Stopped adapting DecimalField values to strings on PostgreSQL.

The psycopg2 backend uses the type information to provide type hints.

django/db/backends/postgresql/operations.py

@@ -257,6 +257,9 @@ class DatabaseOperations(BaseDatabaseOperations):
     def adapt_timefield_value(self, value):
         return value
 
+    def adapt_decimalfield_value(self, value, max_digits=None, decimal_places=None):
+        return value
+
     def adapt_ipaddressfield_value(self, value):
         if value:
             return Inet(value)

tests/expressions_case/tests.py

@@ -6,9 +6,9 @@ from uuid import UUID
 
 from django.core.exceptions import FieldError
 from django.db.models import (
-    BinaryField, BooleanField, Case, CharField, Count, DurationField, F,
-    GenericIPAddressField, IntegerField, Max, Min, Q, Sum, TextField,
-    TimeField, UUIDField, Value, When,
+    BinaryField, BooleanField, Case, CharField, Count, DecimalField,
+    DurationField, F, GenericIPAddressField, IntegerField, Max, Min, Q, Sum,
+    TextField, TimeField, UUIDField, Value, When,
 )
 from django.test import SimpleTestCase, TestCase
 
@@ -705,7 +705,7 @@ class CaseExpressionTests(TestCase):
         CaseTestModel.objects.update(
             decimal=Case(
                 When(integer=1, then=Decimal('1.1')),
-                When(integer=2, then=Decimal('2.2')),
+                When(integer=2, then=Value(Decimal('2.2'), output_field=DecimalField())),
             ),
         )
         self.assertQuerysetEqual(