[4.2.x] Fixed #34840 -- Avoided casting string base fields on PostgreSQL.

Thanks Alex Vandiver for the report.

Regression in 09ffc5c1212d4ced58b708cbbf3dfbfb77b782ca.

Backport of 779cd28acb1f7eb06f629c0ea4ded99b5ebb670a from main.

2 files changed, 8 insertions, 15 deletions

diff --git a/django/db/backends/postgresql/operations.py b/django/db/backends/postgresql/operations.py
index 18cfcb29cb..c4d90b56ab 100644
--- a/django/db/backends/postgresql/operations.py
+++ b/django/db/backends/postgresql/operations.py
@@ -153,17 +153,6 @@ class DatabaseOperations(BaseDatabaseOperations):
 
     def lookup_cast(self, lookup_type, internal_type=None):
         lookup = "%s"
-
-        if lookup_type == "isnull" and internal_type in (
-            "CharField",
-            "EmailField",
-            "TextField",
-            "CICharField",
-            "CIEmailField",
-            "CITextField",
-        ):
-            return "%s::text"
-
         # Cast text lookups to text to allow things like filter(x__contains=4)
         if lookup_type in (
             "iexact",
diff --git a/django/db/models/lookups.py b/django/db/models/lookups.py
index d3697b2003..a4729c640a 100644
--- a/django/db/models/lookups.py
+++ b/django/db/models/lookups.py
@@ -568,11 +568,15 @@ class IsNull(BuiltinLookup):
             raise ValueError(
                 "The QuerySet value for an isnull lookup must be True or False."
             )
-        if isinstance(self.lhs, Value) and self.lhs.value is None:
-            if self.rhs:
-                raise FullResultSet
+        if isinstance(self.lhs, Value):
+            if self.lhs.value is None or (
+                self.lhs.value == ""
+                and connection.features.interprets_empty_strings_as_nulls
+            ):
+                result_exception = FullResultSet if self.rhs else EmptyResultSet
             else:
-                raise EmptyResultSet
+                result_exception = EmptyResultSet if self.rhs else FullResultSet
+            raise result_exception
         sql, params = self.process_lhs(compiler, connection)
         if self.rhs:
             return "%s IS NULL" % sql, params