms of
 the model field, or the names of model fields. They will be converted to a
 string and used as the ``expressions`` placeholder within the ``template``.
@@ -950,6 +958,15 @@ calling the appropriate methods on the wrapped expression.
         in :class:`~django.db.models.expressions.Window`. Defaults to
         ``False``.
 
+    .. attribute:: empty_aggregate_value
+
+        .. versionadded:: 4.0
+
+        Tells Django which value should be returned when the expression is used
+        to :meth:`aggregate <django.db.models.query.QuerySet.aggregate>` over
+        an empty result set. Defaults to :py:data:`NotImplemented` which forces
+        the expression to be computed on the database.
+
     .. method:: resolve_expression(query=None, allow_joins=True, reuse=None, summarize=False, for_save=False)
 
         Provides the chance to do any pre-processing or validation of
diff --git a/docs/releases/4.0.txt b/docs/releases/4.0.txt
index a9b29d7ce4..c1e64c499a 100644
--- a/docs/releases/4.0.txt
+++ b/docs/releases/4.0.txt
@@ -265,6 +265,9 @@ Models
 
 * :meth:`.QuerySet.bulk_update` now returns the number of objects updated.
 
+* The new :attr:`.Aggregate.empty_aggregate_value` attribute allows specifying
+  a value to return when the aggregation is used over an empty result set.
+
 Requests and Responses
 ~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/tests/aggregation/tests.py b/tests/aggregation/tests.py
index db24f36a79..f7b2331d34 100644
--- a/tests/aggregation/tests.py
+++ b/tests/aggregation/tests.py
@@ -8,7 +8,7 @@ from django.db.models import (
     Avg, Case, Count, DecimalField, DurationField, Exists, F, FloatField,
     IntegerField, Max, Min, OuterRef, Subquery, Sum, Value, When,
 )
-from django.db.models.expressions import RawSQL
+from django.db.models.expressions import Func, RawSQL
 from django.db.models.functions import Coalesce, Greatest
 from django.test import TestCase
 from django.test.testcases import skipUnlessDBFeature
@@ -1342,33 +1342,63 @@ class AggregateTestCase(TestCase):
             ('Peter Norvig', 2),
         ], lambda a: (a.name, a.contact_count), ordered=False)
 
+    def test_empty_result_optimization(self):
+        with self.assertNumQueries(0):
+            self.assertEqual(
+                Publisher.objects.none().aggregate(
+                    sum_awards=Sum('num_awards'),
+                    books_count=Count('book'),
+                ), {
+                    'sum_awards': None,
+                    'books_count': 0,
+                }
+            )
+        # Expression without empty_aggregate_value forces queries to be
+        # executed even if they would return an empty result set.
+        raw_books_count = Func('book', function='COUNT')
+        raw_books_count.contains_aggregate = True
+        with self.assertNumQueries(1):
+            self.assertEqual(
+                Publisher.objects.none().aggregate(
+                    sum_awards=Sum('num_awards'),
+                    books_count=raw_books_count,
+                ), {
+                    'sum_awards': None,
+                    'books_count': 0,
+                }
+            )
+
     def test_coalesced_empty_result_set(self):
-        self.assertEqual(
-            Publisher.objects.none().aggregate(
-                sum_awards=Coalesce(Sum('num_awards'), 0),
-            )['sum_awards'],
-            0,
-        )
+        with self.assertNumQueries(0):
+            self.assertEqual(
+                Publisher.objects.none().aggregate(
+                    sum_awards=Coalesce(Sum('num_awards'), 0),
+                )['sum_awards'],
+                0,
+            )
         # Multiple expressions.
-        self.assertEqual(
-            Publisher.objects.none().aggregate(
-                sum_awards=Coalesce(Sum('num_awards'), None, 0),
-            )['sum_awards'],
-            0,
-        )
+        with self.assertNumQueries(0):
+            self.assertEqual(
+                Publisher.objects.none().aggregate(
+                    sum_awards=Coalesce(Sum('num_awards'), None, 0),
+                )['sum_awards'],
+                0,
+            )
         # Nested coalesce.
-        self.assertEqual(
-            Publisher.objects.none().aggregate(
-                sum_awards=Coalesce(Coalesce(Sum('num_awards'), None), 0),
-            )['sum_awards'],
-            0,
-        )
+        with self.assertNumQueries(0):
+            self.assertEqual(
+                Publisher.objects.none().aggregate(
+                    sum_awards=Coalesce(Coalesce(Sum('num_awards'), None), 0),
+                )['sum_awards'],
+                0,
+            )
         # Expression coalesce.
-        self.assertIsInstance(
-            Store.objects.none().aggregate(
-                latest_opening=Coalesce(
-                    Max('original_opening'), RawSQL('CURRENT_TIMESTAMP', []),
-                ),
-            )['latest_opening'],
-            datetime.datetime,
-        )
+        with self.assertNumQueries(1):
+            self.assertIsInstance(
+                Store.objects.none().aggregate(
+                    latest_opening=Coalesce(
+                        Max('original_opening'), RawSQL('CURRENT_TIMESTAMP', []),
+                    ),
+                )['latest_opening'],
+                datetime.datetime,
+            )