98 lines
4.0 KiB
Plaintext
98 lines
4.0 KiB
Plaintext
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?
|
|
-----------------------------------------------------------
|
|
|
|
Take a look at Django's support for :mod:`schema migrations
|
|
<django.db.migrations>`.
|
|
|
|
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:`migrate` was executed.
|
|
|
|
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()
|