Fixed #31615 -- Made migrations skip extension operations if not needed.

- Don't try to create an existing extension.
- Don't try to drop a nonexistent extension.

4 files changed, 54 insertions, 6 deletions

diff --git a/django/contrib/postgres/operations.py b/django/contrib/postgres/operations.py
index 0bb131ddf2..740911d059 100644
--- a/django/contrib/postgres/operations.py
+++ b/django/contrib/postgres/operations.py
@@ -21,7 +21,10 @@ class CreateExtension(Operation):
             not router.allow_migrate(schema_editor.connection.alias, app_label)
         ):
             return
-        schema_editor.execute("CREATE EXTENSION IF NOT EXISTS %s" % schema_editor.quote_name(self.name))
+        if not self.extension_exists(schema_editor, self.name):
+            schema_editor.execute(
+                'CREATE EXTENSION %s' % schema_editor.quote_name(self.name)
+            )
         # Clear cached, stale oids.
         get_hstore_oids.cache_clear()
         get_citext_oids.cache_clear()
@@ -33,11 +36,22 @@ class CreateExtension(Operation):
     def database_backwards(self, app_label, schema_editor, from_state, to_state):
         if not router.allow_migrate(schema_editor.connection.alias, app_label):
             return
-        schema_editor.execute("DROP EXTENSION %s" % schema_editor.quote_name(self.name))
+        if self.extension_exists(schema_editor, self.name):
+            schema_editor.execute(
+                'DROP EXTENSION %s' % schema_editor.quote_name(self.name)
+            )
         # Clear cached, stale oids.
         get_hstore_oids.cache_clear()
         get_citext_oids.cache_clear()
 
+    def extension_exists(self, schema_editor, extension):
+        with schema_editor.connection.cursor() as cursor:
+            cursor.execute(
+                'SELECT 1 FROM pg_extension WHERE extname = %s',
+                [extension],
+            )
+            return bool(cursor.fetchone())
+
     def describe(self):
         return "Creates extension %s" % self.name
 
diff --git a/docs/ref/contrib/postgres/operations.txt b/docs/ref/contrib/postgres/operations.txt
index 8696e4e81f..8571cc84e8 100644
--- a/docs/ref/contrib/postgres/operations.txt
+++ b/docs/ref/contrib/postgres/operations.txt
@@ -30,12 +30,19 @@ For example::
             ...
         ]
 
+Django checks that the extension already exists in the database and skips the
+migration if so.
+
 For most extensions, this requires a database user with superuser privileges.
 If the Django database user doesn't have the appropriate privileges, you'll
 have to create the extension outside of Django migrations with a user that has
 them. In that case, connect to your Django database and run the query
 ``CREATE EXTENSION IF NOT EXISTS hstore;``.
 
+.. versionchanged:: 3.2
+
+    In older versions, the existence of an extension isn't checked.
+
 .. currentmodule:: django.contrib.postgres.operations
 
 ``CreateExtension``
diff --git a/docs/releases/3.2.txt b/docs/releases/3.2.txt
index 84f757cd2b..bd8db085a9 100644
--- a/docs/releases/3.2.txt
+++ b/docs/releases/3.2.txt
@@ -63,7 +63,9 @@ Minor features
 :mod:`django.contrib.messages`
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-* ...
+* The :class:`~django.contrib.postgres.operations.CreateExtension` operation
+  now checks that the extension already exists in the database and skips the
+  migration if so.
 
 :mod:`django.contrib.postgres`
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/tests/postgres_tests/test_operations.py b/tests/postgres_tests/test_operations.py
index 0a9d8040ef..8cc9a2b66e 100644
--- a/tests/postgres_tests/test_operations.py
+++ b/tests/postgres_tests/test_operations.py
@@ -12,7 +12,8 @@ from . import PostgreSQLTestCase
 
 try:
     from django.contrib.postgres.operations import (
-        AddIndexConcurrently, CreateExtension, RemoveIndexConcurrently,
+        AddIndexConcurrently, BloomExtension, CreateExtension,
+        RemoveIndexConcurrently,
     )
     from django.contrib.postgres.indexes import BrinIndex, BTreeIndex
 except ImportError:
@@ -180,9 +181,33 @@ class CreateExtensionTests(PostgreSQLTestCase):
         with CaptureQueriesContext(connection) as captured_queries:
             with connection.schema_editor(atomic=False) as editor:
                 operation.database_forwards(self.app_label, editor, project_state, new_state)
-        self.assertIn('CREATE EXTENSION', captured_queries[0]['sql'])
+        self.assertEqual(len(captured_queries), 4)
+        self.assertIn('CREATE EXTENSION', captured_queries[1]['sql'])
         # Reversal.
         with CaptureQueriesContext(connection) as captured_queries:
             with connection.schema_editor(atomic=False) as editor:
                 operation.database_backwards(self.app_label, editor, new_state, project_state)
-        self.assertIn('DROP EXTENSION', captured_queries[0]['sql'])
+        self.assertEqual(len(captured_queries), 2)
+        self.assertIn('DROP EXTENSION', captured_queries[1]['sql'])
+
+    def test_create_existing_extension(self):
+        operation = BloomExtension()
+        project_state = ProjectState()
+        new_state = project_state.clone()
+        # Don't create an existing extension.
+        with CaptureQueriesContext(connection) as captured_queries:
+            with connection.schema_editor(atomic=False) as editor:
+                operation.database_forwards(self.app_label, editor, project_state, new_state)
+        self.assertEqual(len(captured_queries), 3)
+        self.assertIn('SELECT', captured_queries[0]['sql'])
+
+    def test_drop_nonexistent_extension(self):
+        operation = CreateExtension('tablefunc')
+        project_state = ProjectState()
+        new_state = project_state.clone()
+        # Don't drop a nonexistent extension.
+        with CaptureQueriesContext(connection) as captured_queries:
+            with connection.schema_editor(atomic=False) as editor:
+                operation.database_backwards(self.app_label, editor, project_state, new_state)
+        self.assertEqual(len(captured_queries), 1)
+        self.assertIn('SELECT', captured_queries[0]['sql'])