.. _ref-databases: =============================== Notes about supported 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. .. postgresql-notes: PostgreSQL notes ================ 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 an database backend 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://developer.postgresql.org/pgdocs/postgres/release-8-2-5.html Transaction handling --------------------- :ref:`By default `, Django starts a transaction when a database connection if first used and commits the result at the end of the request/response handling. The PostgreSQL backends normally operate the same as any other Django backend in this respect. Autocommit mode ~~~~~~~~~~~~~~~ .. versionadded:: 1.1 If your application is particularly read-heavy and doesn't make many database writes, the overhead of a constantly open transaction can sometimes be noticeable. For those situations, if you're using the ``postgresql_psycopg2`` backend, you can configure Django to use *"autocommit"* behavior for the connection, meaning that each database operation will normally be in its own transaction, rather than having the transaction extend over multiple operations. In this case, you can still manually start a transaction if you're doing something that requires consistency across multiple database operations. The autocommit behavior is enabled by setting the ``autocommit`` key in the :setting:`DATABASE_OPTIONS` setting:: DATABASE_OPTIONS = { "autocommit": True, } In this configuration, Django still ensures that :ref:`delete() ` and :ref:`update() ` queries run inside a single transaction, so that either all the affected objects are changed or none of them are. .. admonition:: This is database-level autocommit This functionality is not the same as the :ref:`topics-db-transactions-autocommit` decorator. That decorator is a Django-level implementation that commits automatically after data changing operations. The feature enabled using the :setting:`DATABASE_OPTIONS` settings provides autocommit behavior at the database adapter level. It commits after *every* operation. If you are using this feature and performing an operation akin to delete or updating that requires multiple operations, you are strongly recommended to wrap you operations in manual transaction handling to ensure data consistency. You should also audit your existing code for any instances of this behavior before enabling this feature. It's faster, but it provides less automatic protection for multi-call operations. .. _mysql-notes: MySQL notes =========== Django expects the database to support transactions, referential integrity, and Unicode support (UTF-8 encoding). Fortunately, MySQL_ has all these features as available as far back as 3.23. While it may be possible to use 3.23 or 4.0, you'll probably have less trouble if you use 4.1 or 5.0. MySQL 4.1 --------- `MySQL 4.1`_ has greatly improved support for character sets. It is possible to set different default character sets on the database, table, and column. Previous versions have only a server-wide character set setting. It's also the first version where the character set can be changed on the fly. 4.1 also has support for views, but Django currently doesn't use views. MySQL 5.0 --------- `MySQL 5.0`_ adds the ``information_schema`` database, which contains detailed data on all database schema. Django's ``inspectdb`` feature uses this ``information_schema`` if it's available. 5.0 also has support for stored procedures, but Django currently doesn't use stored procedures. .. _MySQL: http://www.mysql.com/ .. _MySQL 4.1: http://dev.mysql.com/doc/refman/4.1/en/index.html .. _MySQL 5.0: http://dev.mysql.com/doc/refman/5.0/en/index.html Storage engines --------------- MySQL has several `storage engines`_ (previously called table types). You can change the default storage engine in the server configuration. The default engine is MyISAM_ [#]_. The main drawback of MyISAM is that it doesn't currently support transactions or foreign keys. On the plus side, it's currently the only engine that supports full-text indexing and searching. The InnoDB_ engine is fully transactional and supports foreign key references. The BDB_ engine, like InnoDB, is also fully transactional and supports foreign key references. However, its use seems to be deprecated. `Other storage engines`_, including SolidDB_ and Falcon_, are on the horizon. For now, InnoDB is probably your best choice. .. _storage engines: http://dev.mysql.com/doc/refman/5.0/en/storage-engines.html .. _MyISAM: http://dev.mysql.com/doc/refman/5.0/en/myisam-storage-engine.html .. _BDB: http://dev.mysql.com/doc/refman/5.0/en/bdb-storage-engine.html .. _InnoDB: http://dev.mysql.com/doc/refman/5.0/en/innodb.html .. _Other storage engines: http://dev.mysql.com/doc/refman/5.1/en/storage-engines-other.html .. _SolidDB: http://forge.mysql.com/projects/project.php?id=139 .. _Falcon: http://dev.mysql.com/doc/falcon/en/index.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 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 returns 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 work smoothly with such columns, but if your code must be prepared to call ``django.utils.encoding.smart_unicode()`` 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) is the recommended solution. Should you decide to use ``utf8_bin`` collation for some of your tables with MySQLdb 1.2.1p2, you should still use ``utf8_collation_ci_swedish`` (the default) collation for the :class:`django.contrib.sessions.models.Session` table (usually called ``django_session`` and the table :class:`django.contrib.admin.models.LogEntry` table (usually called ``django_admin_log``). Those are the two standard tables that use :class:`~django.db.model.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 :ref:`settings documentation `. Connection settings are used in this order: 1. :setting:`DATABASE_OPTIONS`. 2. :setting:`DATABASE_NAME`, :setting:`DATABASE_USER`, :setting:`DATABASE_PASSWORD`, :setting:`DATABASE_HOST`, :setting:`DATABASE_PORT` 3. MySQL option files. In other words, if you set the name of the database in ``DATABASE_OPTIONS``, this will take precedence over ``DATABASE_NAME``, which would override anything in a `MySQL option file`_. Here's a sample configuration which uses a MySQL option file:: # settings.py DATABASE_ENGINE = "mysql" DATABASE_OPTIONS = { 'read_default_file': '/path/to/my.cnf', } # my.cnf [client] database = DATABASE_NAME user = DATABASE_USER password = DATABASE_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 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:: DATABASE_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. * Another method for changing the storage engine is described in AlterModelOnSyncDB_. .. _AlterModelOnSyncDB: http://code.djangoproject.com/wiki/AlterModelOnSyncDB Notes on specific fields ------------------------ Boolean fields ~~~~~~~~~~~~~~ Since MySQL doesn't have a direct ``BOOLEAN`` column type, Django uses a ``TINYINT`` column with values of ``1`` and ``0`` to store values for the :class:`~django.db.models.BooleanField` model field. Refer to the documentation of that field for more details, but usually this won't be something that will matter unless you're printing out the field values and are expecting to see ``True`` and ``False.``. 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`. Furthermore, if you are using a version of MySQL prior to 5.0.3, all of those column types have a maximum length restriction of 255 characters, regardless of whether ``unique=True`` is specified or not. .. _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: String matching for non-ASCII strings -------------------------------------- SQLite doesn't support case-insensitive matching for non-ASCII strings. Some possible workarounds for this are `documented at sqlite.org`_, but they are not utilised by the default SQLite backend in Django. Therefore, if you are using the ``iexact`` lookup type in your queryset filters, be aware that it will not work as expected for non-ASCII strings. .. _documented at sqlite.org: http://www.sqlite.org/faq.html#q18 Versions prior to 3.3.6 ------------------------ Versions of SQLite 3.3.5 and older `contain 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``. The problem can be solved updating SQLite to version 3.3.6 or newer, possibly also updating the ``pysqlite2`` Python module in the process. .. _contain a bug: http://www.sqlite.org/cvstrac/tktview?tn=1768 This has a very low impact because 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. However, in the case of Windows, the official binary distribution of the stable release of Python 2.5 (2.5.2, as of this writing) includes SQLite 3.3.4, so the bug can make itself evident in that platform. There are (as of Django 1.0) even three tests in the Django test suite that will fail when run under this setup. As described above, this can be solved by downloading and installing a newer version of ``pysqlite2`` (``pysqlite-2.x.x.win32-py2.5.exe``) that includes and uses a newer version of SQLite. Python 2.6 ships with a newer version of SQLite and is not affected by this issue. If you are in such platform and find yourself in the need to update ``pysqlite``/SQLite, you will also need to manually modify the ``django/db/backends/sqlite3/base.py`` file in the Django source tree so it attempts to import ``pysqlite2`` before than ``sqlite3`` and so it can take advantage of the new ``pysqlite2``/SQLite versions. Version 3.5.9 ------------- The Ubuntu "Intrepid Ibex" 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 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. .. _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. .. _`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 Connecting to the database -------------------------- Your Django settings.py file should look something like this for Oracle:: DATABASE_ENGINE = 'oracle' DATABASE_NAME = 'xe' DATABASE_USER = 'a_user' DATABASE_PASSWORD = 'a_password' DATABASE_HOST = '' DATABASE_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:`DATABASE_HOST` and :setting:`DATABASE_PORT` like so:: DATABASE_ENGINE = 'oracle' DATABASE_NAME = 'xe' DATABASE_USER = 'a_user' DATABASE_PASSWORD = 'a_password' DATABASE_HOST = 'dbprod01ned.mycompany.com' DATABASE_PORT = '1540' You should supply both :setting:`DATABASE_HOST` and :setting:`DATABASE_PORT`, or leave both as empty strings. Tablespace options ------------------ A common paradigm for optimizing performance in Oracle-based systems is the use of `tablespaces`_ to organize disk layout. The Oracle backend supports this use case by adding ``db_tablespace`` options to the ``Meta`` and ``Field`` classes. (When you use a backend that lacks support for tablespaces, Django ignores these options.) .. _`tablespaces`: http://en.wikipedia.org/wiki/Tablespace A tablespace can be specified for the table(s) generated by a model by supplying the ``db_tablespace`` option inside the model's ``class Meta``. Additionally, you can pass the ``db_tablespace`` option to a ``Field`` constructor to specify an alternate tablespace for the ``Field``'s column index. If no index would be created for the column, the ``db_tablespace`` option is ignored:: class TablespaceExample(models.Model): name = models.CharField(max_length=30, db_index=True, db_tablespace="indexes") data = models.CharField(max_length=255, db_index=True) edges = models.ManyToManyField(to="self", db_tablespace="indexes") class Meta: db_tablespace = "tables" In this example, the tables generated by the ``TablespaceExample`` model (i.e., the model table and the many-to-many table) would be stored in the ``tables`` tablespace. The index for the name field and the indexes on the many-to-many table would be stored in the ``indexes`` tablespace. The ``data`` field would also generate an index, but no tablespace for it is specified, so it would be stored in the model tablespace ``tables`` by default. .. versionadded:: 1.0 Use the :setting:`DEFAULT_TABLESPACE` and :setting:`DEFAULT_INDEX_TABLESPACE` settings to specify default values for the db_tablespace options. These are useful for setting a tablespace for the built-in Django apps and other applications whose code you cannot control. Django does not create the tablespaces for you. Please refer to `Oracle's documentation`_ for details on creating and managing tablespaces. .. _`Oracle's documentation`: http://download.oracle.com/docs/cd/B19306_01/server.102/b14200/statements_7003.htm#SQLRF01403 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. 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 coerces the ``null=True`` option on fields that have the empty string as a possible value. 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. A workaround to this is to keep ``TextField`` columns out of any models that you foresee performing ``distinct()`` queries on, and to include the ``TextField`` in a related model instead.