=========

Databases

=========

Django officially supports the following databases:

* :ref:`PostgreSQL <postgresql-notes>`

* :ref:`MariaDB <mariadb-notes>`

* :ref:`MySQL <mysql-notes>`

* :ref:`Oracle <oracle-notes>`

* :ref:`SQLite <sqlite-notes>`

There are also a number of :ref:`database backends provided by third parties

<third-party-notes>`.

Django attempts to support as many features as possible on all database

backends. However, not all database backends are alike, and we've had to make

design decisions on which features to support and which assumptions we can make

safely.

This file describes some of the features that might be relevant to Django

usage. It is not intended as a replacement for server-specific documentation or

reference manuals.

General notes

=============

.. _persistent-database-connections:

Persistent connections

----------------------

Persistent connections avoid the overhead of reestablishing a connection to

the database in each request. They're controlled by the

:setting:`CONN_MAX_AGE` parameter which defines the maximum lifetime of a

connection. It can be set independently for each database.

The default value is ``0``, preserving the historical behavior of closing the

database connection at the end of each request. To enable persistent

connections, set :setting:`CONN_MAX_AGE` to a positive integer of seconds. For

unlimited persistent connections, set it to ``None``.

Connection management

~~~~~~~~~~~~~~~~~~~~~

Django opens a connection to the database when it first makes a database

query. It keeps this connection open and reuses it in subsequent requests.

Django closes the connection once it exceeds the maximum age defined by

:setting:`CONN_MAX_AGE` or when it isn't usable any longer.

In detail, Django automatically opens a connection to the database whenever it

needs one and doesn't have one already — either because this is the first

connection, or because the previous connection was closed.

At the beginning of each request, Django closes the connection if it has

reached its maximum age. If your database terminates idle connections after

some time, you should set :setting:`CONN_MAX_AGE` to a lower value, so that

Django doesn't attempt to use a connection that has been terminated by the

database server. (This problem may only affect very low traffic sites.)

At the end of each request, Django closes the connection if it has reached its

maximum age or if it is in an unrecoverable error state. If any database

errors have occurred while processing the requests, Django checks whether the

connection still works, and closes it if it doesn't. Thus, database errors

affect at most one request per each application's worker thread; if the

connection becomes unusable, the next request gets a fresh connection.

Setting :setting:`CONN_HEALTH_CHECKS` to ``True`` can be used to improve the

robustness of connection reuse and prevent errors when a connection has been

closed by the database server which is now ready to accept and serve new

connections, e.g. after database server restart. The health check is performed

only once per request and only if the database is being accessed during the

handling of the request.

.. versionchanged:: 4.1

    The :setting:`CONN_HEALTH_CHECKS` setting was added.

Caveats

~~~~~~~

Since each thread maintains its own connection, your database must support at

least as many simultaneous connections as you have worker threads.

Sometimes a database won't be accessed by the majority of your views, for

example because it's the database of an external system, or thanks to caching.

In such cases, you should set :setting:`CONN_MAX_AGE` to a low value or even

``0``, because it doesn't make sense to maintain a connection that's unlikely

to be reused. This will help keep the number of simultaneous connections to

this database small.

The development server creates a new thread for each request it handles,

negating the effect of persistent connections. Don't enable them during

development.

When Django establishes a connection to the database, it sets up appropriate

parameters, depending on the backend being used. If you enable persistent

connections, this setup is no longer repeated every request. If you modify

parameters such as the connection's isolation level or time zone, you should

either restore Django's defaults at the end of each request, force an

appropriate value at the beginning of each request, or disable persistent

connections.

Encoding

--------

Django assumes that all databases use UTF-8 encoding. Using other encodings may

result in unexpected behavior such as "value too long" errors from your

database for data that is valid in Django. See the database specific notes

below for information on how to set up your database correctly.

.. _postgresql-notes:

PostgreSQL notes

================

Django supports PostgreSQL 11 and higher. `psycopg2`_ 2.8.4 or higher is

required, though the latest release is recommended.

.. _psycopg2: https://www.psycopg.org/

.. _postgresql-connection-settings:

PostgreSQL connection settings

-------------------------------

See :setting:`HOST` for details.

To connect using a service name from the `connection service file`_ and a

password from the `password file`_, you must specify them in the

:setting:`OPTIONS` part of your database configuration in :setting:`DATABASES`:

.. code-block:: python

    :caption: ``settings.py``

    DATABASES = {

        'default': {

            'ENGINE': 'django.db.backends.postgresql',

            'OPTIONS': {

                'service': 'my_service',

                'passfile': '.my_pgpass',

            },

        }

    }

.. code-block:: text

    :caption: ``.pg_service.conf``

    [my_service]

    host=localhost

    user=USER

    dbname=NAME

    port=5432

.. code-block:: text

    :caption: ``.my_pgpass``

    localhost:5432:NAME:USER:PASSWORD

.. _connection service file: https://www.postgresql.org/docs/current/libpq-pgservice.html

.. _password file: https://www.postgresql.org/docs/current/libpq-pgpass.html

.. versionchanged:: 4.0

    Support for connecting by a service name, and specifying a password file

    was added.

.. warning::

    Using a service name for testing purposes is not supported. This

    :ticket:`may be implemented later <33685>`.

Optimizing PostgreSQL's configuration

-------------------------------------

Django needs the following parameters for its database connections:

- ``client_encoding``: ``'UTF8'``,

- ``default_transaction_isolation``: ``'read committed'`` by default,

  or the value set in the connection options (see below),

- ``timezone``:

    - when :setting:`USE_TZ` is ``True``, ``'UTC'`` by default, or the

      :setting:`TIME_ZONE <DATABASE-TIME_ZONE>` value set for the connection,

    - when :setting:`USE_TZ` is ``False``, the value of the global

      :setting:`TIME_ZONE` setting.

If these parameters already have the correct values, Django won't set them for

every new connection, which improves performance slightly. You can configure

them directly in :file:`postgresql.conf` or more conveniently per database

user with `ALTER ROLE`_.

Django will work just fine without this optimization, but each new connection

will do some additional queries to set these parameters.

.. _ALTER ROLE: https://www.postgresql.org/docs/current/sql-alterrole.html

.. _database-isolation-level:

Isolation level

---------------

Like PostgreSQL itself, Django defaults to the ``READ COMMITTED`` `isolation

level`_. If you need a higher isolation level such as ``REPEATABLE READ`` or

``SERIALIZABLE``, set it in the :setting:`OPTIONS` part of your database

configuration in :setting:`DATABASES`::

    import psycopg2.extensions

    DATABASES = {

        # ...

        'OPTIONS': {

            'isolation_level': psycopg2.extensions.ISOLATION_LEVEL_SERIALIZABLE,

        },

    }

.. note::

    Under higher isolation levels, your application should be prepared to

    handle exceptions raised on serialization failures. This option is

    designed for advanced uses.

.. _isolation level: https://www.postgresql.org/docs/current/transaction-iso.html

Indexes for ``varchar`` and ``text`` columns

--------------------------------------------

When specifying ``db_index=True`` on your model fields, Django typically

outputs a single ``CREATE INDEX`` statement.  However, if the database type

for the field is either ``varchar`` or ``text`` (e.g., used by ``CharField``,

``FileField``, and ``TextField``), then Django will create

an additional index that uses an appropriate `PostgreSQL operator class`_

for the column.  The extra index is necessary to correctly perform

lookups that use the ``LIKE`` operator in their SQL, as is done with the

``contains`` and ``startswith`` lookup types.

.. _PostgreSQL operator class: https://www.postgresql.org/docs/current/indexes-opclass.html

Migration operation for adding extensions

-----------------------------------------

If you need to add a PostgreSQL extension (like ``hstore``, ``postgis``, etc.)

using a migration, use the

:class:`~django.contrib.postgres.operations.CreateExtension` operation.

.. _postgresql-server-side-cursors:

Server-side cursors

-------------------

When using :meth:`QuerySet.iterator()

<django.db.models.query.QuerySet.iterator>`, Django opens a :ref:`server-side

cursor <psycopg2:server-side-cursors>`. By default, PostgreSQL assumes that

only the first 10% of the results of cursor queries will be fetched. The query

planner spends less time planning the query and starts returning results

faster, but this could diminish performance if more than 10% of the results are

retrieved. PostgreSQL's assumptions on the number of rows retrieved for a

cursor query is controlled with the `cursor_tuple_fraction`_ option.

.. _cursor_tuple_fraction: https://www.postgresql.org/docs/current/runtime-config-query.html#GUC-CURSOR-TUPLE-FRACTION

.. _transaction-pooling-server-side-cursors:

Transaction pooling and server-side cursors

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Using a connection pooler in transaction pooling mode (e.g. `PgBouncer`_)

requires disabling server-side cursors for that connection.

Server-side cursors are local to a connection and remain open at the end of a

transaction when :setting:`AUTOCOMMIT <DATABASE-AUTOCOMMIT>` is ``True``. A

subsequent transaction may attempt to fetch more results from a server-side

cursor. In transaction pooling mode, there's no guarantee that subsequent

transactions will use the same connection. If a different connection is used,

an error is raised when the transaction references the server-side cursor,

because server-side cursors are only accessible in the connection in which they

were created.

One solution is to disable server-side cursors for a connection in

:setting:`DATABASES` by setting :setting:`DISABLE_SERVER_SIDE_CURSORS

<DATABASE-DISABLE_SERVER_SIDE_CURSORS>` to ``True``.

To benefit from server-side cursors in transaction pooling mode, you could set

up :doc:`another connection to the database </topics/db/multi-db>` in order to

perform queries that use server-side cursors. This connection needs to either

be directly to the database or to a connection pooler in session pooling mode.

Another option is to wrap each ``QuerySet`` using server-side cursors in an

:func:`~django.db.transaction.atomic` block, because it disables ``autocommit``

for the duration of the transaction. This way, the server-side cursor will only

live for the duration of the transaction.

.. _PgBouncer: https://www.pgbouncer.org/

.. _manually-specified-autoincrement-pk:

Manually-specifying values of auto-incrementing primary keys

------------------------------------------------------------

Django uses PostgreSQL's identity columns to store auto-incrementing primary

keys. An identity column is populated with values from a `sequence`_ that keeps

track of the next available value. Manually assigning a value to an

auto-incrementing field doesn't update the field's sequence, which might later

cause a conflict. For example::

    >>> from django.contrib.auth.models import User

    >>> User.objects.create(username='alice', pk=1)

    <User: alice>

    >>> # The sequence hasn't been updated; its next value is 1.

    >>> User.objects.create(username='bob')

    ...

    IntegrityError: duplicate key value violates unique constraint

    "auth_user_pkey" DETAIL:  Key (id)=(1) already exists.

If you need to specify such values, reset the sequence afterward to avoid

reusing a value that's already in the table. The :djadmin:`sqlsequencereset`

management command generates the SQL statements to do that.

.. versionchanged:: 4.1

    In older versions, PostgreSQL’s ``SERIAL`` data type was used instead of

    identity columns.

.. _sequence: https://www.postgresql.org/docs/current/sql-createsequence.html

Test database templates

-----------------------

You can use the :setting:`TEST['TEMPLATE'] <TEST_TEMPLATE>` setting to specify

a `template`_ (e.g. ``'template0'``) from which to create a test database.

.. _template: https://www.postgresql.org/docs/current/sql-createdatabase.html

Speeding up test execution with non-durable settings

----------------------------------------------------

You can speed up test execution times by `configuring PostgreSQL to be

non-durable <https://www.postgresql.org/docs/current/non-durability.html>`_.

.. warning::

    This is dangerous: it will make your database more susceptible to data loss

    or corruption in the case of a server crash or power loss. Only use this on

    a development machine where you can easily restore the entire contents of

    all databases in the cluster.

.. _mariadb-notes:

MariaDB notes

=============

Django supports MariaDB 10.3 and higher.

To use MariaDB, use the MySQL backend, which is shared between the two. See the

:ref:`MySQL notes <mysql-notes>` for more details.

.. _mysql-notes:

MySQL notes

===========

Version support

---------------

Django supports MySQL 5.7 and higher.

Django's ``inspectdb`` feature uses the ``information_schema`` database, which

contains detailed data on all database schemas.

Django expects the database to support Unicode (UTF-8 encoding) and delegates to

it the task of enforcing transactions and referential integrity. It is important

to be aware of the fact that the two latter ones aren't actually enforced by

MySQL when using the MyISAM storage engine, see the next section.

.. _mysql-storage-engines:

Storage engines

---------------

MySQL has several `storage engines`_. You can change the default storage engine

in the server configuration.

MySQL's default storage engine is InnoDB_. This engine is fully transactional

and supports foreign key references. It's the recommended choice. However, the

InnoDB autoincrement counter is lost on a MySQL restart because it does not

remember the ``AUTO_INCREMENT`` value, instead recreating it as "max(id)+1".

This may result in an inadvertent reuse of :class:`~django.db.models.AutoField`

values.

The main drawbacks of MyISAM_ are that it doesn't support transactions or

enforce foreign-key constraints.

.. _storage engines: https://dev.mysql.com/doc/refman/en/storage-engines.html

.. _MyISAM: https://dev.mysql.com/doc/refman/en/myisam-storage-engine.html

.. _InnoDB: https://dev.mysql.com/doc/refman/en/innodb-storage-engine.html

.. _mysql-db-api-drivers:

MySQL DB API Drivers

--------------------

MySQL has a couple drivers that implement the Python Database API described in

:pep:`249`:

- `mysqlclient`_ is a native driver. It's **the recommended choice**.

- `MySQL Connector/Python`_ is a pure Python driver from Oracle that does not

  require the MySQL client library or any Python modules outside the standard

  library.

.. _mysqlclient: https://pypi.org/project/mysqlclient/

.. _MySQL Connector/Python: https://dev.mysql.com/downloads/connector/python/

These drivers are thread-safe and provide connection pooling.

In addition to a DB API driver, Django needs an adapter to access the database

drivers from its ORM. Django provides an adapter for mysqlclient while MySQL

Connector/Python includes `its own`_.

.. _its own: https://dev.mysql.com/doc/connector-python/en/connector-python-django-backend.html

mysqlclient

~~~~~~~~~~~

Django requires `mysqlclient`_ 1.4.0 or later.

MySQL Connector/Python

~~~~~~~~~~~~~~~~~~~~~~

MySQL Connector/Python is available from the `download page`_.

The Django adapter is available in versions 1.1.X and later. It may not

support the most recent releases of Django.

.. _download page: https://dev.mysql.com/downloads/connector/python/

.. _mysql-time-zone-definitions:

Time zone definitions

---------------------

If you plan on using Django's :doc:`timezone support </topics/i18n/timezones>`,

use `mysql_tzinfo_to_sql`_ to load time zone tables into the MySQL database.

This needs to be done just once for your MySQL server, not per database.

.. _mysql_tzinfo_to_sql: https://dev.mysql.com/doc/refman/en/mysql-tzinfo-to-sql.html

Creating your database

----------------------

You can `create your database`_ using the command-line tools and this SQL::

  CREATE DATABASE <dbname> CHARACTER SET utf8;

This ensures all tables and columns will use UTF-8 by default.

.. _create your database: https://dev.mysql.com/doc/refman/en/create-database.html

.. _mysql-collation:

Collation settings

~~~~~~~~~~~~~~~~~~

The collation setting for a column controls the order in which data is sorted

as well as what strings compare as equal. You can specify the ``db_collation``

parameter to set the collation name of the column for

:attr:`CharField <django.db.models.CharField.db_collation>` and

:attr:`TextField <django.db.models.TextField.db_collation>`.

The collation can also be set on a database-wide level and per-table. This is

`documented thoroughly`_ in the MySQL documentation. In such cases, you must

set the collation by directly manipulating the database settings or tables.

Django doesn't provide an API to change them.

.. _documented thoroughly: https://dev.mysql.com/doc/refman/en/charset.html

By default, with a UTF-8 database, MySQL will use the

``utf8_general_ci`` collation. This results in all string equality

comparisons being done in a *case-insensitive* manner. That is, ``"Fred"`` and

``"freD"`` are considered equal at the database level. If you have a unique

constraint on a field, it would be illegal to try to insert both ``"aa"`` and

``"AA"`` into the same column, since they compare as equal (and, hence,

non-unique) with the default collation. If you want case-sensitive comparisons

on a particular column or table, change the column or table to use the

``utf8_bin`` collation.

Please note that according to `MySQL Unicode Character Sets`_, comparisons for

the ``utf8_general_ci`` collation are faster, but slightly less correct, than

comparisons for ``utf8_unicode_ci``. If this is acceptable for your application,

you should use ``utf8_general_ci`` because it is faster. If this is not acceptable

(for example, if you require German dictionary order), use ``utf8_unicode_ci``

because it is more accurate.

.. _MySQL Unicode Character Sets: https://dev.mysql.com/doc/refman/en/charset-unicode-sets.html

.. warning::

    Model formsets validate unique fields in a case-sensitive manner. Thus when

    using a case-insensitive collation, a formset with unique field values that

    differ only by case will pass validation, but upon calling ``save()``, an

    ``IntegrityError`` will be raised.

Connecting to the database

--------------------------

Refer to the :doc:`settings documentation </ref/settings>`.

Connection settings are used in this order:

#. :setting:`OPTIONS`.

#. :setting:`NAME`, :setting:`USER`, :setting:`PASSWORD`, :setting:`HOST`,

   :setting:`PORT`

#. MySQL option files.

In other words, if you set the name of the database in :setting:`OPTIONS`,

this will take precedence over :setting:`NAME`, which would override

anything in a `MySQL option file`_.

Here's a sample configuration which uses a MySQL option file::

    # settings.py

    DATABASES = {

        'default': {

            'ENGINE': 'django.db.backends.mysql',

            'OPTIONS': {

                'read_default_file': '/path/to/my.cnf',

            },

        }

    }

    # my.cnf

    [client]

    database = NAME

    user = USER

    password = PASSWORD

    default-character-set = utf8

Several other `MySQLdb connection options`_ may be useful, such as ``ssl``,

``init_command``, and ``sql_mode``.

.. _MySQL option file: https://dev.mysql.com/doc/refman/en/option-files.html

.. _MySQLdb connection options: https://mysqlclient.readthedocs.io/user_guide.html#functions-and-attributes

.. _mysql-sql-mode:

Setting ``sql_mode``

~~~~~~~~~~~~~~~~~~~~

From MySQL 5.7 onward, the default value of the ``sql_mode`` option contains

``STRICT_TRANS_TABLES``. That option escalates warnings into errors when data

are truncated upon insertion, so Django highly recommends activating a

`strict mode`_ for MySQL to prevent data loss (either ``STRICT_TRANS_TABLES``

or ``STRICT_ALL_TABLES``).

.. _strict mode: https://dev.mysql.com/doc/refman/en/sql-mode.html#sql-mode-strict

If you need to customize the SQL mode, you can set the ``sql_mode`` variable

like other MySQL options: either in a config file or with the entry

``'init_command': "SET sql_mode='STRICT_TRANS_TABLES'"`` in the

:setting:`OPTIONS` part of your database configuration in :setting:`DATABASES`.

.. _mysql-isolation-level:

Isolation level

~~~~~~~~~~~~~~~

When running concurrent loads, database transactions from different sessions

(say, separate threads handling different requests) may interact with each

other. These interactions are affected by each session's `transaction isolation

level`_. You can set a connection's isolation level with an

``'isolation_level'`` entry in the :setting:`OPTIONS` part of your database

configuration in :setting:`DATABASES`. Valid values for

this entry are the four standard isolation levels:

* ``'read uncommitted'``

* ``'read committed'``

* ``'repeatable read'``

* ``'serializable'``

or ``None`` to use the server's configured isolation level. However, Django

works best with and defaults to read committed rather than MySQL's default,

repeatable read. Data loss is possible with repeatable read. In particular,

you may see cases where :meth:`~django.db.models.query.QuerySet.get_or_create`

will raise an :exc:`~django.db.IntegrityError` but the object won't appear in

a subsequent :meth:`~django.db.models.query.QuerySet.get` call.

.. _transaction isolation level: https://dev.mysql.com/doc/refman/en/innodb-transaction-isolation-levels.html

Creating your tables

--------------------

When Django generates the schema, it doesn't specify a storage engine, so

tables will be created with whatever default storage engine your database

server is configured for. The easiest solution is to set your database server's

default storage engine to the desired engine.

If you're using a hosting service and can't change your server's default

storage engine, you have a couple of options.

* After the tables are created, execute an ``ALTER TABLE`` statement to

  convert a table to a new storage engine (such as InnoDB)::

      ALTER TABLE <tablename> ENGINE=INNODB;

  This can be tedious if you have a lot of tables.

* Another option is to use the ``init_command`` option for MySQLdb prior to

  creating your tables::

      'OPTIONS': {

         'init_command': 'SET default_storage_engine=INNODB',

      }

  This sets the default storage engine upon connecting to the database.

  After your tables have been created, you should remove this option as it

  adds a query that is only needed during table creation to each database

  connection.

Table names

-----------

There are `known issues`_ in even the latest versions of MySQL that can cause the

case of a table name to be altered when certain SQL statements are executed

under certain conditions. It is recommended that you use lowercase table

names, if possible, to avoid any problems that might arise from this behavior.

Django uses lowercase table names when it auto-generates table names from

models, so this is mainly a consideration if you are overriding the table name

via the :class:`~django.db.models.Options.db_table` parameter.

.. _known issues: https://bugs.mysql.com/bug.php?id=48875

Savepoints

----------

Both the Django ORM and MySQL (when using the InnoDB :ref:`storage engine

<mysql-storage-engines>`) support database :ref:`savepoints

<topics-db-transactions-savepoints>`.

If you use the MyISAM storage engine please be aware of the fact that you will

receive database-generated errors if you try to use the :ref:`savepoint-related

methods of the transactions API <topics-db-transactions-savepoints>`. The reason

for this is that detecting the storage engine of a MySQL database/table is an

expensive operation so it was decided it isn't worth to dynamically convert

these methods in no-op's based in the results of such detection.

Notes on specific fields

------------------------

.. _mysql-character-fields:

Character fields

~~~~~~~~~~~~~~~~

Any fields that are stored with ``VARCHAR`` column types may have their

``max_length`` restricted to 255 characters if you are using ``unique=True``

for the field. This affects :class:`~django.db.models.CharField`,

:class:`~django.db.models.SlugField`. See `the MySQL documentation`_ for more

details.

.. _the MySQL documentation: https://dev.mysql.com/doc/refman/en/create-index.html#create-index-column-prefixes

``TextField`` limitations

~~~~~~~~~~~~~~~~~~~~~~~~~

MySQL can index only the first N chars of a ``BLOB`` or ``TEXT`` column. Since

``TextField`` doesn't have a defined length, you can't mark it as

``unique=True``. MySQL will report: "BLOB/TEXT column '<db_column>' used in key

specification without a key length".

.. _mysql-fractional-seconds:

Fractional seconds support for Time and DateTime fields

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

MySQL can store fractional seconds, provided that the column definition

includes a fractional indication (e.g. ``DATETIME(6)``).

Django will not upgrade existing columns to include fractional seconds if the

database server supports it. If you want to enable them on an existing database,

it's up to you to either manually update the column on the target database, by

executing a command like::

    ALTER TABLE `your_table` MODIFY `your_datetime_column` DATETIME(6)

or using a :class:`~django.db.migrations.operations.RunSQL` operation in a

:ref:`data migration <data-migrations>`.

``TIMESTAMP`` columns

~~~~~~~~~~~~~~~~~~~~~

If you are using a legacy database that contains ``TIMESTAMP`` columns, you must

set :setting:`USE_TZ = False <USE_TZ>` to avoid data corruption.

:djadmin:`inspectdb` maps these columns to

:class:`~django.db.models.DateTimeField` and if you enable timezone support,

both MySQL and Django will attempt to convert the values from UTC to local time.

Row locking with ``QuerySet.select_for_update()``

-------------------------------------------------

MySQL and MariaDB do not support some options to the ``SELECT ... FOR UPDATE``

statement. If ``select_for_update()`` is used with an unsupported option, then

a :exc:`~django.db.NotSupportedError` is raised.

=============== ========= ==========

Option          MariaDB   MySQL

=============== ========= ==========

``SKIP LOCKED`` X (≥10.6) X (≥8.0.1)

``NOWAIT``      X         X (≥8.0.1)

``OF``                    X (≥8.0.1)

``NO KEY``

=============== ========= ==========

When using ``select_for_update()`` on MySQL, make sure you filter a queryset

against at least a set of fields contained in unique constraints or only

against fields covered by indexes. Otherwise, an exclusive write lock will be

acquired over the full table for the duration of the transaction.

Automatic typecasting can cause unexpected results

--------------------------------------------------

When performing a query on a string type, but with an integer value, MySQL will

coerce the types of all values in the table to an integer before performing the

comparison. If your table contains the values ``'abc'``, ``'def'`` and you

query for ``WHERE mycolumn=0``, both rows will match. Similarly, ``WHERE mycolumn=1``

will match the value ``'abc1'``. Therefore, string type fields included in Django

will always cast the value to a string before using it in a query.

If you implement custom model fields that inherit from

:class:`~django.db.models.Field` directly, are overriding

:meth:`~django.db.models.Field.get_prep_value`, or use

:class:`~django.db.models.expressions.RawSQL`,

:meth:`~django.db.models.query.QuerySet.extra`, or

:meth:`~django.db.models.Manager.raw`, you should ensure that you perform

appropriate typecasting.

.. _sqlite-notes:

SQLite notes

============

Django supports SQLite 3.9.0 and later.

SQLite_ provides an excellent development alternative for applications that

are predominantly read-only or require a smaller installation footprint. As

with all database servers, though, there are some differences that are

specific to SQLite that you should be aware of.

.. _SQLite: https://www.sqlite.org/

.. _sqlite-string-matching:

Substring matching and case sensitivity

---------------------------------------

For all SQLite versions, there is some slightly counter-intuitive behavior when

attempting to match some types of strings.  These are triggered when using the

:lookup:`iexact` or :lookup:`contains` filters in Querysets. The behavior

splits into two cases:

1. For substring matching, all matches are done case-insensitively. That is a

filter such as ``filter(name__contains="aa")`` will match a name of ``"Aabb"``.

2. For strings containing characters outside the ASCII range, all exact string

matches are performed case-sensitively, even when the case-insensitive options

are passed into the query. So the :lookup:`iexact` filter will behave exactly

the same as the :lookup:`exact` filter in these cases.

Some possible workarounds for this are `documented at sqlite.org`_, but they

aren't utilized by the default SQLite backend in Django, as incorporating them

would be fairly difficult to do robustly. Thus, Django exposes the default

SQLite behavior and you should be aware of this when doing case-insensitive or

substring filtering.

.. _documented at sqlite.org: https://www.sqlite.org/faq.html#q18

.. _sqlite-decimal-handling:

Decimal handling

----------------

SQLite has no real decimal internal type. Decimal values are internally

converted to the ``REAL`` data type (8-byte IEEE floating point number), as

explained in the `SQLite datatypes documentation`__, so they don't support

correctly-rounded decimal floating point arithmetic.

__ https://www.sqlite.org/datatype3.html#storage_classes_and_datatypes

"Database is locked" errors

---------------------------

SQLite is meant to be a lightweight database, and thus can't support a high

level of concurrency. ``OperationalError: database is locked`` errors indicate

that your application is experiencing more concurrency than ``sqlite`` can

handle in default configuration. This error means that one thread or process has

an exclusive lock on the database connection and another thread timed out

waiting for the lock the be released.

Python's SQLite wrapper has

a default timeout value that determines how long the second thread is allowed to

wait on the lock before it times out and raises the ``OperationalError: database

is locked`` error.

If you're getting this error, you can solve it by:

* Switching to another database backend. At a certain point SQLite becomes

  too "lite" for real-world applications, and these sorts of concurrency

  errors indicate you've reached that point.

* Rewriting your code to reduce concurrency and ensure that database

  transactions are short-lived.

* Increase the default timeout value by setting the ``timeout`` database

  option::

      'OPTIONS': {

          # ...

          'timeout': 20,

          # ...

      }

  This will make SQLite wait a bit longer before throwing "database is locked"

  errors; it won't really do anything to solve them.

``QuerySet.select_for_update()`` not supported

----------------------------------------------

SQLite does not support the ``SELECT ... FOR UPDATE`` syntax. Calling it will

have no effect.

"pyformat" parameter style in raw queries not supported

-------------------------------------------------------

For most backends, raw queries (``Manager.raw()`` or ``cursor.execute()``)

can use the "pyformat" parameter style, where placeholders in the query

are given as ``'%(name)s'`` and the parameters are passed as a dictionary

rather than a list. SQLite does not support this.

.. _sqlite-isolation:

Isolation when using ``QuerySet.iterator()``

--------------------------------------------

There are special considerations described in `Isolation In SQLite`_ when

modifying a table while iterating over it using :meth:`.QuerySet.iterator`. If

a row is added, changed, or deleted within the loop, then that row may or may

not appear, or may appear twice, in subsequent results fetched from the

iterator. Your code must handle this.

.. _`Isolation in SQLite`: https://www.sqlite.org/isolation.html

.. _sqlite-json1:

Enabling JSON1 extension on SQLite

----------------------------------

To use :class:`~django.db.models.JSONField` on SQLite, you need to enable the

`JSON1 extension`_ on Python's :py:mod:`sqlite3` library. If the extension is

not enabled on your installation, a system error (``fields.E180``) will be

raised.

To enable the JSON1 extension you can follow the instruction on

`the wiki page`_.

.. _JSON1 extension: https://www.sqlite.org/json1.html

.. _the wiki page: https://code.djangoproject.com/wiki/JSON1Extension

.. _oracle-notes:

Oracle notes

============

Django supports `Oracle Database Server`_ versions 19c and higher. Version 7.0

or higher of the `cx_Oracle`_ Python driver is required.

.. _`Oracle Database Server`: https://www.oracle.com/

.. _`cx_Oracle`: https://oracle.github.io/python-cx_Oracle/

In order for the ``python manage.py migrate`` command to work, your Oracle

database user must have privileges to run the following commands:

* CREATE TABLE

* CREATE SEQUENCE

* CREATE PROCEDURE

* CREATE TRIGGER

To run a project's test suite, the user usually needs these *additional*

privileges:

* CREATE USER

* ALTER USER

* DROP USER

* CREATE TABLESPACE

* DROP TABLESPACE

* CREATE SESSION WITH ADMIN OPTION

* CREATE TABLE WITH ADMIN OPTION

* CREATE SEQUENCE WITH ADMIN OPTION

* CREATE PROCEDURE WITH ADMIN OPTION

* CREATE TRIGGER WITH ADMIN OPTION

While the ``RESOURCE`` role has the required ``CREATE TABLE``,

``CREATE SEQUENCE``, ``CREATE PROCEDURE``, and ``CREATE TRIGGER`` privileges,

and a user granted ``RESOURCE WITH ADMIN OPTION`` can grant ``RESOURCE``, such

a user cannot grant the individual privileges (e.g. ``CREATE TABLE``), and thus

``RESOURCE WITH ADMIN OPTION`` is not usually sufficient for running tests.

Some test suites also create views or materialized views; to run these, the

user also needs ``CREATE VIEW WITH ADMIN OPTION`` and

``CREATE MATERIALIZED VIEW WITH ADMIN OPTION`` privileges. In particular, this

is needed for Django's own test suite.

All of these privileges are included in the DBA role, which is appropriate

for use on a private developer's database.

The Oracle database backend uses the ``SYS.DBMS_LOB`` and ``SYS.DBMS_RANDOM``

packages, so your user will require execute permissions on it. It's normally

accessible to all users by default, but in case it is not, you'll need to grant

permissions like so:

.. code-block:: sql

    GRANT EXECUTE ON SYS.DBMS_LOB TO user;

    GRANT EXECUTE ON SYS.DBMS_RANDOM TO user;

Connecting to the database

--------------------------

To connect using the service name of your Oracle database, your ``settings.py``

file should look something like this::

    DATABASES = {

        'default': {

            'ENGINE': 'django.db.backends.oracle',

            'NAME': 'xe',

            'USER': 'a_user',

            'PASSWORD': 'a_password',

            'HOST': '',

            'PORT': '',

        }

    }

In this case, you should leave both :setting:`HOST` and :setting:`PORT` empty.

However, if you don't use a ``tnsnames.ora`` file or a similar naming method

and want to connect using the SID ("xe" in this example), then fill in both

:setting:`HOST` and :setting:`PORT` like so::

    DATABASES = {

        'default': {

            'ENGINE': 'django.db.backends.oracle',

            'NAME': 'xe',

            'USER': 'a_user',

            'PASSWORD': 'a_password',

            'HOST': 'dbprod01ned.mycompany.com',

            'PORT': '1540',

        }

    }

You should either supply both :setting:`HOST` and :setting:`PORT`, or leave

both as empty strings. Django will use a different connect descriptor depending

on that choice.

Full DSN and Easy Connect

~~~~~~~~~~~~~~~~~~~~~~~~~

A Full DSN or Easy Connect string can be used in :setting:`NAME` if both

:setting:`HOST` and :setting:`PORT` are empty. This format is required when

using RAC or pluggable databases without ``tnsnames.ora``, for example.

Example of an Easy Connect string::

    'NAME': 'localhost:1521/orclpdb1',

Example of a full DSN string::

    'NAME': (

        '(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=localhost)(PORT=1521))'

        '(CONNECT_DATA=(SERVICE_NAME=orclpdb1)))'

    ),

Threaded option

---------------

If you plan to run Django in a multithreaded environment (e.g. Apache using the

default MPM module on any modern operating system), then you **must** set

the ``threaded`` option of your Oracle database configuration to ``True``::

    'OPTIONS': {

        'threaded': True,

    },

Failure to do this may result in crashes and other odd behavior.

INSERT ... RETURNING INTO

-------------------------

By default, the Oracle backend uses a ``RETURNING INTO`` clause to efficiently

retrieve the value of an ``AutoField`` when inserting new rows.  This behavior

may result in a ``DatabaseError`` in certain unusual setups, such as when

inserting into a remote table, or into a view with an ``INSTEAD OF`` trigger.

The ``RETURNING INTO`` clause can be disabled by setting the

``use_returning_into`` option of the database configuration to ``False``::

    'OPTIONS': {

        'use_returning_into': False,

    },

In this case, the Oracle backend will use a separate ``SELECT`` query to

retrieve ``AutoField`` values.

Naming issues

-------------

Oracle imposes a name length limit of 30 characters. To accommodate this, the

backend truncates database identifiers to fit, replacing the final four

characters of the truncated name with a repeatable MD5 hash value.

Additionally, the backend turns database identifiers to all-uppercase.

To prevent these transformations (this is usually required only when dealing

with legacy databases or accessing tables which belong to other users), use

a quoted name as the value for ``db_table``::

    class LegacyModel(models.Model):

        class Meta:

            db_table = '"name_left_in_lowercase"'

    class ForeignModel(models.Model):

        class Meta:

            db_table = '"OTHER_USER"."NAME_ONLY_SEEMS_OVER_30"'

Quoted names can also be used with Django's other supported database

backends; except for Oracle, however, the quotes have no effect.

When running ``migrate``, an ``ORA-06552`` error may be encountered if

certain Oracle keywords are used as the name of a model field or the

value of a ``db_column`` option.  Django quotes all identifiers used

in queries to prevent most such problems, but this error can still

occur when an Oracle datatype is used as a column name.  In

particular, take care to avoid using the names ``date``,

``timestamp``, ``number`` or ``float`` as a field name.

.. _oracle-null-empty-strings:

NULL and empty strings

----------------------

Django generally prefers to use the empty string (``''``) rather than

``NULL``, but Oracle treats both identically. To get around this, the

Oracle backend ignores an explicit ``null`` option on fields that

have the empty string as a possible value and generates DDL as if

``null=True``. When fetching from the database, it is assumed that

a ``NULL`` value in one of these fields really means the empty

string, and the data is silently converted to reflect this assumption.

``TextField`` limitations

-------------------------

The Oracle backend stores ``TextFields`` as ``NCLOB`` columns. Oracle imposes

some limitations on the usage of such LOB columns in general:

* LOB columns may not be used as primary keys.

* LOB columns may not be used in indexes.

* LOB columns may not be used in a ``SELECT DISTINCT`` list. This means that

  attempting to use the ``QuerySet.distinct`` method on a model that

  includes ``TextField`` columns will result in an ``ORA-00932`` error when

  run against Oracle. As a workaround, use the ``QuerySet.defer`` method in

  conjunction with ``distinct()`` to prevent ``TextField`` columns from being

  included in the ``SELECT DISTINCT`` list.

.. _subclassing-database-backends:

Subclassing the built-in database backends

==========================================

Django comes with built-in database backends. You may subclass an existing

database backends to modify its behavior, features, or configuration.

Consider, for example, that you need to change a single database feature.

First, you have to create a new directory with a ``base`` module in it. For

example::

    mysite/

        ...

        mydbengine/

            __init__.py

            base.py

The ``base.py`` module must contain a class named ``DatabaseWrapper`` that

subclasses an existing engine from the ``django.db.backends`` module. Here's an

example of subclassing the PostgreSQL engine to change a feature class

``allows_group_by_selected_pks_on_model``:

.. code-block:: python

    :caption: ``mysite/mydbengine/base.py``

    from django.db.backends.postgresql import base, features

    class DatabaseFeatures(features.DatabaseFeatures):

        def allows_group_by_selected_pks_on_model(self, model):

            return True

    class DatabaseWrapper(base.DatabaseWrapper):

        features_class = DatabaseFeatures

Finally, you must specify a :setting:`DATABASE-ENGINE` in your ``settings.py``

file::

    DATABASES = {

        'default': {

            'ENGINE': 'mydbengine',

            ...

        },

    }

You can see the current list of database engines by looking in

:source:`django/db/backends`.

.. _third-party-notes:

Using a 3rd-party database backend

==================================

In addition to the officially supported databases, there are backends provided

by 3rd parties that allow you to use other databases with Django:

* `CockroachDB`_

* `Firebird`_

* `Google Cloud Spanner`_

* `Microsoft SQL Server`_

* `TiDB`_

* `YugabyteDB`_

The Django versions and ORM features supported by these unofficial backends

vary considerably. Queries regarding the specific capabilities of these

unofficial backends, along with any support queries, should be directed to

the support channels provided by each 3rd party project.

.. _CockroachDB: https://pypi.org/project/django-cockroachdb/

.. _Firebird: https://pypi.org/project/django-firebird/

.. _Google Cloud Spanner: https://pypi.org/project/django-google-spanner/

.. _Microsoft SQL Server: https://pypi.org/project/mssql-django/

.. _TiDB: https://pypi.org/project/django-tidb/

.. _YugabyteDB: https://pypi.org/project/django-yugabytedb/