Databases¶

Persistent connections¶

Persistent connections avoid the overhead of reestablishing a connection to the database in each HTTP 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.

To connect using a service name from the connection service file and a password from the password file, you must specify them in the OPTIONS part of your database configuration in DATABASES:

DATABASES = {
    "default": {
        "ENGINE": "django.db.backends.postgresql",
        "OPTIONS": {
            "service": "my_service",
            "passfile": ".my_pgpass",
        },
    }
}

[my_service]
host=localhost
user=USER
dbname=NAME
port=5432

localhost:5432:NAME:USER:PASSWORD

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:

from django.db.backends.postgresql.psycopg_any import IsolationLevel

DATABASES = {
    # ...
    "OPTIONS": {
        "isolation_level": IsolationLevel.SERIALIZABLE,
    },
}

Role¶

If you need to use a different role for database connections than the role use to establish the connection, set it in the OPTIONS part of your database configuration in DATABASES:

DATABASES = {
    "default": {
        "ENGINE": "django.db.backends.postgresql",
        # ...
        "OPTIONS": {
            "assume_role": "my_application_role",
        },
    },
}

Server-side parameters binding¶

With psycopg 3.1.8+, Django defaults to the client-side binding cursors. If you want to use the server-side binding set it in the OPTIONS part of your database configuration in DATABASES:

DATABASES = {
    "default": {
        "ENGINE": "django.db.backends.postgresql",
        # ...
        "OPTIONS": {
            "server_side_binding": True,
        },
    },
}

This option is ignored with psycopg2.

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 identity columns to store auto-incrementing primary keys. An identity 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 afterward 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 8 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¶

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.

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, init_command, and sql_mode.

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 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.

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.

Savepoints¶

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.

Notes on specific fields¶

ALTER TABLE `your_table` MODIFY `your_datetime_column` DATETIME(6)

Row locking with QuerySet.select_for_update()¶

MySQL and MariaDB do not support some options to the SELECT ... FOR UPDATE statement. If select_for_update() is used with an unsupported option, then a NotSupportedError is raised.

When using select_for_update() on MySQL, make sure you filter a queryset against at least a set of fields contained in unique constraints or only against fields covered by indexes. Otherwise, an exclusive write lock will be acquired over the full table for the duration of the transaction.

Automatic typecasting can cause unexpected results¶

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.

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 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.

Decimal handling¶

SQLite has no real decimal internal type. Decimal values are internally converted to the REAL data type (8-byte IEEE floating point number), as explained in the SQLite datatypes documentation, so they don’t support correctly-rounded decimal floating point arithmetic.

«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:

  "OPTIONS": {
      # ...
      "timeout": 20,
      # ...
  }
  

  This will 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.

Isolation when using QuerySet.iterator()¶

There are special considerations described in Isolation In SQLite when modifying a table while iterating over it using QuerySet.iterator(). If a row is added, changed, or deleted within the loop, then that row may or may not appear, or may appear twice, in subsequent results fetched from the iterator. Your code must handle this.

Enabling JSON1 extension on SQLite¶

To use JSONField on SQLite, you need to enable the JSON1 extension on Python’s sqlite3 library. If the extension is not enabled on your installation, a system error (fields.E180) will be raised.

To enable the JSON1 extension you can follow the instruction on the wiki page.

Connecting to the database¶

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.

"NAME": "localhost:1521/orclpdb1"

"NAME": (
    "(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=localhost)(PORT=1521))"
    "(CONNECT_DATA=(SERVICE_NAME=orclpdb1)))"
)

Threaded option¶

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.

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. 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.

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 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:

• 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 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.