Fixed #27859 -- Ignored db_index for TextField/BinaryField on Oracle and MySQL.

Thanks Zubair Alam for the initial patch and Tim Graham for the review.

django/db/backends/base/features.py

@@ -227,6 +227,9 @@ class BaseDatabaseFeatures:
     supports_select_difference = True
     supports_slicing_ordering_in_compound = False
 
+    # Does the backend support indexing a TextField?
+    supports_index_on_text_field = True
+
     def __init__(self, connection):
         self.connection = connection
 

django/db/backends/mysql/base.py

@@ -143,6 +143,14 @@ class DatabaseWrapper(BaseDatabaseWrapper):
         else:
             return self._data_types
 
+    # For these columns, MySQL doesn't:
+    # - accept default values and implicitly treats these columns as nullable
+    # - support a database index
+    _limited_data_types = (
+        'tinyblob', 'blob', 'mediumblob', 'longblob', 'tinytext', 'text',
+        'mediumtext', 'longtext', 'json',
+    )
+
     operators = {
         'exact': '= %s',
         'iexact': 'LIKE %s',

django/db/backends/mysql/features.py

@@ -31,6 +31,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     supports_select_intersection = False
     supports_select_difference = False
     supports_slicing_ordering_in_compound = True
+    supports_index_on_text_field = False
 
     @cached_property
     def _mysql_storage_engine(self):

django/db/backends/mysql/schema.py

@@ -29,20 +29,12 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         import MySQLdb.converters
         return MySQLdb.escape(value, MySQLdb.converters.conversions)
 
-    def skip_default(self, field):
+    def _is_limited_data_type(self, field):
-        """
-        MySQL doesn't accept default values for some data types and implicitly
-        treats these columns as nullable.
-        """
         db_type = field.db_type(self.connection)
-        return (
+        return db_type is not None and db_type.lower() in self.connection._limited_data_types
-            db_type is not None and
+
-            db_type.lower() in {
+    def skip_default(self, field):
-                'tinyblob', 'blob', 'mediumblob', 'longblob',
+        return self._is_limited_data_type(field)
-                'tinytext', 'text', 'mediumtext', 'longtext',
-                'json',
-            }
-        )
 
     def add_field(self, model, field):
         super().add_field(model, field)
@@ -69,6 +61,8 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
                 field.get_internal_type() == 'ForeignKey' and
                 field.db_constraint):
             return False
+        if self._is_limited_data_type(field):
+            return False
         return create_index
 
     def _delete_composed_index(self, model, fields, *args):

django/db/backends/mysql/validation.py

@@ -31,6 +31,7 @@ class DatabaseValidation(BaseDatabaseValidation):
         MySQL has the following field length restriction:
         No character (varchar) fields can have a length exceeding 255
         characters if they have a unique index on them.
+        MySQL doesn't support a database index on some data types.
         """
         errors = []
         if (field_type.startswith('varchar') and field.unique and
@@ -42,4 +43,18 @@ class DatabaseValidation(BaseDatabaseValidation):
                     id='mysql.E001',
                 )
             )
+
+        if field.db_index and field_type.lower() in self.connection._limited_data_types:
+            errors.append(
+                checks.Warning(
+                    'MySQL does not support a database index on %s columns.'
+                    % field_type,
+                    hint=(
+                        "An index won't be created. Silence this warning if "
+                        "you don't care about it."
+                    ),
+                    obj=field,
+                    id='fields.W162',
+                )
+            )
         return errors

django/db/backends/oracle/base.py

@@ -55,6 +55,7 @@ from .introspection import DatabaseIntrospection            # NOQA isort:skip
 from .operations import DatabaseOperations                  # NOQA isort:skip
 from .schema import DatabaseSchemaEditor                    # NOQA isort:skip
 from .utils import Oracle_datetime                          # NOQA isort:skip
+from .validation import DatabaseValidation                  # NOQA isort:skip
 
 
 class _UninitializedOperatorsDescriptor:
@@ -115,6 +116,9 @@ class DatabaseWrapper(BaseDatabaseWrapper):
         'PositiveSmallIntegerField': '%(qn_column)s >= 0',
     }
 
+    # Oracle doesn't support a database index on these columns.
+    _limited_data_types = ('clob', 'nclob', 'blob')
+
     operators = _UninitializedOperatorsDescriptor()
 
     _standard_operators = {
@@ -174,6 +178,7 @@ class DatabaseWrapper(BaseDatabaseWrapper):
     features_class = DatabaseFeatures
     introspection_class = DatabaseIntrospection
     ops_class = DatabaseOperations
+    validation_class = DatabaseValidation
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)

django/db/backends/oracle/features.py

@@ -35,3 +35,4 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     # Oracle doesn't ignore quoted identifiers case but the current backend
     # does by uppercasing all identifiers.
     ignores_table_name_case = True
+    supports_index_on_text_field = False

django/db/backends/oracle/schema.py

@@ -119,3 +119,10 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
 
     def prepare_default(self, value):
         return self.quote_value(value)
+
+    def _field_should_be_indexed(self, model, field):
+        create_index = super()._field_should_be_indexed(model, field)
+        db_type = field.db_type(self.connection)
+        if db_type is not None and db_type.lower() in self.connection._limited_data_types:
+            return False
+        return create_index

django/db/backends/oracle/validation.py

@@ -0,0 +1,22 @@
+from django.core import checks
+from django.db.backends.base.validation import BaseDatabaseValidation
+
+
+class DatabaseValidation(BaseDatabaseValidation):
+    def check_field_type(self, field, field_type):
+        """Oracle doesn't support a database index on some data types."""
+        errors = []
+        if field.db_index and field_type.lower() in self.connection._limited_data_types:
+            errors.append(
+                checks.Warning(
+                    'Oracle does not support a database index on %s columns.'
+                    % field_type,
+                    hint=(
+                        "An index won't be created. Silence this warning if "
+                        "you don't care about it."
+                    ),
+                    obj=field,
+                    id='fields.W162',
+                )
+            )
+        return errors

docs/ref/checks.txt

@@ -172,6 +172,8 @@ Model fields
 * **fields.E160**: The options ``auto_now``, ``auto_now_add``, and ``default``
   are mutually exclusive. Only one of these options may be present.
 * **fields.W161**: Fixed default value provided.
+* **fields.W162**: ``<database>`` does not support a database index on
+  ``<field data type>`` columns.
 * **fields.E900**: ``IPAddressField`` has been removed except for support in
   historical migrations.
 * **fields.W900**: ``IPAddressField`` has been deprecated. Support for it

tests/invalid_models_tests/test_ordinary_fields.py

@@ -2,7 +2,7 @@ import unittest
 
 from django.core.checks import Error, Warning as DjangoWarning
 from django.db import connection, models
-from django.test import SimpleTestCase, TestCase
+from django.test import SimpleTestCase, TestCase, skipIfDBFeature
 from django.test.utils import isolate_apps, override_settings
 from django.utils.timezone import now
 
@@ -646,3 +646,26 @@ class TimeFieldTests(TestCase):
     @override_settings(USE_TZ=True)
     def test_fix_default_value_tz(self):
         self.test_fix_default_value()
+
+
+@isolate_apps('invalid_models_tests')
+class TextFieldTests(TestCase):
+
+    @skipIfDBFeature('supports_index_on_text_field')
+    def test_max_length_warning(self):
+        class Model(models.Model):
+            value = models.TextField(db_index=True)
+        field = Model._meta.get_field('value')
+        field_type = field.db_type(connection)
+        self.assertEqual(field.check(), [
+            DjangoWarning(
+                '%s does not support a database index on %s columns.'
+                % (connection.display_name, field_type),
+                hint=(
+                    "An index won't be created. Silence this warning if you "
+                    "don't care about it."
+                ),
+                obj=field,
+                id='fields.W162',
+            )
+        ])

tests/schema/models.py

@@ -17,6 +17,13 @@ class Author(models.Model):
         apps = new_apps
 
 
+class AuthorTextFieldWithIndex(models.Model):
+    text_field = models.TextField(db_index=True)
+
+    class Meta:
+        apps = new_apps
+
+
 class AuthorWithDefaultHeight(models.Model):
     name = models.CharField(max_length=255)
     height = models.PositiveIntegerField(null=True, blank=True, default=42)

tests/schema/tests.py

@@ -29,11 +29,11 @@ from .fields import (
     CustomManyToManyField, InheritedManyToManyField, MediumBlobField,
 )
 from .models import (
-    Author, AuthorWithDefaultHeight, AuthorWithEvenLongerName,
+    Author, AuthorTextFieldWithIndex, AuthorWithDefaultHeight,
-    AuthorWithIndexedName, Book, BookForeignObj, BookWeak, BookWithLongName,
+    AuthorWithEvenLongerName, AuthorWithIndexedName, Book, BookForeignObj,
-    BookWithO2O, BookWithoutAuthor, BookWithSlug, IntegerPK, Node, Note,
+    BookWeak, BookWithLongName, BookWithO2O, BookWithoutAuthor, BookWithSlug,
-    NoteRename, Tag, TagIndexed, TagM2MTest, TagUniqueRename, Thing,
+    IntegerPK, Node, Note, NoteRename, Tag, TagIndexed, TagM2MTest,
-    UniqueTest, new_apps,
+    TagUniqueRename, Thing, UniqueTest, new_apps,
 )
 
 
@@ -1749,6 +1749,13 @@ class SchemaTests(TransactionTestCase):
             self.get_indexes(Book._meta.db_table),
         )
 
+    def test_text_field_with_db_index(self):
+        with connection.schema_editor() as editor:
+            editor.create_model(AuthorTextFieldWithIndex)
+        # The text_field index is present if the database supports it.
+        assertion = self.assertIn if connection.features.supports_index_on_text_field else self.assertNotIn
+        assertion('text_field', self.get_indexes(AuthorTextFieldWithIndex._meta.db_table))
+
     def test_primary_key(self):
         """
         Tests altering of the primary key