# Τεκμηρίωση του Django - 1.10.7

# 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.

## General notes

### 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 number of seconds. For unlimited persistent connections, set it to `None`.

#### Connection management

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.

#### Caveats

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.

### 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 notes

Django supports PostgreSQL 9.2 and higher. It requires the use of psycopg2 2.4.5 or higher (or 2.5+ if you want to use `django.contrib.postgres`).

### 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`: `'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.

### 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,
        },
    }

Σημείωση

Under higher isolation levels, your application should be prepared to handle exceptions raised on serialization failures. This option is designed for advanced uses.

### 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.

### Speeding up test execution with non-durable settings

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

Προειδοποίηση

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.

## MySQL notes

### Version support

Django supports MySQL 5.5 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.

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.

### MySQL DB API Drivers

The Python Database API is described in PEP 249. MySQL has three prominent drivers that implement this API:

- MySQLdb is a native driver that has been developed and supported for over a decade by Andy Dustman.
- mysqlclient is a fork of `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.
- 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.

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.

#### MySQLdb

Django requires MySQLdb version 1.2.1p2 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.

Σημείωση

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`.

#### mysqlclient

Django requires mysqlclient 1.3.3 or later. Note that Python 3.2 is not supported. Except for the Python 3.3+ support, mysqlclient should mostly behave the same as MySQLDB.

#### MySQL Connector/Python

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.

### 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.

#### 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` 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_text()` 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_general_ci` (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.

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.

Προειδοποίηση

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.

### 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`. Consult the MySQLdb documentation for more details.

#### Setting `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`.

### 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

#### 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`.

#### `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”.

#### Fractional seconds support for Time and DateTime fields

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.

### 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.

### 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.

## 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 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.

### Old SQLite and `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.

### Using newer versions of the SQLite DB-API 2.0 driver

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.

### “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 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.

### “pyformat” parameter style in raw queries not supported

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.

## Oracle notes

Django supports Oracle Database Server versions 11.2 and higher. Versions 4.3.1 through 5.2.1 of the cx_Oracle Python driver are supported, although 5.1.3 or later is recommended as these versions support Python 3.

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 migrate` 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 a project’s test suite, the user usually needs these additional privileges:

- CREATE USER
- ALTER USER
- DROP USER
- CREATE TABLESPACE
- DROP TABLESPACE
- CREATE SESSION WITH ADMIN OPTION
- CREATE TABLE WITH ADMIN OPTION
- CREATE SEQUENCE WITH ADMIN OPTION
- CREATE PROCEDURE WITH ADMIN OPTION
- CREATE TRIGGER WITH ADMIN OPTION

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;

### 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.

### 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.

## 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:

- SAP SQL Anywhere
- IBM DB2
- Microsoft SQL Server
- Firebird
- ODBC

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.