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Lib/site-packages/django/db/backends/base/base.py

@@ -0,0 +1,674 @@
+import _thread
+import copy
+import threading
+import time
+import warnings
+from collections import deque
+from contextlib import contextmanager
+
+import pytz
+
+from django.conf import settings
+from django.core.exceptions import ImproperlyConfigured
+from django.db import DEFAULT_DB_ALIAS, DatabaseError
+from django.db.backends import utils
+from django.db.backends.base.validation import BaseDatabaseValidation
+from django.db.backends.signals import connection_created
+from django.db.transaction import TransactionManagementError
+from django.db.utils import DatabaseErrorWrapper
+from django.utils import timezone
+from django.utils.asyncio import async_unsafe
+from django.utils.functional import cached_property
+
+NO_DB_ALIAS = '__no_db__'
+
+
+class BaseDatabaseWrapper:
+    """Represent a database connection."""
+    # Mapping of Field objects to their column types.
+    data_types = {}
+    # Mapping of Field objects to their SQL suffix such as AUTOINCREMENT.
+    data_types_suffix = {}
+    # Mapping of Field objects to their SQL for CHECK constraints.
+    data_type_check_constraints = {}
+    ops = None
+    vendor = 'unknown'
+    display_name = 'unknown'
+    SchemaEditorClass = None
+    # Classes instantiated in __init__().
+    client_class = None
+    creation_class = None
+    features_class = None
+    introspection_class = None
+    ops_class = None
+    validation_class = BaseDatabaseValidation
+
+    queries_limit = 9000
+
+    def __init__(self, settings_dict, alias=DEFAULT_DB_ALIAS):
+        # Connection related attributes.
+        # The underlying database connection.
+        self.connection = None
+        # `settings_dict` should be a dictionary containing keys such as
+        # NAME, USER, etc. It's called `settings_dict` instead of `settings`
+        # to disambiguate it from Django settings modules.
+        self.settings_dict = settings_dict
+        self.alias = alias
+        # Query logging in debug mode or when explicitly enabled.
+        self.queries_log = deque(maxlen=self.queries_limit)
+        self.force_debug_cursor = False
+
+        # Transaction related attributes.
+        # Tracks if the connection is in autocommit mode. Per PEP 249, by
+        # default, it isn't.
+        self.autocommit = False
+        # Tracks if the connection is in a transaction managed by 'atomic'.
+        self.in_atomic_block = False
+        # Increment to generate unique savepoint ids.
+        self.savepoint_state = 0
+        # List of savepoints created by 'atomic'.
+        self.savepoint_ids = []
+        # Tracks if the outermost 'atomic' block should commit on exit,
+        # ie. if autocommit was active on entry.
+        self.commit_on_exit = True
+        # Tracks if the transaction should be rolled back to the next
+        # available savepoint because of an exception in an inner block.
+        self.needs_rollback = False
+
+        # Connection termination related attributes.
+        self.close_at = None
+        self.closed_in_transaction = False
+        self.errors_occurred = False
+
+        # Thread-safety related attributes.
+        self._thread_sharing_lock = threading.Lock()
+        self._thread_sharing_count = 0
+        self._thread_ident = _thread.get_ident()
+
+        # A list of no-argument functions to run when the transaction commits.
+        # Each entry is an (sids, func) tuple, where sids is a set of the
+        # active savepoint IDs when this function was registered.
+        self.run_on_commit = []
+
+        # Should we run the on-commit hooks the next time set_autocommit(True)
+        # is called?
+        self.run_commit_hooks_on_set_autocommit_on = False
+
+        # A stack of wrappers to be invoked around execute()/executemany()
+        # calls. Each entry is a function taking five arguments: execute, sql,
+        # params, many, and context. It's the function's responsibility to
+        # call execute(sql, params, many, context).
+        self.execute_wrappers = []
+
+        self.client = self.client_class(self)
+        self.creation = self.creation_class(self)
+        self.features = self.features_class(self)
+        self.introspection = self.introspection_class(self)
+        self.ops = self.ops_class(self)
+        self.validation = self.validation_class(self)
+
+    def ensure_timezone(self):
+        """
+        Ensure the connection's timezone is set to `self.timezone_name` and
+        return whether it changed or not.
+        """
+        return False
+
+    @cached_property
+    def timezone(self):
+        """
+        Return a tzinfo of the database connection time zone.
+
+        This is only used when time zone support is enabled. When a datetime is
+        read from the database, it is always returned in this time zone.
+
+        When the database backend supports time zones, it doesn't matter which
+        time zone Django uses, as long as aware datetimes are used everywhere.
+        Other users connecting to the database can choose their own time zone.
+
+        When the database backend doesn't support time zones, the time zone
+        Django uses may be constrained by the requirements of other users of
+        the database.
+        """
+        if not settings.USE_TZ:
+            return None
+        elif self.settings_dict['TIME_ZONE'] is None:
+            return timezone.utc
+        else:
+            return pytz.timezone(self.settings_dict['TIME_ZONE'])
+
+    @cached_property
+    def timezone_name(self):
+        """
+        Name of the time zone of the database connection.
+        """
+        if not settings.USE_TZ:
+            return settings.TIME_ZONE
+        elif self.settings_dict['TIME_ZONE'] is None:
+            return 'UTC'
+        else:
+            return self.settings_dict['TIME_ZONE']
+
+    @property
+    def queries_logged(self):
+        return self.force_debug_cursor or settings.DEBUG
+
+    @property
+    def queries(self):
+        if len(self.queries_log) == self.queries_log.maxlen:
+            warnings.warn(
+                "Limit for query logging exceeded, only the last {} queries "
+                "will be returned.".format(self.queries_log.maxlen))
+        return list(self.queries_log)
+
+    # ##### Backend-specific methods for creating connections and cursors #####
+
+    def get_connection_params(self):
+        """Return a dict of parameters suitable for get_new_connection."""
+        raise NotImplementedError('subclasses of BaseDatabaseWrapper may require a get_connection_params() method')
+
+    def get_new_connection(self, conn_params):
+        """Open a connection to the database."""
+        raise NotImplementedError('subclasses of BaseDatabaseWrapper may require a get_new_connection() method')
+
+    def init_connection_state(self):
+        """Initialize the database connection settings."""
+        raise NotImplementedError('subclasses of BaseDatabaseWrapper may require an init_connection_state() method')
+
+    def create_cursor(self, name=None):
+        """Create a cursor. Assume that a connection is established."""
+        raise NotImplementedError('subclasses of BaseDatabaseWrapper may require a create_cursor() method')
+
+    # ##### Backend-specific methods for creating connections #####
+
+    @async_unsafe
+    def connect(self):
+        """Connect to the database. Assume that the connection is closed."""
+        # Check for invalid configurations.
+        self.check_settings()
+        # In case the previous connection was closed while in an atomic block
+        self.in_atomic_block = False
+        self.savepoint_ids = []
+        self.needs_rollback = False
+        # Reset parameters defining when to close the connection
+        max_age = self.settings_dict['CONN_MAX_AGE']
+        self.close_at = None if max_age is None else time.monotonic() + max_age
+        self.closed_in_transaction = False
+        self.errors_occurred = False
+        # Establish the connection
+        conn_params = self.get_connection_params()
+        self.connection = self.get_new_connection(conn_params)
+        self.set_autocommit(self.settings_dict['AUTOCOMMIT'])
+        self.init_connection_state()
+        connection_created.send(sender=self.__class__, connection=self)
+
+        self.run_on_commit = []
+
+    def check_settings(self):
+        if self.settings_dict['TIME_ZONE'] is not None and not settings.USE_TZ:
+            raise ImproperlyConfigured(
+                "Connection '%s' cannot set TIME_ZONE because USE_TZ is False."
+                % self.alias
+            )
+
+    @async_unsafe
+    def ensure_connection(self):
+        """Guarantee that a connection to the database is established."""
+        if self.connection is None:
+            with self.wrap_database_errors:
+                self.connect()
+
+    # ##### Backend-specific wrappers for PEP-249 connection methods #####
+
+    def _prepare_cursor(self, cursor):
+        """
+        Validate the connection is usable and perform database cursor wrapping.
+        """
+        self.validate_thread_sharing()
+        if self.queries_logged:
+            wrapped_cursor = self.make_debug_cursor(cursor)
+        else:
+            wrapped_cursor = self.make_cursor(cursor)
+        return wrapped_cursor
+
+    def _cursor(self, name=None):
+        self.ensure_connection()
+        with self.wrap_database_errors:
+            return self._prepare_cursor(self.create_cursor(name))
+
+    def _commit(self):
+        if self.connection is not None:
+            with self.wrap_database_errors:
+                return self.connection.commit()
+
+    def _rollback(self):
+        if self.connection is not None:
+            with self.wrap_database_errors:
+                return self.connection.rollback()
+
+    def _close(self):
+        if self.connection is not None:
+            with self.wrap_database_errors:
+                return self.connection.close()
+
+    # ##### Generic wrappers for PEP-249 connection methods #####
+
+    @async_unsafe
+    def cursor(self):
+        """Create a cursor, opening a connection if necessary."""
+        return self._cursor()
+
+    @async_unsafe
+    def commit(self):
+        """Commit a transaction and reset the dirty flag."""
+        self.validate_thread_sharing()
+        self.validate_no_atomic_block()
+        self._commit()
+        # A successful commit means that the database connection works.
+        self.errors_occurred = False
+        self.run_commit_hooks_on_set_autocommit_on = True
+
+    @async_unsafe
+    def rollback(self):
+        """Roll back a transaction and reset the dirty flag."""
+        self.validate_thread_sharing()
+        self.validate_no_atomic_block()
+        self._rollback()
+        # A successful rollback means that the database connection works.
+        self.errors_occurred = False
+        self.needs_rollback = False
+        self.run_on_commit = []
+
+    @async_unsafe
+    def close(self):
+        """Close the connection to the database."""
+        self.validate_thread_sharing()
+        self.run_on_commit = []
+
+        # Don't call validate_no_atomic_block() to avoid making it difficult
+        # to get rid of a connection in an invalid state. The next connect()
+        # will reset the transaction state anyway.
+        if self.closed_in_transaction or self.connection is None:
+            return
+        try:
+            self._close()
+        finally:
+            if self.in_atomic_block:
+                self.closed_in_transaction = True
+                self.needs_rollback = True
+            else:
+                self.connection = None
+
+    # ##### Backend-specific savepoint management methods #####
+
+    def _savepoint(self, sid):
+        with self.cursor() as cursor:
+            cursor.execute(self.ops.savepoint_create_sql(sid))
+
+    def _savepoint_rollback(self, sid):
+        with self.cursor() as cursor:
+            cursor.execute(self.ops.savepoint_rollback_sql(sid))
+
+    def _savepoint_commit(self, sid):
+        with self.cursor() as cursor:
+            cursor.execute(self.ops.savepoint_commit_sql(sid))
+
+    def _savepoint_allowed(self):
+        # Savepoints cannot be created outside a transaction
+        return self.features.uses_savepoints and not self.get_autocommit()
+
+    # ##### Generic savepoint management methods #####
+
+    @async_unsafe
+    def savepoint(self):
+        """
+        Create a savepoint inside the current transaction. Return an
+        identifier for the savepoint that will be used for the subsequent
+        rollback or commit. Do nothing if savepoints are not supported.
+        """
+        if not self._savepoint_allowed():
+            return
+
+        thread_ident = _thread.get_ident()
+        tid = str(thread_ident).replace('-', '')
+
+        self.savepoint_state += 1
+        sid = "s%s_x%d" % (tid, self.savepoint_state)
+
+        self.validate_thread_sharing()
+        self._savepoint(sid)
+
+        return sid
+
+    @async_unsafe
+    def savepoint_rollback(self, sid):
+        """
+        Roll back to a savepoint. Do nothing if savepoints are not supported.
+        """
+        if not self._savepoint_allowed():
+            return
+
+        self.validate_thread_sharing()
+        self._savepoint_rollback(sid)
+
+        # Remove any callbacks registered while this savepoint was active.
+        self.run_on_commit = [
+            (sids, func) for (sids, func) in self.run_on_commit if sid not in sids
+        ]
+
+    @async_unsafe
+    def savepoint_commit(self, sid):
+        """
+        Release a savepoint. Do nothing if savepoints are not supported.
+        """
+        if not self._savepoint_allowed():
+            return
+
+        self.validate_thread_sharing()
+        self._savepoint_commit(sid)
+
+    @async_unsafe
+    def clean_savepoints(self):
+        """
+        Reset the counter used to generate unique savepoint ids in this thread.
+        """
+        self.savepoint_state = 0
+
+    # ##### Backend-specific transaction management methods #####
+
+    def _set_autocommit(self, autocommit):
+        """
+        Backend-specific implementation to enable or disable autocommit.
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseWrapper may require a _set_autocommit() method')
+
+    # ##### Generic transaction management methods #####
+
+    def get_autocommit(self):
+        """Get the autocommit state."""
+        self.ensure_connection()
+        return self.autocommit
+
+    def set_autocommit(self, autocommit, force_begin_transaction_with_broken_autocommit=False):
+        """
+        Enable or disable autocommit.
+
+        The usual way to start a transaction is to turn autocommit off.
+        SQLite does not properly start a transaction when disabling
+        autocommit. To avoid this buggy behavior and to actually enter a new
+        transaction, an explicit BEGIN is required. Using
+        force_begin_transaction_with_broken_autocommit=True will issue an
+        explicit BEGIN with SQLite. This option will be ignored for other
+        backends.
+        """
+        self.validate_no_atomic_block()
+        self.ensure_connection()
+
+        start_transaction_under_autocommit = (
+            force_begin_transaction_with_broken_autocommit and not autocommit and
+            hasattr(self, '_start_transaction_under_autocommit')
+        )
+
+        if start_transaction_under_autocommit:
+            self._start_transaction_under_autocommit()
+        else:
+            self._set_autocommit(autocommit)
+
+        self.autocommit = autocommit
+
+        if autocommit and self.run_commit_hooks_on_set_autocommit_on:
+            self.run_and_clear_commit_hooks()
+            self.run_commit_hooks_on_set_autocommit_on = False
+
+    def get_rollback(self):
+        """Get the "needs rollback" flag -- for *advanced use* only."""
+        if not self.in_atomic_block:
+            raise TransactionManagementError(
+                "The rollback flag doesn't work outside of an 'atomic' block.")
+        return self.needs_rollback
+
+    def set_rollback(self, rollback):
+        """
+        Set or unset the "needs rollback" flag -- for *advanced use* only.
+        """
+        if not self.in_atomic_block:
+            raise TransactionManagementError(
+                "The rollback flag doesn't work outside of an 'atomic' block.")
+        self.needs_rollback = rollback
+
+    def validate_no_atomic_block(self):
+        """Raise an error if an atomic block is active."""
+        if self.in_atomic_block:
+            raise TransactionManagementError(
+                "This is forbidden when an 'atomic' block is active.")
+
+    def validate_no_broken_transaction(self):
+        if self.needs_rollback:
+            raise TransactionManagementError(
+                "An error occurred in the current transaction. You can't "
+                "execute queries until the end of the 'atomic' block.")
+
+    # ##### Foreign key constraints checks handling #####
+
+    @contextmanager
+    def constraint_checks_disabled(self):
+        """
+        Disable foreign key constraint checking.
+        """
+        disabled = self.disable_constraint_checking()
+        try:
+            yield
+        finally:
+            if disabled:
+                self.enable_constraint_checking()
+
+    def disable_constraint_checking(self):
+        """
+        Backends can implement as needed to temporarily disable foreign key
+        constraint checking. Should return True if the constraints were
+        disabled and will need to be reenabled.
+        """
+        return False
+
+    def enable_constraint_checking(self):
+        """
+        Backends can implement as needed to re-enable foreign key constraint
+        checking.
+        """
+        pass
+
+    def check_constraints(self, table_names=None):
+        """
+        Backends can override this method if they can apply constraint
+        checking (e.g. via "SET CONSTRAINTS ALL IMMEDIATE"). Should raise an
+        IntegrityError if any invalid foreign key references are encountered.
+        """
+        pass
+
+    # ##### Connection termination handling #####
+
+    def is_usable(self):
+        """
+        Test if the database connection is usable.
+
+        This method may assume that self.connection is not None.
+
+        Actual implementations should take care not to raise exceptions
+        as that may prevent Django from recycling unusable connections.
+        """
+        raise NotImplementedError(
+            "subclasses of BaseDatabaseWrapper may require an is_usable() method")
+
+    def close_if_unusable_or_obsolete(self):
+        """
+        Close the current connection if unrecoverable errors have occurred
+        or if it outlived its maximum age.
+        """
+        if self.connection is not None:
+            # If the application didn't restore the original autocommit setting,
+            # don't take chances, drop the connection.
+            if self.get_autocommit() != self.settings_dict['AUTOCOMMIT']:
+                self.close()
+                return
+
+            # If an exception other than DataError or IntegrityError occurred
+            # since the last commit / rollback, check if the connection works.
+            if self.errors_occurred:
+                if self.is_usable():
+                    self.errors_occurred = False
+                else:
+                    self.close()
+                    return
+
+            if self.close_at is not None and time.monotonic() >= self.close_at:
+                self.close()
+                return
+
+    # ##### Thread safety handling #####
+
+    @property
+    def allow_thread_sharing(self):
+        with self._thread_sharing_lock:
+            return self._thread_sharing_count > 0
+
+    def inc_thread_sharing(self):
+        with self._thread_sharing_lock:
+            self._thread_sharing_count += 1
+
+    def dec_thread_sharing(self):
+        with self._thread_sharing_lock:
+            if self._thread_sharing_count <= 0:
+                raise RuntimeError('Cannot decrement the thread sharing count below zero.')
+            self._thread_sharing_count -= 1
+
+    def validate_thread_sharing(self):
+        """
+        Validate that the connection isn't accessed by another thread than the
+        one which originally created it, unless the connection was explicitly
+        authorized to be shared between threads (via the `inc_thread_sharing()`
+        method). Raise an exception if the validation fails.
+        """
+        if not (self.allow_thread_sharing or self._thread_ident == _thread.get_ident()):
+            raise DatabaseError(
+                "DatabaseWrapper objects created in a "
+                "thread can only be used in that same thread. The object "
+                "with alias '%s' was created in thread id %s and this is "
+                "thread id %s."
+                % (self.alias, self._thread_ident, _thread.get_ident())
+            )
+
+    # ##### Miscellaneous #####
+
+    def prepare_database(self):
+        """
+        Hook to do any database check or preparation, generally called before
+        migrating a project or an app.
+        """
+        pass
+
+    @cached_property
+    def wrap_database_errors(self):
+        """
+        Context manager and decorator that re-throws backend-specific database
+        exceptions using Django's common wrappers.
+        """
+        return DatabaseErrorWrapper(self)
+
+    def chunked_cursor(self):
+        """
+        Return a cursor that tries to avoid caching in the database (if
+        supported by the database), otherwise return a regular cursor.
+        """
+        return self.cursor()
+
+    def make_debug_cursor(self, cursor):
+        """Create a cursor that logs all queries in self.queries_log."""
+        return utils.CursorDebugWrapper(cursor, self)
+
+    def make_cursor(self, cursor):
+        """Create a cursor without debug logging."""
+        return utils.CursorWrapper(cursor, self)
+
+    @contextmanager
+    def temporary_connection(self):
+        """
+        Context manager that ensures that a connection is established, and
+        if it opened one, closes it to avoid leaving a dangling connection.
+        This is useful for operations outside of the request-response cycle.
+
+        Provide a cursor: with self.temporary_connection() as cursor: ...
+        """
+        must_close = self.connection is None
+        try:
+            with self.cursor() as cursor:
+                yield cursor
+        finally:
+            if must_close:
+                self.close()
+
+    @contextmanager
+    def _nodb_cursor(self):
+        """
+        Return a cursor from an alternative connection to be used when there is
+        no need to access the main database, specifically for test db
+        creation/deletion. This also prevents the production database from
+        being exposed to potential child threads while (or after) the test
+        database is destroyed. Refs #10868, #17786, #16969.
+        """
+        conn = self.__class__({**self.settings_dict, 'NAME': None}, alias=NO_DB_ALIAS)
+        try:
+            with conn.cursor() as cursor:
+                yield cursor
+        finally:
+            conn.close()
+
+    def schema_editor(self, *args, **kwargs):
+        """
+        Return a new instance of this backend's SchemaEditor.
+        """
+        if self.SchemaEditorClass is None:
+            raise NotImplementedError(
+                'The SchemaEditorClass attribute of this database wrapper is still None')
+        return self.SchemaEditorClass(self, *args, **kwargs)
+
+    def on_commit(self, func):
+        if self.in_atomic_block:
+            # Transaction in progress; save for execution on commit.
+            self.run_on_commit.append((set(self.savepoint_ids), func))
+        elif not self.get_autocommit():
+            raise TransactionManagementError('on_commit() cannot be used in manual transaction management')
+        else:
+            # No transaction in progress and in autocommit mode; execute
+            # immediately.
+            func()
+
+    def run_and_clear_commit_hooks(self):
+        self.validate_no_atomic_block()
+        current_run_on_commit = self.run_on_commit
+        self.run_on_commit = []
+        while current_run_on_commit:
+            sids, func = current_run_on_commit.pop(0)
+            func()
+
+    @contextmanager
+    def execute_wrapper(self, wrapper):
+        """
+        Return a context manager under which the wrapper is applied to suitable
+        database query executions.
+        """
+        self.execute_wrappers.append(wrapper)
+        try:
+            yield
+        finally:
+            self.execute_wrappers.pop()
+
+    def copy(self, alias=None):
+        """
+        Return a copy of this connection.
+
+        For tests that require two connections to the same database.
+        """
+        settings_dict = copy.deepcopy(self.settings_dict)
+        if alias is None:
+            alias = self.alias
+        return type(self)(settings_dict, alias)

Lib/site-packages/django/db/backends/base/client.py

@@ -0,0 +1,12 @@
+class BaseDatabaseClient:
+    """Encapsulate backend-specific methods for opening a client shell."""
+    # This should be a string representing the name of the executable
+    # (e.g., "psql"). Subclasses must override this.
+    executable_name = None
+
+    def __init__(self, connection):
+        # connection is an instance of BaseDatabaseWrapper.
+        self.connection = connection
+
+    def runshell(self, parameters):
+        raise NotImplementedError('subclasses of BaseDatabaseClient must provide a runshell() method')

Lib/site-packages/django/db/backends/base/creation.py

@@ -0,0 +1,314 @@
+import os
+import sys
+from io import StringIO
+
+from django.apps import apps
+from django.conf import settings
+from django.core import serializers
+from django.db import router
+from django.db.transaction import atomic
+
+# The prefix to put on the default database name when creating
+# the test database.
+TEST_DATABASE_PREFIX = 'test_'
+
+
+class BaseDatabaseCreation:
+    """
+    Encapsulate backend-specific differences pertaining to creation and
+    destruction of the test database.
+    """
+    def __init__(self, connection):
+        self.connection = connection
+
+    def _nodb_cursor(self):
+        return self.connection._nodb_cursor()
+
+    def log(self, msg):
+        sys.stderr.write(msg + os.linesep)
+
+    def create_test_db(self, verbosity=1, autoclobber=False, serialize=True, keepdb=False):
+        """
+        Create a test database, prompting the user for confirmation if the
+        database already exists. Return the name of the test database created.
+        """
+        # Don't import django.core.management if it isn't needed.
+        from django.core.management import call_command
+
+        test_database_name = self._get_test_db_name()
+
+        if verbosity >= 1:
+            action = 'Creating'
+            if keepdb:
+                action = "Using existing"
+
+            self.log('%s test database for alias %s...' % (
+                action,
+                self._get_database_display_str(verbosity, test_database_name),
+            ))
+
+        # We could skip this call if keepdb is True, but we instead
+        # give it the keepdb param. This is to handle the case
+        # where the test DB doesn't exist, in which case we need to
+        # create it, then just not destroy it. If we instead skip
+        # this, we will get an exception.
+        self._create_test_db(verbosity, autoclobber, keepdb)
+
+        self.connection.close()
+        settings.DATABASES[self.connection.alias]["NAME"] = test_database_name
+        self.connection.settings_dict["NAME"] = test_database_name
+
+        try:
+            if self.connection.settings_dict['TEST']['MIGRATE'] is False:
+                # Disable migrations for all apps.
+                old_migration_modules = settings.MIGRATION_MODULES
+                settings.MIGRATION_MODULES = {
+                    app.label: None
+                    for app in apps.get_app_configs()
+                }
+            # We report migrate messages at one level lower than that
+            # requested. This ensures we don't get flooded with messages during
+            # testing (unless you really ask to be flooded).
+            call_command(
+                'migrate',
+                verbosity=max(verbosity - 1, 0),
+                interactive=False,
+                database=self.connection.alias,
+                run_syncdb=True,
+            )
+        finally:
+            if self.connection.settings_dict['TEST']['MIGRATE'] is False:
+                settings.MIGRATION_MODULES = old_migration_modules
+
+        # We then serialize the current state of the database into a string
+        # and store it on the connection. This slightly horrific process is so people
+        # who are testing on databases without transactions or who are using
+        # a TransactionTestCase still get a clean database on every test run.
+        if serialize:
+            self.connection._test_serialized_contents = self.serialize_db_to_string()
+
+        call_command('createcachetable', database=self.connection.alias)
+
+        # Ensure a connection for the side effect of initializing the test database.
+        self.connection.ensure_connection()
+
+        return test_database_name
+
+    def set_as_test_mirror(self, primary_settings_dict):
+        """
+        Set this database up to be used in testing as a mirror of a primary
+        database whose settings are given.
+        """
+        self.connection.settings_dict['NAME'] = primary_settings_dict['NAME']
+
+    def serialize_db_to_string(self):
+        """
+        Serialize all data in the database into a JSON string.
+        Designed only for test runner usage; will not handle large
+        amounts of data.
+        """
+        # Iteratively return every object for all models to serialize.
+        def get_objects():
+            from django.db.migrations.loader import MigrationLoader
+            loader = MigrationLoader(self.connection)
+            for app_config in apps.get_app_configs():
+                if (
+                    app_config.models_module is not None and
+                    app_config.label in loader.migrated_apps and
+                    app_config.name not in settings.TEST_NON_SERIALIZED_APPS
+                ):
+                    for model in app_config.get_models():
+                        if (
+                            model._meta.can_migrate(self.connection) and
+                            router.allow_migrate_model(self.connection.alias, model)
+                        ):
+                            queryset = model._default_manager.using(
+                                self.connection.alias,
+                            ).order_by(model._meta.pk.name)
+                            yield from queryset.iterator()
+        # Serialize to a string
+        out = StringIO()
+        serializers.serialize("json", get_objects(), indent=None, stream=out)
+        return out.getvalue()
+
+    def deserialize_db_from_string(self, data):
+        """
+        Reload the database with data from a string generated by
+        the serialize_db_to_string() method.
+        """
+        data = StringIO(data)
+        table_names = set()
+        # Load data in a transaction to handle forward references and cycles.
+        with atomic(using=self.connection.alias):
+            # Disable constraint checks, because some databases (MySQL) doesn't
+            # support deferred checks.
+            with self.connection.constraint_checks_disabled():
+                for obj in serializers.deserialize('json', data, using=self.connection.alias):
+                    obj.save()
+                    table_names.add(obj.object.__class__._meta.db_table)
+            # Manually check for any invalid keys that might have been added,
+            # because constraint checks were disabled.
+            self.connection.check_constraints(table_names=table_names)
+
+    def _get_database_display_str(self, verbosity, database_name):
+        """
+        Return display string for a database for use in various actions.
+        """
+        return "'%s'%s" % (
+            self.connection.alias,
+            (" ('%s')" % database_name) if verbosity >= 2 else '',
+        )
+
+    def _get_test_db_name(self):
+        """
+        Internal implementation - return the name of the test DB that will be
+        created. Only useful when called from create_test_db() and
+        _create_test_db() and when no external munging is done with the 'NAME'
+        settings.
+        """
+        if self.connection.settings_dict['TEST']['NAME']:
+            return self.connection.settings_dict['TEST']['NAME']
+        return TEST_DATABASE_PREFIX + self.connection.settings_dict['NAME']
+
+    def _execute_create_test_db(self, cursor, parameters, keepdb=False):
+        cursor.execute('CREATE DATABASE %(dbname)s %(suffix)s' % parameters)
+
+    def _create_test_db(self, verbosity, autoclobber, keepdb=False):
+        """
+        Internal implementation - create the test db tables.
+        """
+        test_database_name = self._get_test_db_name()
+        test_db_params = {
+            'dbname': self.connection.ops.quote_name(test_database_name),
+            'suffix': self.sql_table_creation_suffix(),
+        }
+        # Create the test database and connect to it.
+        with self._nodb_cursor() as cursor:
+            try:
+                self._execute_create_test_db(cursor, test_db_params, keepdb)
+            except Exception as e:
+                # if we want to keep the db, then no need to do any of the below,
+                # just return and skip it all.
+                if keepdb:
+                    return test_database_name
+
+                self.log('Got an error creating the test database: %s' % e)
+                if not autoclobber:
+                    confirm = input(
+                        "Type 'yes' if you would like to try deleting the test "
+                        "database '%s', or 'no' to cancel: " % test_database_name)
+                if autoclobber or confirm == 'yes':
+                    try:
+                        if verbosity >= 1:
+                            self.log('Destroying old test database for alias %s...' % (
+                                self._get_database_display_str(verbosity, test_database_name),
+                            ))
+                        cursor.execute('DROP DATABASE %(dbname)s' % test_db_params)
+                        self._execute_create_test_db(cursor, test_db_params, keepdb)
+                    except Exception as e:
+                        self.log('Got an error recreating the test database: %s' % e)
+                        sys.exit(2)
+                else:
+                    self.log('Tests cancelled.')
+                    sys.exit(1)
+
+        return test_database_name
+
+    def clone_test_db(self, suffix, verbosity=1, autoclobber=False, keepdb=False):
+        """
+        Clone a test database.
+        """
+        source_database_name = self.connection.settings_dict['NAME']
+
+        if verbosity >= 1:
+            action = 'Cloning test database'
+            if keepdb:
+                action = 'Using existing clone'
+            self.log('%s for alias %s...' % (
+                action,
+                self._get_database_display_str(verbosity, source_database_name),
+            ))
+
+        # We could skip this call if keepdb is True, but we instead
+        # give it the keepdb param. See create_test_db for details.
+        self._clone_test_db(suffix, verbosity, keepdb)
+
+    def get_test_db_clone_settings(self, suffix):
+        """
+        Return a modified connection settings dict for the n-th clone of a DB.
+        """
+        # When this function is called, the test database has been created
+        # already and its name has been copied to settings_dict['NAME'] so
+        # we don't need to call _get_test_db_name.
+        orig_settings_dict = self.connection.settings_dict
+        return {**orig_settings_dict, 'NAME': '{}_{}'.format(orig_settings_dict['NAME'], suffix)}
+
+    def _clone_test_db(self, suffix, verbosity, keepdb=False):
+        """
+        Internal implementation - duplicate the test db tables.
+        """
+        raise NotImplementedError(
+            "The database backend doesn't support cloning databases. "
+            "Disable the option to run tests in parallel processes.")
+
+    def destroy_test_db(self, old_database_name=None, verbosity=1, keepdb=False, suffix=None):
+        """
+        Destroy a test database, prompting the user for confirmation if the
+        database already exists.
+        """
+        self.connection.close()
+        if suffix is None:
+            test_database_name = self.connection.settings_dict['NAME']
+        else:
+            test_database_name = self.get_test_db_clone_settings(suffix)['NAME']
+
+        if verbosity >= 1:
+            action = 'Destroying'
+            if keepdb:
+                action = 'Preserving'
+            self.log('%s test database for alias %s...' % (
+                action,
+                self._get_database_display_str(verbosity, test_database_name),
+            ))
+
+        # if we want to preserve the database
+        # skip the actual destroying piece.
+        if not keepdb:
+            self._destroy_test_db(test_database_name, verbosity)
+
+        # Restore the original database name
+        if old_database_name is not None:
+            settings.DATABASES[self.connection.alias]["NAME"] = old_database_name
+            self.connection.settings_dict["NAME"] = old_database_name
+
+    def _destroy_test_db(self, test_database_name, verbosity):
+        """
+        Internal implementation - remove the test db tables.
+        """
+        # Remove the test database to clean up after
+        # ourselves. Connect to the previous database (not the test database)
+        # to do so, because it's not allowed to delete a database while being
+        # connected to it.
+        with self._nodb_cursor() as cursor:
+            cursor.execute("DROP DATABASE %s"
+                           % self.connection.ops.quote_name(test_database_name))
+
+    def sql_table_creation_suffix(self):
+        """
+        SQL to append to the end of the test table creation statements.
+        """
+        return ''
+
+    def test_db_signature(self):
+        """
+        Return a tuple with elements of self.connection.settings_dict (a
+        DATABASES setting value) that uniquely identify a database
+        accordingly to the RDBMS particularities.
+        """
+        settings_dict = self.connection.settings_dict
+        return (
+            settings_dict['HOST'],
+            settings_dict['PORT'],
+            settings_dict['ENGINE'],
+            self._get_test_db_name(),
+        )

Lib/site-packages/django/db/backends/base/features.py

@@ -0,0 +1,346 @@
+from django.db import ProgrammingError
+from django.utils.functional import cached_property
+
+
+class BaseDatabaseFeatures:
+    gis_enabled = False
+    allows_group_by_pk = False
+    allows_group_by_selected_pks = False
+    empty_fetchmany_value = []
+    update_can_self_select = True
+
+    # Does the backend distinguish between '' and None?
+    interprets_empty_strings_as_nulls = False
+
+    # Does the backend allow inserting duplicate NULL rows in a nullable
+    # unique field? All core backends implement this correctly, but other
+    # databases such as SQL Server do not.
+    supports_nullable_unique_constraints = True
+
+    # Does the backend allow inserting duplicate rows when a unique_together
+    # constraint exists and some fields are nullable but not all of them?
+    supports_partially_nullable_unique_constraints = True
+    # Does the backend support initially deferrable unique constraints?
+    supports_deferrable_unique_constraints = False
+
+    can_use_chunked_reads = True
+    can_return_columns_from_insert = False
+    can_return_rows_from_bulk_insert = False
+    has_bulk_insert = True
+    uses_savepoints = True
+    can_release_savepoints = False
+
+    # If True, don't use integer foreign keys referring to, e.g., positive
+    # integer primary keys.
+    related_fields_match_type = False
+    allow_sliced_subqueries_with_in = True
+    has_select_for_update = False
+    has_select_for_update_nowait = False
+    has_select_for_update_skip_locked = False
+    has_select_for_update_of = False
+    # Does the database's SELECT FOR UPDATE OF syntax require a column rather
+    # than a table?
+    select_for_update_of_column = False
+
+    # Does the default test database allow multiple connections?
+    # Usually an indication that the test database is in-memory
+    test_db_allows_multiple_connections = True
+
+    # Can an object be saved without an explicit primary key?
+    supports_unspecified_pk = False
+
+    # Can a fixture contain forward references? i.e., are
+    # FK constraints checked at the end of transaction, or
+    # at the end of each save operation?
+    supports_forward_references = True
+
+    # Does the backend truncate names properly when they are too long?
+    truncates_names = False
+
+    # Is there a REAL datatype in addition to floats/doubles?
+    has_real_datatype = False
+    supports_subqueries_in_group_by = True
+
+    # Is there a true datatype for uuid?
+    has_native_uuid_field = False
+
+    # Is there a true datatype for timedeltas?
+    has_native_duration_field = False
+
+    # Does the database driver supports same type temporal data subtraction
+    # by returning the type used to store duration field?
+    supports_temporal_subtraction = False
+
+    # Does the __regex lookup support backreferencing and grouping?
+    supports_regex_backreferencing = True
+
+    # Can date/datetime lookups be performed using a string?
+    supports_date_lookup_using_string = True
+
+    # Can datetimes with timezones be used?
+    supports_timezones = True
+
+    # Does the database have a copy of the zoneinfo database?
+    has_zoneinfo_database = True
+
+    # When performing a GROUP BY, is an ORDER BY NULL required
+    # to remove any ordering?
+    requires_explicit_null_ordering_when_grouping = False
+
+    # Does the backend order NULL values as largest or smallest?
+    nulls_order_largest = False
+
+    # Does the backend support NULLS FIRST and NULLS LAST in ORDER BY?
+    supports_order_by_nulls_modifier = True
+
+    # Does the backend orders NULLS FIRST by default?
+    order_by_nulls_first = False
+
+    # The database's limit on the number of query parameters.
+    max_query_params = None
+
+    # Can an object have an autoincrement primary key of 0? MySQL says No.
+    allows_auto_pk_0 = True
+
+    # Do we need to NULL a ForeignKey out, or can the constraint check be
+    # deferred
+    can_defer_constraint_checks = False
+
+    # date_interval_sql can properly handle mixed Date/DateTime fields and timedeltas
+    supports_mixed_date_datetime_comparisons = True
+
+    # Does the backend support tablespaces? Default to False because it isn't
+    # in the SQL standard.
+    supports_tablespaces = False
+
+    # Does the backend reset sequences between tests?
+    supports_sequence_reset = True
+
+    # Can the backend introspect the default value of a column?
+    can_introspect_default = True
+
+    # Confirm support for introspected foreign keys
+    # Every database can do this reliably, except MySQL,
+    # which can't do it for MyISAM tables
+    can_introspect_foreign_keys = True
+
+    # Can the backend introspect an AutoField, instead of an IntegerField?
+    can_introspect_autofield = False
+
+    # Can the backend introspect a BigIntegerField, instead of an IntegerField?
+    can_introspect_big_integer_field = True
+
+    # Can the backend introspect an BinaryField, instead of an TextField?
+    can_introspect_binary_field = True
+
+    # Can the backend introspect an DecimalField, instead of an FloatField?
+    can_introspect_decimal_field = True
+
+    # Can the backend introspect a DurationField, instead of a BigIntegerField?
+    can_introspect_duration_field = True
+
+    # Can the backend introspect an IPAddressField, instead of an CharField?
+    can_introspect_ip_address_field = False
+
+    # Can the backend introspect a PositiveIntegerField, instead of an IntegerField?
+    can_introspect_positive_integer_field = False
+
+    # Can the backend introspect a SmallIntegerField, instead of an IntegerField?
+    can_introspect_small_integer_field = False
+
+    # Can the backend introspect a TimeField, instead of a DateTimeField?
+    can_introspect_time_field = True
+
+    # Some backends may not be able to differentiate BigAutoField or
+    # SmallAutoField from other fields such as AutoField.
+    introspected_big_auto_field_type = 'BigAutoField'
+    introspected_small_auto_field_type = 'SmallAutoField'
+
+    # Some backends may not be able to differentiate BooleanField from other
+    # fields such as IntegerField.
+    introspected_boolean_field_type = 'BooleanField'
+
+    # Can the backend introspect the column order (ASC/DESC) for indexes?
+    supports_index_column_ordering = True
+
+    # Does the backend support introspection of materialized views?
+    can_introspect_materialized_views = False
+
+    # Support for the DISTINCT ON clause
+    can_distinct_on_fields = False
+
+    # Does the backend prevent running SQL queries in broken transactions?
+    atomic_transactions = True
+
+    # Can we roll back DDL in a transaction?
+    can_rollback_ddl = False
+
+    # Does it support operations requiring references rename in a transaction?
+    supports_atomic_references_rename = True
+
+    # Can we issue more than one ALTER COLUMN clause in an ALTER TABLE?
+    supports_combined_alters = False
+
+    # Does it support foreign keys?
+    supports_foreign_keys = True
+
+    # Can it create foreign key constraints inline when adding columns?
+    can_create_inline_fk = True
+
+    # Does it support CHECK constraints?
+    supports_column_check_constraints = True
+    supports_table_check_constraints = True
+    # Does the backend support introspection of CHECK constraints?
+    can_introspect_check_constraints = True
+
+    # Does the backend support 'pyformat' style ("... %(name)s ...", {'name': value})
+    # parameter passing? Note this can be provided by the backend even if not
+    # supported by the Python driver
+    supports_paramstyle_pyformat = True
+
+    # Does the backend require literal defaults, rather than parameterized ones?
+    requires_literal_defaults = False
+
+    # Does the backend require a connection reset after each material schema change?
+    connection_persists_old_columns = False
+
+    # What kind of error does the backend throw when accessing closed cursor?
+    closed_cursor_error_class = ProgrammingError
+
+    # Does 'a' LIKE 'A' match?
+    has_case_insensitive_like = True
+
+    # Suffix for backends that don't support "SELECT xxx;" queries.
+    bare_select_suffix = ''
+
+    # If NULL is implied on columns without needing to be explicitly specified
+    implied_column_null = False
+
+    # Does the backend support "select for update" queries with limit (and offset)?
+    supports_select_for_update_with_limit = True
+
+    # Does the backend ignore null expressions in GREATEST and LEAST queries unless
+    # every expression is null?
+    greatest_least_ignores_nulls = False
+
+    # Can the backend clone databases for parallel test execution?
+    # Defaults to False to allow third-party backends to opt-in.
+    can_clone_databases = False
+
+    # Does the backend consider table names with different casing to
+    # be equal?
+    ignores_table_name_case = False
+
+    # Place FOR UPDATE right after FROM clause. Used on MSSQL.
+    for_update_after_from = False
+
+    # Combinatorial flags
+    supports_select_union = True
+    supports_select_intersection = True
+    supports_select_difference = True
+    supports_slicing_ordering_in_compound = False
+    supports_parentheses_in_compound = True
+
+    # Does the database support SQL 2003 FILTER (WHERE ...) in aggregate
+    # expressions?
+    supports_aggregate_filter_clause = False
+
+    # Does the backend support indexing a TextField?
+    supports_index_on_text_field = True
+
+    # Does the backend support window expressions (expression OVER (...))?
+    supports_over_clause = False
+    supports_frame_range_fixed_distance = False
+    only_supports_unbounded_with_preceding_and_following = False
+
+    # Does the backend support CAST with precision?
+    supports_cast_with_precision = True
+
+    # How many second decimals does the database return when casting a value to
+    # a type with time?
+    time_cast_precision = 6
+
+    # SQL to create a procedure for use by the Django test suite. The
+    # functionality of the procedure isn't important.
+    create_test_procedure_without_params_sql = None
+    create_test_procedure_with_int_param_sql = None
+
+    # Does the backend support keyword parameters for cursor.callproc()?
+    supports_callproc_kwargs = False
+
+    # Convert CharField results from bytes to str in database functions.
+    db_functions_convert_bytes_to_str = False
+
+    # What formats does the backend EXPLAIN syntax support?
+    supported_explain_formats = set()
+
+    # Does DatabaseOperations.explain_query_prefix() raise ValueError if
+    # unknown kwargs are passed to QuerySet.explain()?
+    validates_explain_options = True
+
+    # Does the backend support the default parameter in lead() and lag()?
+    supports_default_in_lead_lag = True
+
+    # Does the backend support ignoring constraint or uniqueness errors during
+    # INSERT?
+    supports_ignore_conflicts = True
+
+    # Does this backend require casting the results of CASE expressions used
+    # in UPDATE statements to ensure the expression has the correct type?
+    requires_casted_case_in_updates = False
+
+    # Does the backend support partial indexes (CREATE INDEX ... WHERE ...)?
+    supports_partial_indexes = True
+    supports_functions_in_partial_indexes = True
+
+    # Does the database allow more than one constraint or index on the same
+    # field(s)?
+    allows_multiple_constraints_on_same_fields = True
+
+    # Does the backend support boolean expressions in SELECT and GROUP BY
+    # clauses?
+    supports_boolean_expr_in_select_clause = True
+
+    # Does the backend support JSONField?
+    supports_json_field = True
+    # Can the backend introspect a JSONField?
+    can_introspect_json_field = True
+    # Does the backend support primitives in JSONField?
+    supports_primitives_in_json_field = True
+    # Is there a true datatype for JSON?
+    has_native_json_field = False
+    # Does the backend use PostgreSQL-style JSON operators like '->'?
+    has_json_operators = False
+    # Does the backend support __contains and __contained_by lookups for
+    # a JSONField?
+    supports_json_field_contains = True
+    # Does value__d__contains={'f': 'g'} (without a list around the dict) match
+    # {'d': [{'f': 'g'}]}?
+    json_key_contains_list_matching_requires_list = False
+
+    def __init__(self, connection):
+        self.connection = connection
+
+    @cached_property
+    def supports_explaining_query_execution(self):
+        """Does this backend support explaining query execution?"""
+        return self.connection.ops.explain_prefix is not None
+
+    @cached_property
+    def supports_transactions(self):
+        """Confirm support for transactions."""
+        with self.connection.cursor() as cursor:
+            cursor.execute('CREATE TABLE ROLLBACK_TEST (X INT)')
+            self.connection.set_autocommit(False)
+            cursor.execute('INSERT INTO ROLLBACK_TEST (X) VALUES (8)')
+            self.connection.rollback()
+            self.connection.set_autocommit(True)
+            cursor.execute('SELECT COUNT(X) FROM ROLLBACK_TEST')
+            count, = cursor.fetchone()
+            cursor.execute('DROP TABLE ROLLBACK_TEST')
+        return count == 0
+
+    def allows_group_by_selected_pks_on_model(self, model):
+        if not self.allows_group_by_selected_pks:
+            return False
+        return model._meta.managed

Lib/site-packages/django/db/backends/base/introspection.py

@@ -0,0 +1,190 @@
+from collections import namedtuple
+
+# Structure returned by DatabaseIntrospection.get_table_list()
+TableInfo = namedtuple('TableInfo', ['name', 'type'])
+
+# Structure returned by the DB-API cursor.description interface (PEP 249)
+FieldInfo = namedtuple('FieldInfo', 'name type_code display_size internal_size precision scale null_ok default')
+
+
+class BaseDatabaseIntrospection:
+    """Encapsulate backend-specific introspection utilities."""
+    data_types_reverse = {}
+
+    def __init__(self, connection):
+        self.connection = connection
+
+    def get_field_type(self, data_type, description):
+        """
+        Hook for a database backend to use the cursor description to
+        match a Django field type to a database column.
+
+        For Oracle, the column data_type on its own is insufficient to
+        distinguish between a FloatField and IntegerField, for example.
+        """
+        return self.data_types_reverse[data_type]
+
+    def identifier_converter(self, name):
+        """
+        Apply a conversion to the identifier for the purposes of comparison.
+
+        The default identifier converter is for case sensitive comparison.
+        """
+        return name
+
+    def table_names(self, cursor=None, include_views=False):
+        """
+        Return a list of names of all tables that exist in the database.
+        Sort the returned table list by Python's default sorting. Do NOT use
+        the database's ORDER BY here to avoid subtle differences in sorting
+        order between databases.
+        """
+        def get_names(cursor):
+            return sorted(ti.name for ti in self.get_table_list(cursor)
+                          if include_views or ti.type == 't')
+        if cursor is None:
+            with self.connection.cursor() as cursor:
+                return get_names(cursor)
+        return get_names(cursor)
+
+    def get_table_list(self, cursor):
+        """
+        Return an unsorted list of TableInfo named tuples of all tables and
+        views that exist in the database.
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseIntrospection may require a get_table_list() method')
+
+    def get_table_description(self, cursor, table_name):
+        """
+        Return a description of the table with the DB-API cursor.description
+        interface.
+        """
+        raise NotImplementedError(
+            'subclasses of BaseDatabaseIntrospection may require a '
+            'get_table_description() method.'
+        )
+
+    def get_migratable_models(self):
+        from django.apps import apps
+        from django.db import router
+        return (
+            model
+            for app_config in apps.get_app_configs()
+            for model in router.get_migratable_models(app_config, self.connection.alias)
+            if model._meta.can_migrate(self.connection)
+        )
+
+    def django_table_names(self, only_existing=False, include_views=True):
+        """
+        Return a list of all table names that have associated Django models and
+        are in INSTALLED_APPS.
+
+        If only_existing is True, include only the tables in the database.
+        """
+        tables = set()
+        for model in self.get_migratable_models():
+            if not model._meta.managed:
+                continue
+            tables.add(model._meta.db_table)
+            tables.update(
+                f.m2m_db_table() for f in model._meta.local_many_to_many
+                if f.remote_field.through._meta.managed
+            )
+        tables = list(tables)
+        if only_existing:
+            existing_tables = set(self.table_names(include_views=include_views))
+            tables = [
+                t
+                for t in tables
+                if self.identifier_converter(t) in existing_tables
+            ]
+        return tables
+
+    def installed_models(self, tables):
+        """
+        Return a set of all models represented by the provided list of table
+        names.
+        """
+        tables = set(map(self.identifier_converter, tables))
+        return {
+            m for m in self.get_migratable_models()
+            if self.identifier_converter(m._meta.db_table) in tables
+        }
+
+    def sequence_list(self):
+        """
+        Return a list of information about all DB sequences for all models in
+        all apps.
+        """
+        sequence_list = []
+        with self.connection.cursor() as cursor:
+            for model in self.get_migratable_models():
+                if not model._meta.managed:
+                    continue
+                if model._meta.swapped:
+                    continue
+                sequence_list.extend(self.get_sequences(cursor, model._meta.db_table, model._meta.local_fields))
+                for f in model._meta.local_many_to_many:
+                    # If this is an m2m using an intermediate table,
+                    # we don't need to reset the sequence.
+                    if f.remote_field.through._meta.auto_created:
+                        sequence = self.get_sequences(cursor, f.m2m_db_table())
+                        sequence_list.extend(sequence or [{'table': f.m2m_db_table(), 'column': None}])
+        return sequence_list
+
+    def get_sequences(self, cursor, table_name, table_fields=()):
+        """
+        Return a list of introspected sequences for table_name. Each sequence
+        is a dict: {'table': <table_name>, 'column': <column_name>}. An optional
+        'name' key can be added if the backend supports named sequences.
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseIntrospection may require a get_sequences() method')
+
+    def get_relations(self, cursor, table_name):
+        """
+        Return a dictionary of
+        {field_name: (field_name_other_table, other_table)} representing all
+        relationships to the given table.
+        """
+        raise NotImplementedError(
+            'subclasses of BaseDatabaseIntrospection may require a '
+            'get_relations() method.'
+        )
+
+    def get_key_columns(self, cursor, table_name):
+        """
+        Backends can override this to return a list of:
+            (column_name, referenced_table_name, referenced_column_name)
+        for all key columns in given table.
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseIntrospection may require a get_key_columns() method')
+
+    def get_primary_key_column(self, cursor, table_name):
+        """
+        Return the name of the primary key column for the given table.
+        """
+        for constraint in self.get_constraints(cursor, table_name).values():
+            if constraint['primary_key']:
+                return constraint['columns'][0]
+        return None
+
+    def get_constraints(self, cursor, table_name):
+        """
+        Retrieve any constraints or keys (unique, pk, fk, check, index)
+        across one or more columns.
+
+        Return a dict mapping constraint names to their attributes,
+        where attributes is a dict with keys:
+         * columns: List of columns this covers
+         * primary_key: True if primary key, False otherwise
+         * unique: True if this is a unique constraint, False otherwise
+         * foreign_key: (table, column) of target, or None
+         * check: True if check constraint, False otherwise
+         * index: True if index, False otherwise.
+         * orders: The order (ASC/DESC) defined for the columns of indexes
+         * type: The type of the index (btree, hash, etc.)
+
+        Some backends may return special constraint names that don't exist
+        if they don't name constraints of a certain type (e.g. SQLite)
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseIntrospection may require a get_constraints() method')

Lib/site-packages/django/db/backends/base/operations.py

@@ -0,0 +1,696 @@
+import datetime
+import decimal
+from importlib import import_module
+
+import sqlparse
+
+from django.conf import settings
+from django.db import NotSupportedError, transaction
+from django.db.backends import utils
+from django.utils import timezone
+from django.utils.encoding import force_str
+
+
+class BaseDatabaseOperations:
+    """
+    Encapsulate backend-specific differences, such as the way a backend
+    performs ordering or calculates the ID of a recently-inserted row.
+    """
+    compiler_module = "django.db.models.sql.compiler"
+
+    # Integer field safe ranges by `internal_type` as documented
+    # in docs/ref/models/fields.txt.
+    integer_field_ranges = {
+        'SmallIntegerField': (-32768, 32767),
+        'IntegerField': (-2147483648, 2147483647),
+        'BigIntegerField': (-9223372036854775808, 9223372036854775807),
+        'PositiveBigIntegerField': (0, 9223372036854775807),
+        'PositiveSmallIntegerField': (0, 32767),
+        'PositiveIntegerField': (0, 2147483647),
+        'SmallAutoField': (-32768, 32767),
+        'AutoField': (-2147483648, 2147483647),
+        'BigAutoField': (-9223372036854775808, 9223372036854775807),
+    }
+    set_operators = {
+        'union': 'UNION',
+        'intersection': 'INTERSECT',
+        'difference': 'EXCEPT',
+    }
+    # Mapping of Field.get_internal_type() (typically the model field's class
+    # name) to the data type to use for the Cast() function, if different from
+    # DatabaseWrapper.data_types.
+    cast_data_types = {}
+    # CharField data type if the max_length argument isn't provided.
+    cast_char_field_without_max_length = None
+
+    # Start and end points for window expressions.
+    PRECEDING = 'PRECEDING'
+    FOLLOWING = 'FOLLOWING'
+    UNBOUNDED_PRECEDING = 'UNBOUNDED ' + PRECEDING
+    UNBOUNDED_FOLLOWING = 'UNBOUNDED ' + FOLLOWING
+    CURRENT_ROW = 'CURRENT ROW'
+
+    # Prefix for EXPLAIN queries, or None EXPLAIN isn't supported.
+    explain_prefix = None
+
+    def __init__(self, connection):
+        self.connection = connection
+        self._cache = None
+
+    def autoinc_sql(self, table, column):
+        """
+        Return any SQL needed to support auto-incrementing primary keys, or
+        None if no SQL is necessary.
+
+        This SQL is executed when a table is created.
+        """
+        return None
+
+    def bulk_batch_size(self, fields, objs):
+        """
+        Return the maximum allowed batch size for the backend. The fields
+        are the fields going to be inserted in the batch, the objs contains
+        all the objects to be inserted.
+        """
+        return len(objs)
+
+    def cache_key_culling_sql(self):
+        """
+        Return an SQL query that retrieves the first cache key greater than the
+        n smallest.
+
+        This is used by the 'db' cache backend to determine where to start
+        culling.
+        """
+        return "SELECT cache_key FROM %s ORDER BY cache_key LIMIT 1 OFFSET %%s"
+
+    def unification_cast_sql(self, output_field):
+        """
+        Given a field instance, return the SQL that casts the result of a union
+        to that type. The resulting string should contain a '%s' placeholder
+        for the expression being cast.
+        """
+        return '%s'
+
+    def date_extract_sql(self, lookup_type, field_name):
+        """
+        Given a lookup_type of 'year', 'month', or 'day', return the SQL that
+        extracts a value from the given date field field_name.
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseOperations may require a date_extract_sql() method')
+
+    def date_interval_sql(self, timedelta):
+        """
+        Implement the date interval functionality for expressions.
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseOperations may require a date_interval_sql() method')
+
+    def date_trunc_sql(self, lookup_type, field_name):
+        """
+        Given a lookup_type of 'year', 'month', or 'day', return the SQL that
+        truncates the given date field field_name to a date object with only
+        the given specificity.
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseOperations may require a date_trunc_sql() method.')
+
+    def datetime_cast_date_sql(self, field_name, tzname):
+        """
+        Return the SQL to cast a datetime value to date value.
+        """
+        raise NotImplementedError(
+            'subclasses of BaseDatabaseOperations may require a '
+            'datetime_cast_date_sql() method.'
+        )
+
+    def datetime_cast_time_sql(self, field_name, tzname):
+        """
+        Return the SQL to cast a datetime value to time value.
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseOperations may require a datetime_cast_time_sql() method')
+
+    def datetime_extract_sql(self, lookup_type, field_name, tzname):
+        """
+        Given a lookup_type of 'year', 'month', 'day', 'hour', 'minute', or
+        'second', return the SQL that extracts a value from the given
+        datetime field field_name.
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseOperations may require a datetime_extract_sql() method')
+
+    def datetime_trunc_sql(self, lookup_type, field_name, tzname):
+        """
+        Given a lookup_type of 'year', 'month', 'day', 'hour', 'minute', or
+        'second', return the SQL that truncates the given datetime field
+        field_name to a datetime object with only the given specificity.
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseOperations may require a datetime_trunc_sql() method')
+
+    def time_trunc_sql(self, lookup_type, field_name):
+        """
+        Given a lookup_type of 'hour', 'minute' or 'second', return the SQL
+        that truncates the given time field field_name to a time object with
+        only the given specificity.
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseOperations may require a time_trunc_sql() method')
+
+    def time_extract_sql(self, lookup_type, field_name):
+        """
+        Given a lookup_type of 'hour', 'minute', or 'second', return the SQL
+        that extracts a value from the given time field field_name.
+        """
+        return self.date_extract_sql(lookup_type, field_name)
+
+    def deferrable_sql(self):
+        """
+        Return the SQL to make a constraint "initially deferred" during a
+        CREATE TABLE statement.
+        """
+        return ''
+
+    def distinct_sql(self, fields, params):
+        """
+        Return an SQL DISTINCT clause which removes duplicate rows from the
+        result set. If any fields are given, only check the given fields for
+        duplicates.
+        """
+        if fields:
+            raise NotSupportedError('DISTINCT ON fields is not supported by this database backend')
+        else:
+            return ['DISTINCT'], []
+
+    def fetch_returned_insert_columns(self, cursor, returning_params):
+        """
+        Given a cursor object that has just performed an INSERT...RETURNING
+        statement into a table, return the newly created data.
+        """
+        return cursor.fetchone()
+
+    def field_cast_sql(self, db_type, internal_type):
+        """
+        Given a column type (e.g. 'BLOB', 'VARCHAR') and an internal type
+        (e.g. 'GenericIPAddressField'), return the SQL to cast it before using
+        it in a WHERE statement. The resulting string should contain a '%s'
+        placeholder for the column being searched against.
+        """
+        return '%s'
+
+    def force_no_ordering(self):
+        """
+        Return a list used in the "ORDER BY" clause to force no ordering at
+        all. Return an empty list to include nothing in the ordering.
+        """
+        return []
+
+    def for_update_sql(self, nowait=False, skip_locked=False, of=()):
+        """
+        Return the FOR UPDATE SQL clause to lock rows for an update operation.
+        """
+        return 'FOR UPDATE%s%s%s' % (
+            ' OF %s' % ', '.join(of) if of else '',
+            ' NOWAIT' if nowait else '',
+            ' SKIP LOCKED' if skip_locked else '',
+        )
+
+    def _get_limit_offset_params(self, low_mark, high_mark):
+        offset = low_mark or 0
+        if high_mark is not None:
+            return (high_mark - offset), offset
+        elif offset:
+            return self.connection.ops.no_limit_value(), offset
+        return None, offset
+
+    def limit_offset_sql(self, low_mark, high_mark):
+        """Return LIMIT/OFFSET SQL clause."""
+        limit, offset = self._get_limit_offset_params(low_mark, high_mark)
+        return ' '.join(sql for sql in (
+            ('LIMIT %d' % limit) if limit else None,
+            ('OFFSET %d' % offset) if offset else None,
+        ) if sql)
+
+    def last_executed_query(self, cursor, sql, params):
+        """
+        Return a string of the query last executed by the given cursor, with
+        placeholders replaced with actual values.
+
+        `sql` is the raw query containing placeholders and `params` is the
+        sequence of parameters. These are used by default, but this method
+        exists for database backends to provide a better implementation
+        according to their own quoting schemes.
+        """
+        # Convert params to contain string values.
+        def to_string(s):
+            return force_str(s, strings_only=True, errors='replace')
+        if isinstance(params, (list, tuple)):
+            u_params = tuple(to_string(val) for val in params)
+        elif params is None:
+            u_params = ()
+        else:
+            u_params = {to_string(k): to_string(v) for k, v in params.items()}
+
+        return "QUERY = %r - PARAMS = %r" % (sql, u_params)
+
+    def last_insert_id(self, cursor, table_name, pk_name):
+        """
+        Given a cursor object that has just performed an INSERT statement into
+        a table that has an auto-incrementing ID, return the newly created ID.
+
+        `pk_name` is the name of the primary-key column.
+        """
+        return cursor.lastrowid
+
+    def lookup_cast(self, lookup_type, internal_type=None):
+        """
+        Return the string to use in a query when performing lookups
+        ("contains", "like", etc.). It should contain a '%s' placeholder for
+        the column being searched against.
+        """
+        return "%s"
+
+    def max_in_list_size(self):
+        """
+        Return the maximum number of items that can be passed in a single 'IN'
+        list condition, or None if the backend does not impose a limit.
+        """
+        return None
+
+    def max_name_length(self):
+        """
+        Return the maximum length of table and column names, or None if there
+        is no limit.
+        """
+        return None
+
+    def no_limit_value(self):
+        """
+        Return the value to use for the LIMIT when we are wanting "LIMIT
+        infinity". Return None if the limit clause can be omitted in this case.
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseOperations may require a no_limit_value() method')
+
+    def pk_default_value(self):
+        """
+        Return the value to use during an INSERT statement to specify that
+        the field should use its default value.
+        """
+        return 'DEFAULT'
+
+    def prepare_sql_script(self, sql):
+        """
+        Take an SQL script that may contain multiple lines and return a list
+        of statements to feed to successive cursor.execute() calls.
+
+        Since few databases are able to process raw SQL scripts in a single
+        cursor.execute() call and PEP 249 doesn't talk about this use case,
+        the default implementation is conservative.
+        """
+        return [
+            sqlparse.format(statement, strip_comments=True)
+            for statement in sqlparse.split(sql) if statement
+        ]
+
+    def process_clob(self, value):
+        """
+        Return the value of a CLOB column, for backends that return a locator
+        object that requires additional processing.
+        """
+        return value
+
+    def return_insert_columns(self, fields):
+        """
+        For backends that support returning columns as part of an insert query,
+        return the SQL and params to append to the INSERT query. The returned
+        fragment should contain a format string to hold the appropriate column.
+        """
+        pass
+
+    def compiler(self, compiler_name):
+        """
+        Return the SQLCompiler class corresponding to the given name,
+        in the namespace corresponding to the `compiler_module` attribute
+        on this backend.
+        """
+        if self._cache is None:
+            self._cache = import_module(self.compiler_module)
+        return getattr(self._cache, compiler_name)
+
+    def quote_name(self, name):
+        """
+        Return a quoted version of the given table, index, or column name. Do
+        not quote the given name if it's already been quoted.
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseOperations may require a quote_name() method')
+
+    def random_function_sql(self):
+        """Return an SQL expression that returns a random value."""
+        return 'RANDOM()'
+
+    def regex_lookup(self, lookup_type):
+        """
+        Return the string to use in a query when performing regular expression
+        lookups (using "regex" or "iregex"). It should contain a '%s'
+        placeholder for the column being searched against.
+
+        If the feature is not supported (or part of it is not supported), raise
+        NotImplementedError.
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseOperations may require a regex_lookup() method')
+
+    def savepoint_create_sql(self, sid):
+        """
+        Return the SQL for starting a new savepoint. Only required if the
+        "uses_savepoints" feature is True. The "sid" parameter is a string
+        for the savepoint id.
+        """
+        return "SAVEPOINT %s" % self.quote_name(sid)
+
+    def savepoint_commit_sql(self, sid):
+        """
+        Return the SQL for committing the given savepoint.
+        """
+        return "RELEASE SAVEPOINT %s" % self.quote_name(sid)
+
+    def savepoint_rollback_sql(self, sid):
+        """
+        Return the SQL for rolling back the given savepoint.
+        """
+        return "ROLLBACK TO SAVEPOINT %s" % self.quote_name(sid)
+
+    def set_time_zone_sql(self):
+        """
+        Return the SQL that will set the connection's time zone.
+
+        Return '' if the backend doesn't support time zones.
+        """
+        return ''
+
+    def sql_flush(self, style, tables, *, reset_sequences=False, allow_cascade=False):
+        """
+        Return a list of SQL statements required to remove all data from
+        the given database tables (without actually removing the tables
+        themselves).
+
+        The `style` argument is a Style object as returned by either
+        color_style() or no_style() in django.core.management.color.
+
+        If `reset_sequences` is True, the list includes SQL statements required
+        to reset the sequences.
+
+        The `allow_cascade` argument determines whether truncation may cascade
+        to tables with foreign keys pointing the tables being truncated.
+        PostgreSQL requires a cascade even if these tables are empty.
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseOperations must provide an sql_flush() method')
+
+    def execute_sql_flush(self, sql_list):
+        """Execute a list of SQL statements to flush the database."""
+        with transaction.atomic(
+            using=self.connection.alias,
+            savepoint=self.connection.features.can_rollback_ddl,
+        ):
+            with self.connection.cursor() as cursor:
+                for sql in sql_list:
+                    cursor.execute(sql)
+
+    def sequence_reset_by_name_sql(self, style, sequences):
+        """
+        Return a list of the SQL statements required to reset sequences
+        passed in `sequences`.
+
+        The `style` argument is a Style object as returned by either
+        color_style() or no_style() in django.core.management.color.
+        """
+        return []
+
+    def sequence_reset_sql(self, style, model_list):
+        """
+        Return a list of the SQL statements required to reset sequences for
+        the given models.
+
+        The `style` argument is a Style object as returned by either
+        color_style() or no_style() in django.core.management.color.
+        """
+        return []  # No sequence reset required by default.
+
+    def start_transaction_sql(self):
+        """Return the SQL statement required to start a transaction."""
+        return "BEGIN;"
+
+    def end_transaction_sql(self, success=True):
+        """Return the SQL statement required to end a transaction."""
+        if not success:
+            return "ROLLBACK;"
+        return "COMMIT;"
+
+    def tablespace_sql(self, tablespace, inline=False):
+        """
+        Return the SQL that will be used in a query to define the tablespace.
+
+        Return '' if the backend doesn't support tablespaces.
+
+        If `inline` is True, append the SQL to a row; otherwise append it to
+        the entire CREATE TABLE or CREATE INDEX statement.
+        """
+        return ''
+
+    def prep_for_like_query(self, x):
+        """Prepare a value for use in a LIKE query."""
+        return str(x).replace("\\", "\\\\").replace("%", r"\%").replace("_", r"\_")
+
+    # Same as prep_for_like_query(), but called for "iexact" matches, which
+    # need not necessarily be implemented using "LIKE" in the backend.
+    prep_for_iexact_query = prep_for_like_query
+
+    def validate_autopk_value(self, value):
+        """
+        Certain backends do not accept some values for "serial" fields
+        (for example zero in MySQL). Raise a ValueError if the value is
+        invalid, otherwise return the validated value.
+        """
+        return value
+
+    def adapt_unknown_value(self, value):
+        """
+        Transform a value to something compatible with the backend driver.
+
+        This method only depends on the type of the value. It's designed for
+        cases where the target type isn't known, such as .raw() SQL queries.
+        As a consequence it may not work perfectly in all circumstances.
+        """
+        if isinstance(value, datetime.datetime):   # must be before date
+            return self.adapt_datetimefield_value(value)
+        elif isinstance(value, datetime.date):
+            return self.adapt_datefield_value(value)
+        elif isinstance(value, datetime.time):
+            return self.adapt_timefield_value(value)
+        elif isinstance(value, decimal.Decimal):
+            return self.adapt_decimalfield_value(value)
+        else:
+            return value
+
+    def adapt_datefield_value(self, value):
+        """
+        Transform a date value to an object compatible with what is expected
+        by the backend driver for date columns.
+        """
+        if value is None:
+            return None
+        return str(value)
+
+    def adapt_datetimefield_value(self, value):
+        """
+        Transform a datetime value to an object compatible with what is expected
+        by the backend driver for datetime columns.
+        """
+        if value is None:
+            return None
+        return str(value)
+
+    def adapt_timefield_value(self, value):
+        """
+        Transform a time value to an object compatible with what is expected
+        by the backend driver for time columns.
+        """
+        if value is None:
+            return None
+        if timezone.is_aware(value):
+            raise ValueError("Django does not support timezone-aware times.")
+        return str(value)
+
+    def adapt_decimalfield_value(self, value, max_digits=None, decimal_places=None):
+        """
+        Transform a decimal.Decimal value to an object compatible with what is
+        expected by the backend driver for decimal (numeric) columns.
+        """
+        return utils.format_number(value, max_digits, decimal_places)
+
+    def adapt_ipaddressfield_value(self, value):
+        """
+        Transform a string representation of an IP address into the expected
+        type for the backend driver.
+        """
+        return value or None
+
+    def year_lookup_bounds_for_date_field(self, value):
+        """
+        Return a two-elements list with the lower and upper bound to be used
+        with a BETWEEN operator to query a DateField value using a year
+        lookup.
+
+        `value` is an int, containing the looked-up year.
+        """
+        first = datetime.date(value, 1, 1)
+        second = datetime.date(value, 12, 31)
+        first = self.adapt_datefield_value(first)
+        second = self.adapt_datefield_value(second)
+        return [first, second]
+
+    def year_lookup_bounds_for_datetime_field(self, value):
+        """
+        Return a two-elements list with the lower and upper bound to be used
+        with a BETWEEN operator to query a DateTimeField value using a year
+        lookup.
+
+        `value` is an int, containing the looked-up year.
+        """
+        first = datetime.datetime(value, 1, 1)
+        second = datetime.datetime(value, 12, 31, 23, 59, 59, 999999)
+        if settings.USE_TZ:
+            tz = timezone.get_current_timezone()
+            first = timezone.make_aware(first, tz)
+            second = timezone.make_aware(second, tz)
+        first = self.adapt_datetimefield_value(first)
+        second = self.adapt_datetimefield_value(second)
+        return [first, second]
+
+    def get_db_converters(self, expression):
+        """
+        Return a list of functions needed to convert field data.
+
+        Some field types on some backends do not provide data in the correct
+        format, this is the hook for converter functions.
+        """
+        return []
+
+    def convert_durationfield_value(self, value, expression, connection):
+        if value is not None:
+            return datetime.timedelta(0, 0, value)
+
+    def check_expression_support(self, expression):
+        """
+        Check that the backend supports the provided expression.
+
+        This is used on specific backends to rule out known expressions
+        that have problematic or nonexistent implementations. If the
+        expression has a known problem, the backend should raise
+        NotSupportedError.
+        """
+        pass
+
+    def conditional_expression_supported_in_where_clause(self, expression):
+        """
+        Return True, if the conditional expression is supported in the WHERE
+        clause.
+        """
+        return True
+
+    def combine_expression(self, connector, sub_expressions):
+        """
+        Combine a list of subexpressions into a single expression, using
+        the provided connecting operator. This is required because operators
+        can vary between backends (e.g., Oracle with %% and &) and between
+        subexpression types (e.g., date expressions).
+        """
+        conn = ' %s ' % connector
+        return conn.join(sub_expressions)
+
+    def combine_duration_expression(self, connector, sub_expressions):
+        return self.combine_expression(connector, sub_expressions)
+
+    def binary_placeholder_sql(self, value):
+        """
+        Some backends require special syntax to insert binary content (MySQL
+        for example uses '_binary %s').
+        """
+        return '%s'
+
+    def modify_insert_params(self, placeholder, params):
+        """
+        Allow modification of insert parameters. Needed for Oracle Spatial
+        backend due to #10888.
+        """
+        return params
+
+    def integer_field_range(self, internal_type):
+        """
+        Given an integer field internal type (e.g. 'PositiveIntegerField'),
+        return a tuple of the (min_value, max_value) form representing the
+        range of the column type bound to the field.
+        """
+        return self.integer_field_ranges[internal_type]
+
+    def subtract_temporals(self, internal_type, lhs, rhs):
+        if self.connection.features.supports_temporal_subtraction:
+            lhs_sql, lhs_params = lhs
+            rhs_sql, rhs_params = rhs
+            return '(%s - %s)' % (lhs_sql, rhs_sql), (*lhs_params, *rhs_params)
+        raise NotSupportedError("This backend does not support %s subtraction." % internal_type)
+
+    def window_frame_start(self, start):
+        if isinstance(start, int):
+            if start < 0:
+                return '%d %s' % (abs(start), self.PRECEDING)
+            elif start == 0:
+                return self.CURRENT_ROW
+        elif start is None:
+            return self.UNBOUNDED_PRECEDING
+        raise ValueError("start argument must be a negative integer, zero, or None, but got '%s'." % start)
+
+    def window_frame_end(self, end):
+        if isinstance(end, int):
+            if end == 0:
+                return self.CURRENT_ROW
+            elif end > 0:
+                return '%d %s' % (end, self.FOLLOWING)
+        elif end is None:
+            return self.UNBOUNDED_FOLLOWING
+        raise ValueError("end argument must be a positive integer, zero, or None, but got '%s'." % end)
+
+    def window_frame_rows_start_end(self, start=None, end=None):
+        """
+        Return SQL for start and end points in an OVER clause window frame.
+        """
+        if not self.connection.features.supports_over_clause:
+            raise NotSupportedError('This backend does not support window expressions.')
+        return self.window_frame_start(start), self.window_frame_end(end)
+
+    def window_frame_range_start_end(self, start=None, end=None):
+        start_, end_ = self.window_frame_rows_start_end(start, end)
+        if (
+            self.connection.features.only_supports_unbounded_with_preceding_and_following and
+            ((start and start < 0) or (end and end > 0))
+        ):
+            raise NotSupportedError(
+                '%s only supports UNBOUNDED together with PRECEDING and '
+                'FOLLOWING.' % self.connection.display_name
+            )
+        return start_, end_
+
+    def explain_query_prefix(self, format=None, **options):
+        if not self.connection.features.supports_explaining_query_execution:
+            raise NotSupportedError('This backend does not support explaining query execution.')
+        if format:
+            supported_formats = self.connection.features.supported_explain_formats
+            normalized_format = format.upper()
+            if normalized_format not in supported_formats:
+                msg = '%s is not a recognized format.' % normalized_format
+                if supported_formats:
+                    msg += ' Allowed formats: %s' % ', '.join(sorted(supported_formats))
+                raise ValueError(msg)
+        if options:
+            raise ValueError('Unknown options: %s' % ', '.join(sorted(options.keys())))
+        return self.explain_prefix
+
+    def insert_statement(self, ignore_conflicts=False):
+        return 'INSERT INTO'
+
+    def ignore_conflicts_suffix_sql(self, ignore_conflicts=None):
+        return ''

Lib/site-packages/django/db/backends/base/validation.py

@@ -0,0 +1,25 @@
+class BaseDatabaseValidation:
+    """Encapsulate backend-specific validation."""
+    def __init__(self, connection):
+        self.connection = connection
+
+    def check(self, **kwargs):
+        return []
+
+    def check_field(self, field, **kwargs):
+        errors = []
+        # Backends may implement a check_field_type() method.
+        if (hasattr(self, 'check_field_type') and
+                # Ignore any related fields.
+                not getattr(field, 'remote_field', None)):
+            # Ignore fields with unsupported features.
+            db_supports_all_required_features = all(
+                getattr(self.connection.features, feature, False)
+                for feature in field.model._meta.required_db_features
+            )
+            if db_supports_all_required_features:
+                field_type = field.db_type(self.connection)
+                # Ignore non-concrete fields.
+                if field_type is not None:
+                    errors.extend(self.check_field_type(field, field_type))
+        return errors