django / docs / faq / models.txt

FAQ: Databases and models

.. _faq-see-raw-sql-queries:

How can I see the raw SQL queries Django is running?

Make sure your Django :setting:`DEBUG` setting is set to ``True``.
Then, just do this::

    >>> from django.db import connection
    >>> connection.queries
    [{'sql': 'SELECT,polls_polls.question,polls_polls.pub_date FROM polls_polls',
    'time': '0.002'}]

``connection.queries`` is only available if :setting:`DEBUG` is ``True``.
It's a list of dictionaries in order of query execution. Each dictionary has
the following::

    ``sql`` -- The raw SQL statement
    ``time`` -- How long the statement took to execute, in seconds.

``connection.queries`` includes all SQL statements -- INSERTs, UPDATES,
SELECTs, etc. Each time your app hits the database, the query will be recorded.
Note that the SQL recorded here may be :ref:`incorrectly quoted under SQLite

.. versionadded:: 1.2

If you are using :doc:`multiple databases</topics/db/multi-db>`, you can use the
same interface on each member of the ``connections`` dictionary::

    >>> from django.db import connections
    >>> connections['my_db_alias'].queries

Can I use Django with a pre-existing database?

Yes. See :doc:`Integrating with a legacy database </howto/legacy-databases>`.

If I make changes to a model, how do I update the database?

If you don't mind clearing data, your project's ```` utility has an
option to reset the SQL for a particular application:: reset appname

This drops any tables associated with ``appname`` and recreates them.

If you do care about deleting data, you'll have to execute the ``ALTER TABLE``
statements manually in your database.

There are `external projects which handle schema updates
<>`_, of which the current
defacto standard is `south <>`_.

Do Django models support multiple-column primary keys?

No. Only single-column primary keys are supported.

But this isn't an issue in practice, because there's nothing stopping you from
adding other constraints (using the ``unique_together`` model option or
creating the constraint directly in your database), and enforcing the
uniqueness at that level. Single-column primary keys are needed for things such
as the admin interface to work; e.g., you need a simple way of being able to
specify an object to edit or delete.

How do I add database-specific options to my CREATE TABLE statements, such as specifying MyISAM as the table type?

We try to avoid adding special cases in the Django code to accommodate all the
database-specific options such as table type, etc. If you'd like to use any of
these options, create an :ref:`SQL initial data file <initial-sql>` that
contains ``ALTER TABLE`` statements that do what you want to do. The initial
data files are executed in your database after the ``CREATE TABLE`` statements.

For example, if you're using MySQL and want your tables to use the MyISAM table
type, create an initial data file and put something like this in it::

    ALTER TABLE myapp_mytable ENGINE=MyISAM;

As explained in the :ref:`SQL initial data file <initial-sql>` documentation,
this SQL file can contain arbitrary SQL, so you can make any sorts of changes
you need to make.

Why is Django leaking memory?

Django isn't known to leak memory. If you find your Django processes are
allocating more and more memory, with no sign of releasing it, check to make
sure your :setting:`DEBUG` setting is set to ``False``. If :setting:`DEBUG`
is ``True``, then Django saves a copy of every SQL statement it has executed.

(The queries are saved in ``django.db.connection.queries``. See
`How can I see the raw SQL queries Django is running?`_.)

To fix the problem, set :setting:`DEBUG` to ``False``.

If you need to clear the query list manually at any point in your functions,
just call ``reset_queries()``, like this::

    from django import db