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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.id, 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
<sqlite-connection-queries>`.
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 ``manage.py`` utility has a
:djadmin:`flush` option to reset the database to the state it was in
immediately after :djadmin:`syncdb` was executed.
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
<http://www.djangopackages.com/grids/g/database-migration/>`_, of which the current
defacto standard is `south <http://south.aeracode.org/>`_.
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
db.reset_queries()
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