Databases¶

Persistent connections¶

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 integer of seconds. For unlimited persistent connections, set it to None.

Encoding¶

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.

PostgreSQL connection settings¶

See HOST for details.

Optimizing PostgreSQL’s configuration¶

Django needs the following parameters for its database connections:

• client_encoding: 'UTF8',
• default_transaction_isolation: 'read committed' by default, or the value set in the connection options (see below),

• timezone:

  • when USE_TZ is True, 'UTC' by default, or the TIME_ZONE value set for the connection,
  • when USE_TZ is False, the value of the global TIME_ZONE setting.

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.

Isolation level¶

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,
    },
}

Indexes for varchar and text columns¶

When specifying db_index=True on your model fields, Django typically outputs a single CREATE INDEX statement. However, if the database type for the field is either varchar or text (e.g., used by CharField, FileField, and TextField), then Django will create an additional index that uses an appropriate PostgreSQL operator class for the column. The extra index is necessary to correctly perform lookups that use the LIKE operator in their SQL, as is done with the contains and startswith lookup types.

Migration operation for adding extensions¶

If you need to add a PostgreSQL extension (like hstore, postgis, etc.) using a migration, use the CreateExtension operation.

Server-side cursors¶

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.

Manually-specifying values of auto-incrementing primary keys¶

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.

Test database templates¶

You can use the TEST['TEMPLATE'] setting to specify a template (e.g. 'template0') from which to create a test database.

Speeding up test execution with non-durable settings¶

You can speed up test execution times by configuring PostgreSQL to be non-durable.

Version support¶

Django supports MySQL 5.6 and higher.

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.

Storage engines¶

MySQL has several storage engines. You can change the default storage engine in the server configuration.

MySQL’s default storage engine is InnoDB. This engine is fully transactional and supports foreign key references. It’s the recommended choice. However, 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.

The main drawbacks of MyISAM are that it doesn’t support transactions or enforce foreign-key constraints.

MySQL DB API Drivers¶

MySQL has a couple drivers that implement the Python Database API described in PEP 249:

• mysqlclient is a native driver. It’s the recommended choice.
• MySQL Connector/Python is a pure Python driver from Oracle that does not require the MySQL client library or any Python modules outside the standard library.

These drivers are thread-safe and provide connection pooling.

In addition to a DB API driver, Django needs an adapter to access the database drivers from its ORM. Django provides an adapter for mysqlclient while MySQL Connector/Python includes its own.

Time zone definitions¶

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.

Creating your database