django/docs/ref/databases.txt

=========
Databases
=========

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. Of course, it is not intended as a replacement for server-specific
documentation or reference manuals.

General notes
=============

.. _persistent-database-connections:

Persistent connections
----------------------

.. versionadded:: 1.6

Persistent connections avoid the overhead of re-establishing a connection to
the database in each request. By default, connections are kept open for up 10
minutes — if not specified, :setting:`CONN_MAX_AGE` defaults to 600 seconds.

Django 1.5 and earlier didn't have persistent connections. To restore the
legacy behavior of closing the connection at the end of every request, set
:setting:`CONN_MAX_AGE` to ``0``.

For unlimited persistent connections, set :setting:`CONN_MAX_AGE` 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; if the connection becomes unusable, the next
request gets a fresh connection.

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 lower 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.

.. _postgresql-notes:

PostgreSQL notes
================

Django supports PostgreSQL 8.2 and higher.

PostgreSQL 8.2 to 8.2.4
-----------------------

The implementation of the population statistics aggregates ``STDDEV_POP`` and
``VAR_POP`` that shipped with PostgreSQL 8.2 to 8.2.4 are `known to be
faulty`_. Users of these releases of PostgreSQL are advised to upgrade to
`Release 8.2.5`_ or later. Django will raise a ``NotImplementedError`` if you
attempt to use the ``StdDev(sample=False)`` or ``Variance(sample=False)``
aggregate with a database backend that falls within the affected release range.

.. _known to be faulty: http://archives.postgresql.org/pgsql-bugs/2007-07/msg00046.php
.. _Release 8.2.5: http://www.postgresql.org/docs/devel/static/release-8-2-5.html

PostgreSQL connection settings
-------------------------------

See :setting:`HOST` for details.

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``: ``'UTC'`` when :setting:`USE_TZ` is ``True``, value of
  :setting:`TIME_ZONE` otherwise.

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: http://www.postgresql.org/docs/current/interactive/sql-alterrole.html

.. _postgresql-autocommit-mode:

Autocommit mode
---------------

.. versionchanged:: 1.6

In previous versions of Django, database-level autocommit could be enabled by
setting the ``autocommit`` key in the :setting:`OPTIONS` part of your database
configuration in :setting:`DATABASES`::

    DATABASES = {
        # ...
        'OPTIONS': {
            'autocommit': True,
        },
    }

Since Django 1.6, autocommit is turned on by default. This configuration is
ignored and can be safely removed.

Isolation level
---------------

.. versionadded:: 1.6

Like PostgreSQL itself, Django defaults to the ``READ COMMITTED`` `isolation
level <postgresql-isolation-levels>`_. 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.

.. _postgresql-isolation-levels: http://www.postgresql.org/docs/devel/static/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: http://www.postgresql.org/docs/8.4/static/indexes-opclass.html

.. _mysql-notes:

MySQL notes
===========

Version support
---------------

Django supports MySQL 5.0.3 and higher.

`MySQL 5.0`_ adds the ``information_schema`` database, which contains detailed
data on all database schema. Django's ``inspectdb`` feature uses it.

.. versionchanged:: 1.5
    The minimum version requirement of MySQL 5.0.3 was set in Django 1.5.

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: http://www.mysql.com/
.. _MySQL 5.0: http://dev.mysql.com/doc/refman/5.0/en/index.html

.. _mysql-storage-engines:

Storage engines
---------------

MySQL has several `storage engines`_ (previously called table types). You can
change the default storage engine in the server configuration.

Until MySQL 5.5.4, the default engine was MyISAM_ [#]_. The main drawbacks of
MyISAM are that it doesn't support transactions or enforce foreign-key
constraints. On the plus side, it's currently the only engine that supports
full-text indexing and searching.

Since MySQL 5.5.5, the default storage engine is InnoDB_. This engine is fully
transactional and supports foreign key references. It's probably the best
choice at this point.

If you upgrade an existing project to MySQL 5.5.5 and subsequently add some
tables, ensure that your tables are using the same storage engine (i.e. MyISAM
vs. InnoDB). Specifically, if tables that have a ``ForeignKey`` between them
use different storage engines, you may see an error like the following when
running ``syncdb``::

    _mysql_exceptions.OperationalError: (
        1005, "Can't create table '\\db_name\\.#sql-4a8_ab' (errno: 150)"
    )

.. _storage engines: http://dev.mysql.com/doc/refman/5.5/en/storage-engines.html
.. _MyISAM: http://dev.mysql.com/doc/refman/5.5/en/myisam-storage-engine.html
.. _InnoDB: http://dev.mysql.com/doc/refman/5.5/en/innodb.html

.. [#] Unless this was changed by the packager of your MySQL package. We've
   had reports that the Windows Community Server installer sets up InnoDB as
   the default storage engine, for example.

MySQLdb
-------

`MySQLdb`_ is the Python interface to MySQL. Version 1.2.1p2 or later is
required for full MySQL support in Django.

.. note::
    If you see ``ImportError: cannot import name ImmutableSet`` when trying to
    use Django, your MySQLdb installation may contain an outdated ``sets.py``
    file that conflicts with the built-in module of the same name from Python
    2.4 and later. To fix this, verify that you have installed MySQLdb version
    1.2.1p2 or newer, then delete the ``sets.py`` file in the MySQLdb
    directory that was left by an earlier version.

.. _MySQLdb: http://sourceforge.net/projects/mysql-python

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: http://dev.mysql.com/doc/refman/5.0/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. It can be set on a database-wide
level and also per-table and per-column. This is `documented thoroughly`_ in
the MySQL documentation. In all cases, you set the collation by directly
manipulating the database tables; Django doesn't provide a way to set this on
the model definition.

.. _documented thoroughly: http://dev.mysql.com/doc/refman/5.0/en/charset.html

By default, with a UTF-8 database, MySQL will use the
``utf8_general_ci_swedish`` 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.

In many cases, this default will not be a problem. However, if you really want
case-sensitive comparisons on a particular column or table, you would change
the column or table to use the ``utf8_bin`` collation. The main thing to be
aware of in this case is that if you are using MySQLdb 1.2.2, the database
backend in Django will then return bytestrings (instead of unicode strings) for
any character fields it receive from the database. This is a strong variation
from Django's normal practice of *always* returning unicode strings. It is up
to you, the developer, to handle the fact that you will receive bytestrings if
you configure your table(s) to use ``utf8_bin`` collation. Django itself should
mostly work smoothly with such columns (except for the ``contrib.sessions``
``Session`` and ``contrib.admin`` ``LogEntry`` tables described below), but
your code must be prepared to call ``django.utils.encoding.smart_text()`` at
times if it really wants to work with consistent data -- Django will not do
this for you (the database backend layer and the model population layer are
separated internally so the database layer doesn't know it needs to make this
conversion in this one particular case).

If you're using MySQLdb 1.2.1p2, Django's standard
:class:`~django.db.models.CharField` class will return unicode strings even
with ``utf8_bin`` collation. However, :class:`~django.db.models.TextField`
fields will be returned as an ``array.array`` instance (from Python's standard
``array`` module). There isn't a lot Django can do about that, since, again,
the information needed to make the necessary conversions isn't available when
the data is read in from the database. This problem was `fixed in MySQLdb
1.2.2`_, so if you want to use :class:`~django.db.models.TextField` with
``utf8_bin`` collation, upgrading to version 1.2.2 and then dealing with the
bytestrings (which shouldn't be too difficult) as described above is the
recommended solution.

Should you decide to use ``utf8_bin`` collation for some of your tables with
MySQLdb 1.2.1p2 or 1.2.2, you should still use ``utf8_collation_ci_swedish``
(the default) collation for the ``django.contrib.sessions.models.Session``
table (usually called ``django_session``) and the
``django.contrib.admin.models.LogEntry`` table (usually called
``django_admin_log``). Those are the two standard tables that use
:class:`~django.db.models.TextField` internally.

.. _fixed in MySQLdb 1.2.2: http://sourceforge.net/tracker/index.php?func=detail&aid=1495765&group_id=22307&atid=374932

Connecting to the database
--------------------------

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

Connection settings are used in this order:

1. :setting:`OPTIONS`.
2. :setting:`NAME`, :setting:`USER`, :setting:`PASSWORD`,
   :setting:`HOST`, :setting:`PORT`
3. 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``,
``use_unicode``, ``init_command``, and ``sql_mode``. Consult the
`MySQLdb documentation`_ for more details.

.. _MySQL option file: http://dev.mysql.com/doc/refman/5.0/en/option-files.html
.. _MySQLdb documentation: http://mysql-python.sourceforge.net/

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 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.

* Another method for changing the storage engine is described in
  AlterModelOnSyncDB_.

.. _AlterModelOnSyncDB: https://code.djangoproject.com/wiki/AlterModelOnSyncDB

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: http://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>`, but this feature wasn't available in
Django until version 1.4 when such support was added.

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

Character fields
~~~~~~~~~~~~~~~~

Any fields that are stored with ``VARCHAR`` column types 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` and
:class:`~django.db.models.CommaSeparatedIntegerField`.

DateTime fields
~~~~~~~~~~~~~~~

MySQL does not have a timezone-aware column type. If an attempt is made to
store a timezone-aware ``time`` or ``datetime`` to a
:class:`~django.db.models.TimeField` or :class:`~django.db.models.DateTimeField`
respectively, a ``ValueError`` is raised rather than truncating data.

MySQL does not store fractions of seconds. Fractions of seconds are truncated
to zero when the time is stored.

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

MySQL does not support the ``NOWAIT`` option to the ``SELECT ... FOR UPDATE``
statement. If ``select_for_update()`` is used with ``nowait=True`` then a
``DatabaseError`` will be raised.

.. _sqlite-notes:

SQLite notes
============

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: http://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 utilised 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: http://www.sqlite.org/faq.html#q18

SQLite 3.3.6 or newer strongly recommended
------------------------------------------

Versions of SQLite 3.3.5 and older contains the following bugs:

* A bug when `handling`_ ``ORDER BY`` parameters. This can cause problems when
  you use the ``select`` parameter for the ``extra()`` QuerySet method. The bug
  can be identified by the error message ``OperationalError: ORDER BY terms
  must not be non-integer constants``.

* A bug when handling `aggregation`_ together with DateFields and
  DecimalFields.

.. _handling: http://www.sqlite.org/cvstrac/tktview?tn=1768
.. _aggregation: https://code.djangoproject.com/ticket/10031

SQLite 3.3.6 was released in April 2006, so most current binary distributions
for different platforms include newer version of SQLite usable from Python
through either the ``pysqlite2`` or the ``sqlite3`` modules.

Version 3.5.9
-------------

The Ubuntu "Intrepid Ibex" (8.10) SQLite 3.5.9-3 package contains a bug that
causes problems with the evaluation of query expressions. If you are using
Ubuntu "Intrepid Ibex", you will need to update the package to version
3.5.9-3ubuntu1 or newer (recommended) or find an alternate source for SQLite
packages, or install SQLite from source.

At one time, Debian Lenny shipped with the same malfunctioning SQLite 3.5.9-3
package. However the Debian project has subsequently issued updated versions
of the SQLite package that correct these bugs. If you find you are getting
unexpected results under Debian, ensure you have updated your SQLite package
to 3.5.9-5 or later.

The problem does not appear to exist with other versions of SQLite packaged
with other operating systems.

Version 3.6.2
--------------

SQLite version 3.6.2 (released August 30, 2008) introduced a bug into ``SELECT
DISTINCT`` handling that is triggered by, amongst other things, Django's
``DateQuerySet`` (returned by the ``dates()`` method on a queryset).

You should avoid using this version of SQLite with Django. Either upgrade to
3.6.3 (released September 22, 2008) or later, or downgrade to an earlier
version of SQLite.

.. _using-newer-versions-of-pysqlite:

Using newer versions of the SQLite DB-API 2.0 driver
----------------------------------------------------

For versions of Python 2.5 or newer that include ``sqlite3`` in the standard
library Django will now use a ``pysqlite2`` interface in preference to
``sqlite3`` if it finds one is available.

This provides the ability to upgrade both the DB-API 2.0 interface or SQLite 3
itself to versions newer than the ones included with your particular Python
binary distribution, if needed.

"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 option::

      'OPTIONS': {
          # ...
          'timeout': 20,
          # ...
      }

  This will simply 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.

.. _sqlite-connection-queries:

Parameters not quoted in ``connection.queries``
-----------------------------------------------

``sqlite3`` does not provide a way to retrieve the SQL after quoting and
substituting the parameters. Instead, the SQL in ``connection.queries`` is
rebuilt with a simple string interpolation. It may be incorrect. Make sure
you add quotes where necessary before copying a query into a SQLite shell.

.. _oracle-notes:

Oracle notes
============

Django supports `Oracle Database Server`_ versions 9i and
higher. Oracle version 10g or later is required to use Django's
``regex`` and ``iregex`` query operators. You will also need at least
version 4.3.1 of the `cx_Oracle`_ Python driver.

Note that due to a Unicode-corruption bug in ``cx_Oracle`` 5.0, that
version of the driver should **not** be used with Django;
``cx_Oracle`` 5.0.1 resolved this issue, so if you'd like to use a
more recent ``cx_Oracle``, use version 5.0.1.

``cx_Oracle`` 5.0.1 or greater can optionally be compiled with the
``WITH_UNICODE`` environment variable.  This is recommended but not
required.

.. _`Oracle Database Server`: http://www.oracle.com/
.. _`cx_Oracle`: http://cx-oracle.sourceforge.net/

In order for the ``python manage.py syncdb`` 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 Django's test suite, the user needs these *additional* privileges:

* CREATE USER
* DROP USER
* CREATE TABLESPACE
* DROP TABLESPACE
* CONNECT WITH ADMIN OPTION
* RESOURCE WITH ADMIN OPTION

Connecting to the database
--------------------------

Your Django settings.py file should look something like this for Oracle::

    DATABASES = {
        'default': {
            'ENGINE': 'django.db.backends.oracle',
            'NAME': 'xe',
            'USER': 'a_user',
            'PASSWORD': 'a_password',
            'HOST': '',
            'PORT': '',
        }
    }


If you don't use a ``tnsnames.ora`` file or a similar naming method that
recognizes 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 supply both :setting:`HOST` and :setting:`PORT`, or leave both
as empty strings.

Threaded option
----------------

If you plan to run Django in a multithreaded environment (e.g. Apache in Windows
using the default MPM module), 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.

When running syncdb, 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.

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 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.

.. _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:

* `Sybase SQL Anywhere`_
* `IBM DB2`_
* `Microsoft SQL Server 2005`_
* Firebird_
* ODBC_
* ADSDB_

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.

.. _Sybase SQL Anywhere: http://code.google.com/p/sqlany-django/
.. _IBM DB2: http://code.google.com/p/ibm-db/
.. _Microsoft SQL Server 2005: http://code.google.com/p/django-mssql/
.. _Firebird: http://code.google.com/p/django-firebird/
.. _ODBC: http://code.google.com/p/django-pyodbc/
.. _ADSDB: http://code.google.com/p/adsdb-django/