[1.8.x] Fixed #25377 -- Changed Count queries to execute COUNT(*) instead of COUNT('*').

Backport of 3fe3887a2e from master

django/db/models/aggregates.py

@@ -2,7 +2,7 @@
 Classes to represent the definitions of aggregate functions.
 """
 from django.core.exceptions import FieldError
-from django.db.models.expressions import Func, Value
+from django.db.models.expressions import Func, Star
 from django.db.models.fields import FloatField, IntegerField
 
 __all__ = [
@@ -90,7 +90,7 @@ class Count(Aggregate):
 
     def __init__(self, expression, distinct=False, **extra):
         if expression == '*':
-            expression = Value(expression)
+            expression = Star()
         super(Count, self).__init__(
             expression, distinct='DISTINCT ' if distinct else '', output_field=IntegerField(), **extra)
 

django/db/models/expressions.py

@@ -584,6 +584,14 @@ class RawSQL(Expression):
         return [self]
 
 
+class Star(Expression):
+    def __repr__(self):
+        return "'*'"
+
+    def as_sql(self, compiler, connection):
+        return '*', []
+
+
 class Random(Expression):
     def __init__(self):
         super(Random, self).__init__(output_field=fields.FloatField())

docs/releases/1.8.5.txt

@@ -32,3 +32,7 @@ Bugfixes
 
 * Fixed migrations crash on MySQL when adding a text or a blob field with an
   unhashable default (:ticket:`25393`).
+
+* Changed ``Count`` queries to execute ``COUNT(*)`` instead of ``COUNT('*')``
+  as versions of Django before 1.8 did (:ticket:`25377`). This may fix a
+  performance regression on some databases.

tests/aggregation/tests.py

@@ -306,6 +306,12 @@ class BaseAggregateTestCase(TestCase):
         vals = Book.objects.aggregate(Count("rating", distinct=True))
         self.assertEqual(vals, {"rating__count": 4})
 
+    def test_count_star(self):
+        with self.assertNumQueries(1) as ctx:
+            Book.objects.aggregate(n=Count("*"))
+        sql = ctx.captured_queries[0]['sql']
+        self.assertIn('SELECT COUNT(*) ', sql)
+
     def test_fkey_aggregate(self):
         explicit = list(Author.objects.annotate(Count('book__id')))
         implicit = list(Author.objects.annotate(Count('book')))

tests/expressions/tests.py

@@ -877,6 +877,7 @@ class ReprTests(TestCase):
     def test_aggregates(self):
         self.assertEqual(repr(Avg('a')), "Avg(F(a))")
         self.assertEqual(repr(Count('a')), "Count(F(a), distinct=False)")
+        self.assertEqual(repr(Count('*')), "Count('*', distinct=False)")
         self.assertEqual(repr(Max('a')), "Max(F(a))")
         self.assertEqual(repr(Min('a')), "Min(F(a))")
         self.assertEqual(repr(StdDev('a')), "StdDev(F(a), sample=False)")

tests/test_runner/test_debug_sql.py

@@ -63,25 +63,25 @@ class TestDebugSQL(unittest.TestCase):
 
     if six.PY3:
         expected_outputs = [
-            ('''QUERY = 'SELECT COUNT(%s) AS "__count" '''
+            ('''QUERY = 'SELECT COUNT(*) AS "__count" '''
                 '''FROM "test_runner_person" WHERE '''
                 '''"test_runner_person"."first_name" = %s' '''
-                '''- PARAMS = ('*', 'error');'''),
-            ('''QUERY = 'SELECT COUNT(%s) AS "__count" '''
+                '''- PARAMS = ('error',);'''),
+            ('''QUERY = 'SELECT COUNT(*) AS "__count" '''
                 '''FROM "test_runner_person" WHERE '''
                 '''"test_runner_person"."first_name" = %s' '''
-                '''- PARAMS = ('*', 'fail');'''),
+                '''- PARAMS = ('fail',);'''),
         ]
     else:
         expected_outputs = [
-            ('''QUERY = u'SELECT COUNT(%s) AS "__count" '''
+            ('''QUERY = u'SELECT COUNT(*) AS "__count" '''
                 '''FROM "test_runner_person" WHERE '''
                 '''"test_runner_person"."first_name" = %s' '''
-                '''- PARAMS = (u'*', u'error');'''),
-            ('''QUERY = u'SELECT COUNT(%s) AS "__count" '''
+                '''- PARAMS = (u'error',);'''),
+            ('''QUERY = u'SELECT COUNT(*) AS "__count" '''
                 '''FROM "test_runner_person" WHERE '''
                 '''"test_runner_person"."first_name" = %s' '''
-                '''- PARAMS = (u'*', u'fail');'''),
+                '''- PARAMS = (u'fail',);'''),
         ]
 
     verbose_expected_outputs = [
@@ -94,15 +94,15 @@ class TestDebugSQL(unittest.TestCase):
     ]
     if six.PY3:
         verbose_expected_outputs += [
-            ('''QUERY = 'SELECT COUNT(%s) AS "__count" '''
+            ('''QUERY = 'SELECT COUNT(*) AS "__count" '''
                 '''FROM "test_runner_person" WHERE '''
                 '''"test_runner_person"."first_name" = %s' '''
-                '''- PARAMS = ('*', 'pass');'''),
+                '''- PARAMS = ('pass',);'''),
         ]
     else:
         verbose_expected_outputs += [
-            ('''QUERY = u'SELECT COUNT(%s) AS "__count" '''
+            ('''QUERY = u'SELECT COUNT(*) AS "__count" '''
                 '''FROM "test_runner_person" WHERE '''
                 '''"test_runner_person"."first_name" = %s' '''
-                '''- PARAMS = (u'*', u'pass');'''),
+                '''- PARAMS = (u'pass',);'''),
         ]