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

Изменено в Django 1.4: Пожалуйста, обратитесь к описанию релиза

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.

Optimizing PostgreSQL’s configuration

Django needs the following parameters for its database connections:

  • client_encoding: 'UTF8',
  • default_transaction_isolation: 'read committed',
  • timezone: 'UTC' when USE_TZ is True, value of 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 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.

Transaction handling

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.

Autocommit mode

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, 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 OPTIONS part of your database configuration in DATABASES:

'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 autocommit decorator. That decorator is a Django-level implementation that commits automatically after data changing operations. The feature enabled using the OPTIONS option 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.

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 perfrom lookups that use the LIKE operator in their SQL, as is done with the contains and startswith lookup types.

MySQL notes

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

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.

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 [1]. 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.

Изменено в Django 1.4: Пожалуйста, обратитесь к описанию релиза

In previous versions of Django, fixtures with forward references (i.e. relations to rows that have not yet been inserted into the database) would fail to load when using the InnoDB storage engine. This was due to the fact that InnoDB deviates from the SQL standard by checking foreign key constraints immediately instead of deferring the check until the transaction is committed. This problem has been resolved in Django 1.4. Fixture data is now loaded with foreign key checks turned off; foreign key checks are then re-enabled when the data has finished loading, at which point the entire table is checked for invalid foreign key references and an IntegrityError is raised if any are found.

[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

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

Примечание

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.

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.

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.

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_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) 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 TextField internally.

Connecting to the database

Refer to the settings documentation.

Connection settings are used in this order:

  1. OPTIONS.
  2. NAME, USER, PASSWORD, HOST, PORT
  3. MySQL option files.

In other words, if you set the name of the database in OPTIONS, this will take precedence over 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.

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.

  • Another method for changing the storage engine is described in 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 db_table parameter.

Notes on specific fields

Boolean fields

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In previous versions of Django when running under MySQL BooleanFields would return their data as ints, instead of true bools. See the release notes for a complete description of the change.

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 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.

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 TimeField or 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 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.

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 iexact or 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 iexact filter will behave exactly the same as the 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.

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 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.

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

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.

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 HOST and 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 HOST and 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 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. As a workaround, use the QuerySet.defer method in conjunction with distinct() to prevent TextField columns from being included in the SELECT DISTINCT list.

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:

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.

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