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.
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.
By default, Django starts a transaction when a database connection is 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.
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 DATABASE_OPTIONS setting:
DATABASE_OPTIONS = {
"autocommit": True,
}
In this configuration, Django still ensures that delete() and update() queries run inside a single transaction, so that either all the affected objects are changed or none of them are.
This is database-level autocommit
This functionality is not the same as the django.db.transaction.autocommit decorator. That decorator is a Django-level implementation that commits automatically after data changing operations. The feature enabled using the 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.
Django expects the database to support transactions, referential integrity, and Unicode (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 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 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 has several storage engines (previously called table types). You can change the default storage engine in the server configuration.
The default engine is MyISAM [1]. 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.
| [1] | 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 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.
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.
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.
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 CharField class will return unicode strings even with utf8_bin collation. However, 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 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 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 TextField internally.
Refer to the settings documentation.
Connection settings are used in this order:
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.
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:
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.
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 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..
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 CharField, SlugField and 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 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 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.
Versions of SQLite 3.3.5 and older contains the following bugs:
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.
However, some platform/Python version combinations include older versions of SQLite (e.g. the official binary distribution of Python 2.5 for Windows, 2.5.4 as of this writing, includes SQLite 3.3.4). There are (as of Django 1.1) even some tests in the Django test suite that will fail when run under this setup.
As described below, this can be solved by downloading and installing a newer version of pysqlite2 (pysqlite-2.x.x.win32-py2.5.exe in the described case) that includes and uses a newer version of SQLite. Python 2.6 for Windows ships with a version of SQLite that is not affected by these issues.
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.
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.
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.
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:
DATABASE_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.
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.
In order for the python manage.py syncdb command to work, your Oracle database user must have privileges to run the following commands:
To run Django's test suite, the user needs these additional privileges:
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 DATABASE_HOST and 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 DATABASE_HOST and DATABASE_PORT, or leave both as empty strings.
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.)
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.
Use the DEFAULT_TABLESPACE and 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 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.
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.
The Oracle backend stores TextFields as NCLOB columns. Oracle imposes some limitations on the usage of such LOB columns in general:
In addition to the officially supported databases, there are backends provided by 3rd parties that allow you to use other databases with Django:
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.
Sep 20, 2009