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.
Persistent connections avoid the overhead of re-establishing a connection to
the database in each request. They’re controlled by the
CONN_MAX_AGE parameter which defines the maximum lifetime of a
connection. It can be set independently for each database.
The default value is 0, preserving the historical behavior of closing the
database connection at the end of each request. To enable persistent
connections, set CONN_MAX_AGE to a positive number of seconds. For
unlimited persistent connections, set it to None.
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
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 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.
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 CONN_MAX_AGE to a low value or even
0, because it doesn’t make sense to maintain a connection that’s unlikely
to be reused. This will help keep the number of simultaneous connections to
this database small.
The development server creates a new thread for each request it handles, negating the effect of persistent connections. Don’t enable them during development.
When Django establishes a connection to the database, it sets up appropriate parameters, depending on the backend being used. If you enable persistent connections, this setup is no longer repeated every request. If you modify parameters such as the connection’s isolation level or time zone, you should either restore Django’s defaults at the end of each request, force an appropriate value at the beginning of each request, or disable persistent connections.
Django assumes that all databases use UTF-8 encoding. Using other encodings may result in unexpected behavior such as “value too long” errors from your database for data that is valid in Django. See the database specific notes below for information on how to set up your database correctly.
Django supports PostgreSQL 9.3 and higher. psycopg2 2.5.4 through 2.7.7 is required, though the 2.7.7 is recommended.
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 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.
Like PostgreSQL itself, Django defaults to the READ COMMITTED isolation
level. If you need a higher isolation level such as REPEATABLE READ or
SERIALIZABLE, set it in the OPTIONS part of your database
configuration in 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.
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.
If you need to add a PostgreSQL extension (like hstore, postgis, etc.)
using a migration, use the
CreateExtension operation.
When using QuerySet.iterator(), Django opens a server-side
cursor. By default, PostgreSQL assumes that
only the first 10% of the results of cursor queries will be fetched. The query
planner spends less time planning the query and starts returning results
faster, but this could diminish performance if more than 10% of the results are
retrieved. PostgreSQL’s assumptions on the number of rows retrieved for a
cursor query is controlled with the cursor_tuple_fraction option.
Using a connection pooler in transaction pooling mode (e.g. pgBouncer) requires disabling server-side cursors for that connection.
Server-side cursors are local to a connection and remain open at the end of a
transaction when AUTOCOMMIT is True. A
subsequent transaction may attempt to fetch more results from a server-side
cursor. In transaction pooling mode, there’s no guarantee that subsequent
transactions will use the same connection. If a different connection is used,
an error is raised when the transaction references the server-side cursor,
because server-side cursors are only accessible in the connection in which they
were created.
One solution is to disable server-side cursors for a connection in
DATABASES by setting DISABLE_SERVER_SIDE_CURSORS to True.
To benefit from server-side cursors in transaction pooling mode, you could set up another connection to the database in order to perform queries that use server-side cursors. This connection needs to either be directly to the database or to a connection pooler in session pooling mode.
Another option is to wrap each QuerySet using server-side cursors in an
atomic() block, because it disables autocommit
for the duration of the transaction. This way, the server-side cursor will only
live for the duration of the transaction.
Django uses PostgreSQL’s SERIAL data type to store auto-incrementing primary
keys. A SERIAL column is populated with values from a sequence that
keeps track of the next available value. Manually assigning a value to an
auto-incrementing field doesn’t update the field’s sequence, which might later
cause a conflict. For example:
>>> from django.contrib.auth.models import User
>>> User.objects.create(username='alice', pk=1)
<User: alice>
>>> # The sequence hasn't been updated; its next value is 1.
>>> User.objects.create(username='bob')
...
IntegrityError: duplicate key value violates unique constraint
"auth_user_pkey" DETAIL: Key (id)=(1) already exists.
If you need to specify such values, reset the sequence afterwards to avoid
reusing a value that’s already in the table. The sqlsequencereset
management command generates the SQL statements to do that.
You can use the TEST['TEMPLATE'] setting to specify
a template (e.g. 'template0') from which to create a test database.
You can speed up test execution times by configuring PostgreSQL to be non-durable.
Warning
This is dangerous: it will make your database more susceptible to data loss or corruption in the case of a server crash or power loss. Only use this on a development machine where you can easily restore the entire contents of all databases in the cluster.
Django supports MySQL 5.5.x - 5.7.x. MySQL 8 and later aren’t supported.
Django’s inspectdb feature uses the information_schema database, which
contains detailed data on all database schemas.
Django expects the database to support Unicode (UTF-8 encoding) and delegates to it the task of enforcing transactions and referential integrity. It is important to be aware of the fact that the two latter ones aren’t actually enforced by MySQL when using the MyISAM storage engine, see the next section.
MySQL has several storage engines. 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 was the only engine that supported full-text indexing and searching until MySQL 5.6.4.
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. However, note that the InnoDB autoincrement counter
is lost on a MySQL restart because it does not remember the
AUTO_INCREMENT value, instead recreating it as “max(id)+1”. This may
result in an inadvertent reuse of AutoField values.
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 migrate:
_mysql_exceptions.OperationalError: (
1005, "Can't create table '\\db_name\\.#sql-4a8_ab' (errno: 150)"
)
| [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. |
The Python Database API is described in PEP 249. MySQL has three prominent drivers that implement this API:
MySQLdb which notably supports Python 3 and
can be used as a drop-in replacement for MySQLdb. At the time of this writing,
this is the recommended choice for using MySQL with Django.All these drivers are thread-safe and provide connection pooling. MySQLdb
is the only one not supporting Python 3 currently.
In addition to a DB API driver, Django needs an adapter to access the database drivers from its ORM. Django provides an adapter for MySQLdb/mysqlclient while MySQL Connector/Python includes its own.
Django requires MySQLdb version 1.2.3 or later.
At the time of writing, the latest release of MySQLdb (1.2.5) doesn’t support
Python 3. In order to use MySQLdb under Python 3, you’ll have to install
mysqlclient instead.
Note
There are known issues with the way MySQLdb converts date strings into
datetime objects. Specifically, date strings with value 0000-00-00 are
valid for MySQL but will be converted into None by MySQLdb.
This means you should be careful while using loaddata and
dumpdata with rows that may have 0000-00-00 values, as they
will be converted to None.
Django supports mysqlclient 1.3.3 through 1.3.13.
MySQL Connector/Python is available from the download page. The Django adapter is available in versions 1.1.X and later. It may not support the most recent releases of Django.
If you plan on using Django’s timezone support, use mysql_tzinfo_to_sql to load time zone tables into the MySQL database. This needs to be done just once for your MySQL server, not per database.
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 collation. This results in all string equality
comparisons being done in a case-insensitive manner. That is, "Fred" and
"freD" are considered equal at the database level. If you have a unique
constraint on a field, it would be illegal to try to insert both "aa" and
"AA" into the same column, since they compare as equal (and, hence,
non-unique) with the default collation. If you want case-sensitive comparisons
on a particular column or table, change the column or table to use the
utf8_bin collation.
Please note that according to MySQL Unicode Character Sets, comparisons for
the utf8_general_ci collation are faster, but slightly less correct, than
comparisons for utf8_unicode_ci. If this is acceptable for your application,
you should use utf8_general_ci because it is faster. If this is not acceptable
(for example, if you require German dictionary order), use utf8_unicode_ci
because it is more accurate.
Warning
Model formsets validate unique fields in a case-sensitive manner. Thus when
using a case-insensitive collation, a formset with unique field values that
differ only by case will pass validation, but upon calling save(), an
IntegrityError will be raised.
Refer to the settings documentation.
Connection settings are used in this order:
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,
init_command, and sql_mode.
sql_mode¶From MySQL 5.7 onwards and on fresh installs of MySQL 5.6, the default value of
the sql_mode option contains STRICT_TRANS_TABLES. That option escalates
warnings into errors when data are truncated upon insertion, so Django highly
recommends activating a strict mode for MySQL to prevent data loss (either
STRICT_TRANS_TABLES or STRICT_ALL_TABLES).
If you need to customize the SQL mode, you can set the sql_mode variable
like other MySQL options: either in a config file or with the entry
'init_command': "SET sql_mode='STRICT_TRANS_TABLES'" in the
OPTIONS part of your database configuration in DATABASES.
When running concurrent loads, database transactions from different sessions
(say, separate threads handling different requests) may interact with each
other. These interactions are affected by each session’s transaction isolation
level. You can set a connection’s isolation level with an
'isolation_level' entry in the OPTIONS part of your database
configuration in DATABASES. Valid values for
this entry are the four standard isolation levels:
'read uncommitted''read committed''repeatable read''serializable'or None to use the server’s configured isolation level. However, Django
works best with read committed rather than MySQL’s default, repeatable read.
Data loss is possible with repeatable read.
When Django generates the schema, it doesn’t specify a storage engine, so tables will be created with whatever default storage engine your database server is configured for. The easiest solution is to set your database server’s default storage engine to the desired engine.
If you’re using a hosting service and can’t change your server’s default storage engine, you have a couple of options.
After the tables are created, execute an ALTER TABLE statement to
convert a table to a new storage engine (such as InnoDB):
ALTER TABLE <tablename> ENGINE=INNODB;
This can be tedious if you have a lot of tables.
Another option is to use the init_command option for MySQLdb prior to
creating your tables:
'OPTIONS': {
'init_command': 'SET default_storage_engine=INNODB',
}
This sets the default storage engine upon connecting to the database. After your tables have been created, you should remove this option as it adds a query that is only needed during table creation to each database connection.
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.
Both the Django ORM and MySQL (when using the InnoDB storage engine) support database savepoints.
If you use the MyISAM storage engine please be aware of the fact that you will receive database-generated errors if you try to use the savepoint-related methods of the transactions API. 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.
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.
TextField limitations¶MySQL can index only the first N chars of a BLOB or TEXT column. Since
TextField doesn’t have a defined length, you can’t mark it as
unique=True. MySQL will report: “BLOB/TEXT column ‘<db_column>’ used in key
specification without a key length”.
MySQL 5.6.4 and later can store fractional seconds, provided that the
column definition includes a fractional indication (e.g. DATETIME(6)).
Earlier versions do not support them at all. In addition, versions of MySQLdb
older than 1.2.5 have a bug that also prevents the use of fractional seconds
with MySQL.
Django will not upgrade existing columns to include fractional seconds if the database server supports it. If you want to enable them on an existing database, it’s up to you to either manually update the column on the target database, by executing a command like:
ALTER TABLE `your_table` MODIFY `your_datetime_column` DATETIME(6)
or using a RunSQL operation in a
data migration.
TIMESTAMP columns¶If you are using a legacy database that contains TIMESTAMP columns, you must
set USE_TZ = False to avoid data corruption.
inspectdb maps these columns to
DateTimeField and if you enable timezone support,
both MySQL and Django will attempt to convert the values from UTC to local time.
QuerySet.select_for_update()¶MySQL does not support the NOWAIT and SKIP LOCKED options to the
SELECT ... FOR UPDATE statement. If select_for_update() is used with
nowait=True or skip_locked=True then a DatabaseError will be raised.
When performing a query on a string type, but with an integer value, MySQL will
coerce the types of all values in the table to an integer before performing the
comparison. If your table contains the values 'abc', 'def' and you
query for WHERE mycolumn=0, both rows will match. Similarly, WHERE mycolumn=1
will match the value 'abc1'. Therefore, string type fields included in Django
will always cast the value to a string before using it in a query.
If you implement custom model fields that inherit from
Field directly, are overriding
get_prep_value(), or use
RawSQL,
extra(), or
raw(), you should ensure that you perform
appropriate typecasting.
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.
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 utilized by the default SQLite backend in Django, as incorporating them would be fairly difficult to do robustly. Thus, Django exposes the default SQLite behavior and you should be aware of this when doing case-insensitive or substring filtering.
CASE expressions¶SQLite 3.6.23.1 and older contains a bug when handling query parameters in
a CASE expression that contains an ELSE and arithmetic.
SQLite 3.6.23.1 was released in March 2010, and most current binary distributions for different platforms include a newer version of SQLite, with the notable exception of the Python 2.7 installers for Windows.
As of this writing, the latest release for Windows - Python 2.7.10 - includes
SQLite 3.6.21. You can install pysqlite2 or replace sqlite3.dll (by
default installed in C:\Python27\DLLs) with a newer version from
https://www.sqlite.org/ to remedy this issue.
Django will use a pysqlite2 module in preference to sqlite3 as shipped
with the Python standard library 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:
'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.
For most backends, raw queries (Manager.raw() or cursor.execute())
can use the “pyformat” parameter style, where placeholders in the query
are given as '%(name)s' and the parameters are passed as a dictionary
rather than a list. SQLite does not support this.
Django supports Oracle Database Server versions 11.2 and higher. Version 5.2 through 6.4.1 of the cx_Oracle Python driver are supported.
In order for the python manage.py migrate command to work, your Oracle
database user must have privileges to run the following commands:
To run a project’s test suite, the user usually needs these additional privileges:
Note that, while the RESOURCE role has the required CREATE TABLE, CREATE SEQUENCE, CREATE PROCEDURE and CREATE TRIGGER privileges, and a user granted RESOURCE WITH ADMIN OPTION can grant RESOURCE, such a user cannot grant the individual privileges (e.g. CREATE TABLE), and thus RESOURCE WITH ADMIN OPTION is not usually sufficient for running tests.
Some test suites also create views; to run these, the user also needs the CREATE VIEW WITH ADMIN OPTION privilege. In particular, this is needed for Django’s own test suite.
All of these privileges are included in the DBA role, which is appropriate for use on a private developer’s database.
The Oracle database backend uses the SYS.DBMS_LOB and SYS.DBMS_RANDOM
packages, so your user will require execute permissions on it. It’s normally
accessible to all users by default, but in case it is not, you’ll need to grant
permissions like so:
GRANT EXECUTE ON SYS.DBMS_LOB TO user;
GRANT EXECUTE ON SYS.DBMS_RANDOM TO user;
To connect using the service name of your Oracle database, your settings.py
file should look something like this:
DATABASES = {
'default': {
'ENGINE': 'django.db.backends.oracle',
'NAME': 'xe',
'USER': 'a_user',
'PASSWORD': 'a_password',
'HOST': '',
'PORT': '',
}
}
In this case, you should leave both HOST and PORT empty.
However, if you don’t use a tnsnames.ora file or a similar naming method
and want to connect using the SID (“xe” in this example), then fill in both
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 either supply both HOST and PORT, or leave
both as empty strings. Django will use a different connect descriptor depending
on that choice.
If you plan to run Django in a multithreaded environment (e.g. Apache using the
default MPM module on any modern operating system), then you must set
the threaded option of your Oracle database configuration to True:
'OPTIONS': {
'threaded': True,
},
Failure to do this may result in crashes and other odd behavior.
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.
Oracle imposes a name length limit of 30 characters. To accommodate this, the backend truncates database identifiers to fit, replacing the final four characters of the truncated name with a repeatable MD5 hash value. Additionally, the backend turns database identifiers to all-uppercase.
To prevent these transformations (this is usually required only when dealing
with legacy databases or accessing tables which belong to other users), use
a quoted name as the value for db_table:
class LegacyModel(models.Model):
class Meta:
db_table = '"name_left_in_lowercase"'
class ForeignModel(models.Model):
class Meta:
db_table = '"OTHER_USER"."NAME_ONLY_SEEMS_OVER_30"'
Quoted names can also be used with Django’s other supported database backends; except for Oracle, however, the quotes have no effect.
When running migrate, an ORA-06552 error may be encountered if
certain Oracle keywords are used as the name of a model field or the
value of a db_column option. Django quotes all identifiers used
in queries to prevent most such problems, but this error can still
occur when an Oracle datatype is used as a column name. In
particular, take care to avoid using the names date,
timestamp, number or float as a field name.
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:
SELECT DISTINCT list. This means that
attempting to use the QuerySet.distinct method on a model that
includes TextField columns will result in an ORA-00932 error when
run against Oracle. As a workaround, use the QuerySet.defer method in
conjunction with distinct() to prevent TextField columns from being
included in the SELECT DISTINCT list.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.
Jun 14, 2020