Fixed #34932 -- Restored varchar_pattern_ops/text_pattern_ops index creation when deterministic collaction is set.

Regression in f3f9d03edf17ccfa17263c7efa0b1350d1ac9278 (4.2) and
8ed25d65ea7546fafd808086fa07e7e5bb5428fc (5.0).

2 files changed, 18 insertions, 3 deletions

diff --git a/django/db/backends/postgresql/features.py b/django/db/backends/postgresql/features.py
index f2c3800307..0fe5d950a5 100644
--- a/django/db/backends/postgresql/features.py
+++ b/django/db/backends/postgresql/features.py
@@ -74,6 +74,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     supports_virtual_generated_columns = False
     can_rename_index = True
     test_collations = {
+        "deterministic": "C",
         "non_default": "sv-x-icu",
         "swedish_ci": "sv-x-icu",
     }
diff --git a/django/db/backends/postgresql/schema.py b/django/db/backends/postgresql/schema.py
index 40fef6660e..5dc93a27d0 100644
--- a/django/db/backends/postgresql/schema.py
+++ b/django/db/backends/postgresql/schema.py
@@ -98,9 +98,10 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
                 return None
             # Non-deterministic collations on Postgresql don't support indexes
             # for operator classes varchar_pattern_ops/text_pattern_ops.
-            if getattr(field, "db_collation", None) or (
-                field.is_relation and getattr(field.target_field, "db_collation", None)
-            ):
+            collation_name = getattr(field, "db_collation", None)
+            if not collation_name and field.is_relation:
+                collation_name = getattr(field.target_field, "db_collation", None)
+            if collation_name and not self._is_collation_deterministic(collation_name):
                 return None
             if db_type.startswith("varchar"):
                 return self._create_index_sql(
@@ -372,3 +373,16 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
             include=include,
             expressions=expressions,
         )
+
+    def _is_collation_deterministic(self, collation_name):
+        with self.connection.cursor() as cursor:
+            cursor.execute(
+                """
+                SELECT collisdeterministic
+                FROM pg_collation
+                WHERE collname = %s
+                """,
+                [collation_name],
+            )
+            row = cursor.fetchone()
+            return row[0] if row else None