Fixed #14180 -- Prevented unneeded index creation on MySQL-InnoDB

Thanks zimnyx for the report and Simon Charette, Tim Graham for
the reviews.

django/db/backends/mysql/introspection.py

@@ -142,13 +142,17 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
 
     def get_storage_engine(self, cursor, table_name):
         """
-        Retrieves the storage engine for a given table.
+        Retrieves the storage engine for a given table. Returns the default
+        storage engine if the table doesn't exist.
         """
         cursor.execute(
             "SELECT engine "
             "FROM information_schema.tables "
             "WHERE table_name = %s", [table_name])
-        return cursor.fetchone()[0]
+        result = cursor.fetchone()
+        if not result:
+            return self.connection.features._mysql_storage_engine
+        return result[0]
 
     def get_constraints(self, cursor, table_name):
         """

django/db/backends/mysql/schema.py

@@ -51,3 +51,15 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
                 'table': self.quote_name(model._meta.db_table),
                 'column': self.quote_name(field.column),
             }, [effective_default])
+
+    def _model_indexes_sql(self, model):
+        storage = self.connection.introspection.get_storage_engine(
+            self.connection.cursor(), model._meta.db_table
+        )
+        if storage == "InnoDB":
+            for field in model._meta.local_fields:
+                if field.db_index and not field.unique and field.get_internal_type() == "ForeignKey":
+                    # Temporary setting db_index to False (in memory) to disable
+                    # index creation for FKs (index automatically created by MySQL)
+                    field.db_index = False
+        return super(DatabaseSchemaEditor, self)._model_indexes_sql(model)

docs/releases/1.8.txt

@@ -249,6 +249,9 @@ Database backends
   and up will support microseconds. See the :ref:`MySQL database notes
   <mysql-fractional-seconds>` for more details.
 
+* The MySQL backend no longer creates explicit indexes for foreign keys when
+  using the InnoDB storage engine, as MySQL already creates them automatically.
+
 Email
 ^^^^^
 

tests/commands_sql/tests.py

@@ -72,14 +72,14 @@ class SQLCommandsTestCase(TestCase):
         with warnings.catch_warnings():
             warnings.simplefilter("ignore", category=RemovedInDjango20Warning)
             output = sql_indexes(app_config, no_style(), connections[DEFAULT_DB_ALIAS])
-        # PostgreSQL creates one additional index for CharField
-        self.assertIn(self.count_ddl(output, 'CREATE INDEX'), [3, 4])
+        # Number of indexes is backend-dependent
+        self.assertTrue(1 <= self.count_ddl(output, 'CREATE INDEX') <= 4)
 
     def test_sql_destroy_indexes(self):
         app_config = apps.get_app_config('commands_sql')
         output = sql_destroy_indexes(app_config, no_style(), connections[DEFAULT_DB_ALIAS])
-        # PostgreSQL creates one additional index for CharField
-        self.assertIn(self.count_ddl(output, 'DROP INDEX'), [3, 4])
+        # Number of indexes is backend-dependent
+        self.assertTrue(1 <= self.count_ddl(output, 'DROP INDEX') <= 4)
 
     def test_sql_all(self):
         app_config = apps.get_app_config('commands_sql')
@@ -88,8 +88,8 @@ class SQLCommandsTestCase(TestCase):
             output = sql_all(app_config, no_style(), connections[DEFAULT_DB_ALIAS])
 
         self.assertEqual(self.count_ddl(output, 'CREATE TABLE'), 3)
-        # PostgreSQL creates one additional index for CharField
-        self.assertIn(self.count_ddl(output, 'CREATE INDEX'), [3, 4])
+        # Number of indexes is backend-dependent
+        self.assertTrue(1 <= self.count_ddl(output, 'CREATE INDEX') <= 4)
 
 
 class TestRouter(object):

tests/indexes/tests.py

@@ -5,7 +5,7 @@ from django.db import connection
 from django.test import TestCase
 from django.test.utils import IgnorePendingDeprecationWarningsMixin
 
-from .models import Article, IndexTogetherSingleList
+from .models import Article, ArticleTranslation, IndexTogetherSingleList
 
 
 class CreationIndexesTests(IgnorePendingDeprecationWarningsMixin, TestCase):
@@ -82,3 +82,17 @@ class SchemaIndexesTests(TestCase):
         """Test indexes are not created for related objects"""
         index_sql = connection.schema_editor()._model_indexes_sql(Article)
         self.assertEqual(len(index_sql), 1)
+
+    @skipUnless(connection.vendor == 'mysql', "This is a mysql-specific issue")
+    def test_no_index_for_foreignkey(self):
+        """
+        MySQL on InnoDB already creates indexes automatically for foreign keys.
+        (#14180).
+        """
+        storage = connection.introspection.get_storage_engine(
+            connection.cursor(), ArticleTranslation._meta.db_table
+        )
+        if storage != "InnoDB":
+            self.skip("This test only applies to the InnoDB storage engine")
+        index_sql = connection.schema_editor()._model_indexes_sql(ArticleTranslation)
+        self.assertEqual(index_sql, [])