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Reference
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<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="1" aria-labelledby="__nav_5_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5">
<span class="md-nav__icon md-icon"></span>
Reference
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_1" >
<label class="md-nav__link" for="__nav_5_1" id="__nav_5_1_label" tabindex="0">
<span class="md-ellipsis">
antispam
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_1_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_1">
<span class="md-nav__icon md-icon"></span>
antispam
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/antispam/models/" class="md-nav__link">
<span class="md-ellipsis">
Models
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/antispam/forms/" class="md-nav__link">
<span class="md-ellipsis">
Forms
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_2" >
<label class="md-nav__link" for="__nav_5_2" id="__nav_5_2_label" tabindex="0">
<span class="md-ellipsis">
api
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_2_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_2">
<span class="md-nav__icon md-icon"></span>
api
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/api/auth/" class="md-nav__link">
<span class="md-ellipsis">
Auth
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/api/hashers/" class="md-nav__link">
<span class="md-ellipsis">
Hashers
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/api/models/" class="md-nav__link">
<span class="md-ellipsis">
Models
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/api/perms/" class="md-nav__link">
<span class="md-ellipsis">
Perms
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_3" >
<label class="md-nav__link" for="__nav_5_3" id="__nav_5_3_label" tabindex="0">
<span class="md-ellipsis">
club
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_3_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_3">
<span class="md-nav__icon md-icon"></span>
club
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/club/models/" class="md-nav__link">
<span class="md-ellipsis">
Models
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/club/views/" class="md-nav__link">
<span class="md-ellipsis">
Views
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_4" >
<label class="md-nav__link" for="__nav_5_4" id="__nav_5_4_label" tabindex="0">
<span class="md-ellipsis">
com
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_4_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_4">
<span class="md-nav__icon md-icon"></span>
com
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/com/models/" class="md-nav__link">
<span class="md-ellipsis">
Models
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/com/views/" class="md-nav__link">
<span class="md-ellipsis">
Views
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_5" >
<label class="md-nav__link" for="__nav_5_5" id="__nav_5_5_label" tabindex="0">
<span class="md-ellipsis">
core
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_5_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_5">
<span class="md-nav__icon md-icon"></span>
core
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/core/models/" class="md-nav__link">
<span class="md-ellipsis">
Models
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/core/model_fields/" class="md-nav__link">
<span class="md-ellipsis">
Champs de modèle
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/core/views/" class="md-nav__link">
<span class="md-ellipsis">
Views
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/core/mixins/" class="md-nav__link">
<span class="md-ellipsis">
Mixins
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/core/schemas/" class="md-nav__link">
<span class="md-ellipsis">
Schemas
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/core/auth/" class="md-nav__link">
<span class="md-ellipsis">
Auth
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_6" >
<label class="md-nav__link" for="__nav_5_6" id="__nav_5_6_label" tabindex="0">
<span class="md-ellipsis">
counter
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_6_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_6">
<span class="md-nav__icon md-icon"></span>
counter
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/counter/models/" class="md-nav__link">
<span class="md-ellipsis">
Models
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/counter/views/" class="md-nav__link">
<span class="md-ellipsis">
Views
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/counter/schemas/" class="md-nav__link">
<span class="md-ellipsis">
Schemas
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_7" >
<label class="md-nav__link" for="__nav_5_7" id="__nav_5_7_label" tabindex="0">
<span class="md-ellipsis">
eboutic
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_7_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_7">
<span class="md-nav__icon md-icon"></span>
eboutic
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/eboutic/models/" class="md-nav__link">
<span class="md-ellipsis">
Models
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/eboutic/views/" class="md-nav__link">
<span class="md-ellipsis">
Views
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_8" >
<label class="md-nav__link" for="__nav_5_8" id="__nav_5_8_label" tabindex="0">
<span class="md-ellipsis">
election
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_8_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_8">
<span class="md-nav__icon md-icon"></span>
election
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/election/models/" class="md-nav__link">
<span class="md-ellipsis">
Models
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/election/views/" class="md-nav__link">
<span class="md-ellipsis">
Views
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_9" >
<label class="md-nav__link" for="__nav_5_9" id="__nav_5_9_label" tabindex="0">
<span class="md-ellipsis">
forum
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_9_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_9">
<span class="md-nav__icon md-icon"></span>
forum
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/forum/models/" class="md-nav__link">
<span class="md-ellipsis">
Models
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/forum/views/" class="md-nav__link">
<span class="md-ellipsis">
Views
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_10" >
<label class="md-nav__link" for="__nav_5_10" id="__nav_5_10_label" tabindex="0">
<span class="md-ellipsis">
galaxy
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_10_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_10">
<span class="md-nav__icon md-icon"></span>
galaxy
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/galaxy/models/" class="md-nav__link">
<span class="md-ellipsis">
Models
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/galaxy/views/" class="md-nav__link">
<span class="md-ellipsis">
Views
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_11" >
<label class="md-nav__link" for="__nav_5_11" id="__nav_5_11_label" tabindex="0">
<span class="md-ellipsis">
matmat
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_11_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_11">
<span class="md-nav__icon md-icon"></span>
matmat
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/matmat/models/" class="md-nav__link">
<span class="md-ellipsis">
Models
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/matmat/views/" class="md-nav__link">
<span class="md-ellipsis">
Views
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_12" >
<label class="md-nav__link" for="__nav_5_12" id="__nav_5_12_label" tabindex="0">
<span class="md-ellipsis">
pedagogy
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_12_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_12">
<span class="md-nav__icon md-icon"></span>
pedagogy
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/pedagogy/models/" class="md-nav__link">
<span class="md-ellipsis">
Models
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/pedagogy/views/" class="md-nav__link">
<span class="md-ellipsis">
Views
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/pedagogy/schemas/" class="md-nav__link">
<span class="md-ellipsis">
Schemas
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_13" >
<label class="md-nav__link" for="__nav_5_13" id="__nav_5_13_label" tabindex="0">
<span class="md-ellipsis">
rootplace
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_13_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_13">
<span class="md-nav__icon md-icon"></span>
rootplace
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/rootplace/models/" class="md-nav__link">
<span class="md-ellipsis">
Models
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/rootplace/forms/" class="md-nav__link">
<span class="md-ellipsis">
Forms
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/rootplace/views/" class="md-nav__link">
<span class="md-ellipsis">
Views
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_14" >
<label class="md-nav__link" for="__nav_5_14" id="__nav_5_14_label" tabindex="0">
<span class="md-ellipsis">
sas
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_14_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_14">
<span class="md-nav__icon md-icon"></span>
sas
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/sas/models/" class="md-nav__link">
<span class="md-ellipsis">
Models
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/sas/views/" class="md-nav__link">
<span class="md-ellipsis">
Views
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/sas/schemas/" class="md-nav__link">
<span class="md-ellipsis">
Schemas
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_15" >
<label class="md-nav__link" for="__nav_5_15" id="__nav_5_15_label" tabindex="0">
<span class="md-ellipsis">
staticfiles
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_15_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_15">
<span class="md-nav__icon md-icon"></span>
staticfiles
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/staticfiles/apps/" class="md-nav__link">
<span class="md-ellipsis">
Apps
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/staticfiles/storage/" class="md-nav__link">
<span class="md-ellipsis">
Storage
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/staticfiles/finders/" class="md-nav__link">
<span class="md-ellipsis">
Finders
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/staticfiles/processors/" class="md-nav__link">
<span class="md-ellipsis">
Processors
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_16" >
<label class="md-nav__link" for="__nav_5_16" id="__nav_5_16_label" tabindex="0">
<span class="md-ellipsis">
subscription
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_16_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_16">
<span class="md-nav__icon md-icon"></span>
subscription
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/subscription/models/" class="md-nav__link">
<span class="md-ellipsis">
Models
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/subscription/views/" class="md-nav__link">
<span class="md-ellipsis">
Views
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item md-nav__item--nested">
<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5_17" >
<label class="md-nav__link" for="__nav_5_17" id="__nav_5_17_label" tabindex="0">
<span class="md-ellipsis">
trombi
</span>
<span class="md-nav__icon md-icon"></span>
</label>
<nav class="md-nav" data-md-level="2" aria-labelledby="__nav_5_17_label" aria-expanded="false">
<label class="md-nav__title" for="__nav_5_17">
<span class="md-nav__icon md-icon"></span>
trombi
</label>
<ul class="md-nav__list" data-md-scrollfix>
<li class="md-nav__item">
<a href="../../reference/trombi/models/" class="md-nav__link">
<span class="md-ellipsis">
Models
</span>
</a>
</li>
<li class="md-nav__item">
<a href="../../reference/trombi/views/" class="md-nav__link">
<span class="md-ellipsis">
Views
</span>
</a>
</li>
</ul>
</nav>
</li>
</ul>
</nav>
</li>
</ul>
</nav>
</div>
</div>
</div>
<div class="md-sidebar md-sidebar--secondary" data-md-component="sidebar" data-md-type="toc" >
<div class="md-sidebar__scrollwrap">
<div class="md-sidebar__inner">
<nav class="md-nav md-nav--secondary" aria-label="Table des matières">
<label class="md-nav__title" for="__toc">
<span class="md-nav__icon md-icon"></span>
Table des matières
</label>
<ul class="md-nav__list" data-md-component="toc" data-md-scrollfix>
<li class="md-nav__item">
<a href="#les-n1-queries" class="md-nav__link">
<span class="md-ellipsis">
Les N+1 queries
</span>
</a>
<nav class="md-nav" aria-label="Les N+1 queries">
<ul class="md-nav__list">
<li class="md-nav__item">
<a href="#le-probleme" class="md-nav__link">
<span class="md-ellipsis">
Le problème
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#select_related" class="md-nav__link">
<span class="md-ellipsis">
select_related
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#prefetch_related" class="md-nav__link">
<span class="md-ellipsis">
prefetch_related
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#recuperer-ce-dont-vous-avez-besoin" class="md-nav__link">
<span class="md-ellipsis">
Récupérer ce dont vous avez besoin
</span>
</a>
</li>
</ul>
</nav>
</li>
<li class="md-nav__item">
<a href="#les-aggregations-manquees" class="md-nav__link">
<span class="md-ellipsis">
Les aggrégations manquées
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#benchmark" class="md-nav__link">
<span class="md-ellipsis">
Benchmark
</span>
</a>
<nav class="md-nav" aria-label="Benchmark">
<ul class="md-nav__list">
<li class="md-nav__item">
<a href="#ce-quil-faut-mesurer" class="md-nav__link">
<span class="md-ellipsis">
Ce qu'il faut mesurer
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#django-debug-toolbar" class="md-nav__link">
<span class="md-ellipsis">
django-debug-toolbar
</span>
</a>
</li>
<li class="md-nav__item">
<a href="#connectionqueries" class="md-nav__link">
<span class="md-ellipsis">
connection.queries
</span>
</a>
</li>
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<h1>L'ORM de Django</h1>
<p>L'ORM de Django est puissant, très puissant, non par parce qu'il
est performant (après tout, ce n'est qu'une interface, le gros du boulot,
c'est la db qui le fait), mais parce qu'il permet d'écrire
de manière relativement simple un grand panel de requêtes.</p>
<p>De manière générale, puisqu'un ORM est un système
consistant à manipuler avec un code orienté-objet
une db relationnelle (c'est-à-dire deux paradigmes
qui ne fonctionnent absolument pas pareil),
on rencontre un des deux problèmes suivants :</p>
<ul>
<li>soit l'ORM n'offre pas assez d'abstraction,
auquel cas, quand on veut faire des requêtes
plus complexes qu'un <code>select</code> avec un <code>where</code>,
on s'emmêle les pinceaux et on se dit que
ça aurait été plus simple de le faire directement
en SQL.</li>
<li>soit l'ORM offre trop d'abstraction,
auquel cas, on a tendance à ne pas prêter
assez attention aux requêtes envoyées en base
de données et on finit par se rendre compte
que les temps d'attente explosent
parce qu'on envoie trop de requêtes.</li>
</ul>
<p>Django est dans ce deuxième cas.</p>
<p>C'est pourquoi nous ne parlerons pas ici
de son fonctionnement exact ni de toutes les fonctions
que l'on peut utiliser
(la <a href="https://docs.djangoproject.com/fr/stable/topics/db/queries/">doc officielle</a> fait déjà ça mieux que nous),
mais plutôt des pièges courants
et des astuces pour les éviter.</p>
<h2 id="les-n1-queries">Les <code>N+1 queries</code><a class="headerlink" href="#les-n1-queries" title="Permanent link">&para;</a></h2>
<h3 id="le-probleme">Le problème<a class="headerlink" href="#le-probleme" title="Permanent link">&para;</a></h3>
<p>Normalement, quand on veut récupérer une liste,
on fait une requête et c'est fini.
Mais des fois, ça n'est pas si simple.
Par exemple, supposons que nous voulons
récupérer les 100 utilisateurs les plus riches,
avec leurs informations client :</p>
<div class="language-python highlight"><pre><span></span><code><span id="__span-0-1"><a id="__codelineno-0-1" name="__codelineno-0-1" href="#__codelineno-0-1"></a><span class="kn">from</span><span class="w"> </span><span class="nn">core.models</span><span class="w"> </span><span class="kn">import</span> <span class="n">User</span>
</span><span id="__span-0-2"><a id="__codelineno-0-2" name="__codelineno-0-2" href="#__codelineno-0-2"></a>
</span><span id="__span-0-3"><a id="__codelineno-0-3" name="__codelineno-0-3" href="#__codelineno-0-3"></a><span class="k">for</span> <span class="n">user</span> <span class="ow">in</span> <span class="n">User</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">order_by</span><span class="p">(</span><span class="s2">&quot;-customer__amount&quot;</span><span class="p">)[:</span><span class="mi">100</span><span class="p">]:</span>
</span><span id="__span-0-4"><a id="__codelineno-0-4" name="__codelineno-0-4" href="#__codelineno-0-4"></a> <span class="nb">print</span><span class="p">(</span><span class="n">user</span><span class="o">.</span><span class="n">customer</span><span class="o">.</span><span class="n">amount</span><span class="p">)</span>
</span></code></pre></div>
<p>Combien de requêtes le bout de code suivant effectue-t-il ?
101.
En deux pauvres lignes de code, nous avons demandé
à la base de données d'effectuer 101 requêtes.
Une requête toute seule n'est déjà une opération anodine,
alors je vous laisse imaginer ce que ça donne pour 101.</p>
<p>Si vous ne comprenez pourquoi ce nombre, c'est très simple :</p>
<ul>
<li>Une requête pour sélectionner nos 100 utilisateurs</li>
<li>Une requête supplémentaire pour récupérer les informations
client de chaque utilisateur, soit 100 requêtes.</li>
</ul>
<p>En effet, les informations client sont stockées dans une
autre table, mais le fait d'établir un lien de clef
étrangère permet de manipuler <code>customer</code>
comme si c'était un membre à part entière de <code>User</code>.</p>
<p>Il est à noter cependant, que Django n'effectue une requête
que pour le premier accès à un membre d'une relation
de clef étrangère.
Toutes les fois suivantes, l'objet est déjà là,
et django le récupère :</p>
<div class="language-python highlight"><pre><span></span><code><span id="__span-1-1"><a id="__codelineno-1-1" name="__codelineno-1-1" href="#__codelineno-1-1"></a><span class="kn">from</span><span class="w"> </span><span class="nn">core.models</span><span class="w"> </span><span class="kn">import</span> <span class="n">User</span>
</span><span id="__span-1-2"><a id="__codelineno-1-2" name="__codelineno-1-2" href="#__codelineno-1-2"></a>
</span><span id="__span-1-3"><a id="__codelineno-1-3" name="__codelineno-1-3" href="#__codelineno-1-3"></a><span class="c1"># l&#39;utilisateur le plus riche</span>
</span><span id="__span-1-4"><a id="__codelineno-1-4" name="__codelineno-1-4" href="#__codelineno-1-4"></a><span class="n">user</span> <span class="o">=</span> <span class="n">User</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">order_by</span><span class="p">(</span><span class="s2">&quot;-customer__amount&quot;</span><span class="p">)</span><span class="o">.</span><span class="n">first</span><span class="p">()</span> <span class="c1"># &lt;-- requête db</span>
</span><span id="__span-1-5"><a id="__codelineno-1-5" name="__codelineno-1-5" href="#__codelineno-1-5"></a><span class="nb">print</span><span class="p">(</span><span class="n">user</span><span class="o">.</span><span class="n">customer</span><span class="o">.</span><span class="n">amount</span><span class="p">)</span> <span class="c1"># &lt;-- requête db</span>
</span><span id="__span-1-6"><a id="__codelineno-1-6" name="__codelineno-1-6" href="#__codelineno-1-6"></a><span class="nb">print</span><span class="p">(</span><span class="n">user</span><span class="o">.</span><span class="n">customer</span><span class="o">.</span><span class="n">account_id</span><span class="p">)</span> <span class="c1"># on a déjà récupéré `customer`, donc pas de requête</span>
</span></code></pre></div>
<p>Ce n'est donc pas gravissime si vous faites cette
erreur quand vous manipulez un seul objet.
En revanche, quand vous en manipulez plusieurs,
il faut régler le problème.
Pour ça, il y a plusieurs méthodes, en fonction de votre cas.</p>
<h3 id="select_related"><code>select_related</code><a class="headerlink" href="#select_related" title="Permanent link">&para;</a></h3>
<p>La méthode la plus basique consiste à annoter le queryset,
avec la méthode <code>select_related()</code>.
En faisant ça, Django fera une jointure sur l'autre table
et demandera des informations en plus
à la db lors de la requête.</p>
<p>De la sorte, lorsque vous appellerez le membre relié,
les informations seront déjà là.</p>
<div class="language-python highlight"><pre><span></span><code><span id="__span-2-1"><a id="__codelineno-2-1" name="__codelineno-2-1" href="#__codelineno-2-1"></a><span class="kn">from</span><span class="w"> </span><span class="nn">core.models</span><span class="w"> </span><span class="kn">import</span> <span class="n">User</span>
</span><span id="__span-2-2"><a id="__codelineno-2-2" name="__codelineno-2-2" href="#__codelineno-2-2"></a>
</span><span id="__span-2-3"><a id="__codelineno-2-3" name="__codelineno-2-3" href="#__codelineno-2-3"></a><span class="n">richest</span> <span class="o">=</span> <span class="n">User</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">order_by</span><span class="p">(</span><span class="s2">&quot;-customer__amount&quot;</span><span class="p">)</span>
</span><span id="__span-2-4"><a id="__codelineno-2-4" name="__codelineno-2-4" href="#__codelineno-2-4"></a><span class="k">for</span> <span class="n">user</span> <span class="ow">in</span> <span class="n">richest</span><span class="o">.</span><span class="n">select_related</span><span class="p">(</span><span class="s2">&quot;customer&quot;</span><span class="p">)[:</span><span class="mi">100</span><span class="p">]:</span>
</span><span id="__span-2-5"><a id="__codelineno-2-5" name="__codelineno-2-5" href="#__codelineno-2-5"></a> <span class="nb">print</span><span class="p">(</span><span class="n">user</span><span class="o">.</span><span class="n">customer</span><span class="p">)</span>
</span></code></pre></div>
<p>Le code ci-dessus effectue une seule requête.
Chaque fois qu'on veut accéder à <code>customer</code>, c'est bon,
ça a déjà été récupéré à travers le <code>select_related</code>.</p>
<h3 id="prefetch_related"><code>prefetch_related</code><a class="headerlink" href="#prefetch_related" title="Permanent link">&para;</a></h3>
<p>Maintenant, un cas plus compliqué.
Supposons que vous ne vouliez pas récupérer des informations
reliées par une relation One-to-One,
mais par une relation One-to-Many.</p>
<p>Par exemple, un utilisateur a un seul compte client,
mais il peut avoir plusieurs cotisations à son actif.
Et dans ces cas-là, <code>annotate</code> ne marche plus.
En effet, s'il peut exister plusieurs cotisations,
comment savoir laquelle on veut ?</p>
<p>Il faut alors utiliser un <code>prefetch_related</code>.
C'est un mécanisme un peu différent :
au lieu de faire une jointure et d'ajouter les informations
voulues dans la même requête, Django va effectuer
une deuxième requête pour récupérer les éléments de l'autre table,
puis, à partir de ces éléments, peupler la relation
de son côté.</p>
<p>C'est un mécanisme qui peut être un peu coûteux en mémoire
et qui demande une deuxième requête,
mais qui reste quand même largement préférable
à faire N requêtes en plus.</p>
<div class="language-python highlight"><pre><span></span><code><span id="__span-3-1"><a id="__codelineno-3-1" name="__codelineno-3-1" href="#__codelineno-3-1"></a><span class="kn">from</span><span class="w"> </span><span class="nn">core.models</span><span class="w"> </span><span class="kn">import</span> <span class="n">User</span>
</span><span id="__span-3-2"><a id="__codelineno-3-2" name="__codelineno-3-2" href="#__codelineno-3-2"></a>
</span><span id="__span-3-3"><a id="__codelineno-3-3" name="__codelineno-3-3" href="#__codelineno-3-3"></a><span class="k">for</span> <span class="n">user</span> <span class="ow">in</span> <span class="n">User</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">prefetch_related</span><span class="p">(</span><span class="s2">&quot;subscriptions&quot;</span><span class="p">)[:</span><span class="mi">100</span><span class="p">]:</span>
</span><span id="__span-3-4"><a id="__codelineno-3-4" name="__codelineno-3-4" href="#__codelineno-3-4"></a> <span class="c1"># c&#39;est bon, la méthode prefetch a récupéré en avance les `subscriptions`</span>
</span><span id="__span-3-5"><a id="__codelineno-3-5" name="__codelineno-3-5" href="#__codelineno-3-5"></a> <span class="nb">print</span><span class="p">(</span><span class="n">user</span><span class="o">.</span><span class="n">subscriptions</span><span class="o">.</span><span class="n">all</span><span class="p">())</span>
</span></code></pre></div>
<div class="admonition danger">
<p class="admonition-title">Danger</p>
<p>La méthode <code>prefetch_related</code> ne marche que si vous
utilisez la méthode <code>all()</code> pour accéder au membre.
Si vous utilisez une autre méthode (comme <code>filter</code> ou <code>annotate</code>),
alors Django effectuera une nouvelle requête,
et vous retomberez dans le problème initial.</p>
<div class="language-python highlight"><pre><span></span><code><span id="__span-4-1"><a id="__codelineno-4-1" name="__codelineno-4-1" href="#__codelineno-4-1"></a><span class="kn">from</span><span class="w"> </span><span class="nn">core.models</span><span class="w"> </span><span class="kn">import</span> <span class="n">User</span>
</span><span id="__span-4-2"><a id="__codelineno-4-2" name="__codelineno-4-2" href="#__codelineno-4-2"></a><span class="kn">from</span><span class="w"> </span><span class="nn">django.db.models</span><span class="w"> </span><span class="kn">import</span> <span class="n">Count</span>
</span><span id="__span-4-3"><a id="__codelineno-4-3" name="__codelineno-4-3" href="#__codelineno-4-3"></a>
</span><span id="__span-4-4"><a id="__codelineno-4-4" name="__codelineno-4-4" href="#__codelineno-4-4"></a><span class="k">for</span> <span class="n">user</span> <span class="ow">in</span> <span class="n">User</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">prefetch_related</span><span class="p">(</span><span class="s2">&quot;subscriptions&quot;</span><span class="p">)[:</span><span class="mi">100</span><span class="p">]:</span>
</span><span id="__span-4-5"><a id="__codelineno-4-5" name="__codelineno-4-5" href="#__codelineno-4-5"></a> <span class="c1"># Le prefetch_related ne marche plus !</span>
</span><span id="__span-4-6"><a id="__codelineno-4-6" name="__codelineno-4-6" href="#__codelineno-4-6"></a> <span class="nb">print</span><span class="p">(</span><span class="n">user</span><span class="o">.</span><span class="n">subscriptions</span><span class="o">.</span><span class="n">annotate</span><span class="p">(</span><span class="n">count</span><span class="o">=</span><span class="n">Count</span><span class="p">(</span><span class="s2">&quot;*&quot;</span><span class="p">)))</span>
</span></code></pre></div>
</div>
<h3 id="recuperer-ce-dont-vous-avez-besoin">Récupérer ce dont vous avez besoin<a class="headerlink" href="#recuperer-ce-dont-vous-avez-besoin" title="Permanent link">&para;</a></h3>
<p>Des fois (souvent, même), penser explicitement
à la jointure est le meilleur choix.</p>
<p>En effet, vous remarquerez que dans tous
les exemples précédents, nous n'utilisions
qu'une partie des informations
(par exemple, nous ne récupérions que la somme
d'argent sur les comptes, et éventuellement le numéro de compte).</p>
<p>Nous pouvons utiliser la méthode <code>annotate</code>
pour spécifier explicitement les données que l'on veut
joindre à notre requête.</p>
<p>Quand nous voulions récupérer les informations utilisateur,
nous aurions tout aussi bien pu écrire :</p>
<div class="language-python highlight"><pre><span></span><code><span id="__span-5-1"><a id="__codelineno-5-1" name="__codelineno-5-1" href="#__codelineno-5-1"></a><span class="kn">from</span><span class="w"> </span><span class="nn">core.models</span><span class="w"> </span><span class="kn">import</span> <span class="n">User</span>
</span><span id="__span-5-2"><a id="__codelineno-5-2" name="__codelineno-5-2" href="#__codelineno-5-2"></a><span class="kn">from</span><span class="w"> </span><span class="nn">django.db.models</span><span class="w"> </span><span class="kn">import</span> <span class="n">F</span>
</span><span id="__span-5-3"><a id="__codelineno-5-3" name="__codelineno-5-3" href="#__codelineno-5-3"></a>
</span><span id="__span-5-4"><a id="__codelineno-5-4" name="__codelineno-5-4" href="#__codelineno-5-4"></a><span class="n">richest</span> <span class="o">=</span> <span class="n">User</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">order_by</span><span class="p">(</span><span class="s2">&quot;-customer__amount&quot;</span><span class="p">)</span>
</span><span id="__span-5-5"><a id="__codelineno-5-5" name="__codelineno-5-5" href="#__codelineno-5-5"></a><span class="k">for</span> <span class="n">user</span> <span class="ow">in</span> <span class="n">richest</span><span class="o">.</span><span class="n">annotate</span><span class="p">(</span><span class="n">amount</span><span class="o">=</span><span class="n">F</span><span class="p">(</span><span class="s2">&quot;customer__amount&quot;</span><span class="p">))[:</span><span class="mi">100</span><span class="p">]:</span>
</span><span id="__span-5-6"><a id="__codelineno-5-6" name="__codelineno-5-6" href="#__codelineno-5-6"></a> <span class="nb">print</span><span class="p">(</span><span class="n">user</span><span class="o">.</span><span class="n">amount</span><span class="p">)</span>
</span></code></pre></div>
<p>On aurait même pu réorganiser ça :
<div class="language-python highlight"><pre><span></span><code><span id="__span-6-1"><a id="__codelineno-6-1" name="__codelineno-6-1" href="#__codelineno-6-1"></a><span class="kn">from</span><span class="w"> </span><span class="nn">core.models</span><span class="w"> </span><span class="kn">import</span> <span class="n">User</span>
</span><span id="__span-6-2"><a id="__codelineno-6-2" name="__codelineno-6-2" href="#__codelineno-6-2"></a><span class="kn">from</span><span class="w"> </span><span class="nn">django.db.models</span><span class="w"> </span><span class="kn">import</span> <span class="n">F</span>
</span><span id="__span-6-3"><a id="__codelineno-6-3" name="__codelineno-6-3" href="#__codelineno-6-3"></a>
</span><span id="__span-6-4"><a id="__codelineno-6-4" name="__codelineno-6-4" href="#__codelineno-6-4"></a><span class="n">richest</span> <span class="o">=</span> <span class="n">User</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">annotate</span><span class="p">(</span><span class="n">amount</span><span class="o">=</span><span class="n">F</span><span class="p">(</span><span class="s2">&quot;customer__amount&quot;</span><span class="p">))</span><span class="o">.</span><span class="n">order_by</span><span class="p">(</span><span class="s2">&quot;-amount&quot;</span><span class="p">)</span>
</span><span id="__span-6-5"><a id="__codelineno-6-5" name="__codelineno-6-5" href="#__codelineno-6-5"></a><span class="k">for</span> <span class="n">user</span> <span class="ow">in</span> <span class="n">richest</span><span class="p">[:</span><span class="mi">100</span><span class="p">]:</span>
</span><span id="__span-6-6"><a id="__codelineno-6-6" name="__codelineno-6-6" href="#__codelineno-6-6"></a> <span class="nb">print</span><span class="p">(</span><span class="n">user</span><span class="o">.</span><span class="n">amount</span><span class="p">)</span>
</span></code></pre></div></p>
<p>Ça peut sembler moins bien qu'un <code>select_related</code>, comme ça.
Des fois, c'est en effet moins bien, et des fois c'est mieux.
La comparaison est plus évidente avec le <code>prefetch_related</code>.</p>
<p>En effet, quand nous voulions récupérer
le nombre de cotisations des utilisateurs,
le <code>prefetch_related</code> ne marchait plus.
Pourtant, nous voulions récupérer une seule information.</p>
<p>Il aurait donc été suffisant d'écrire :
<div class="language-python highlight"><pre><span></span><code><span id="__span-7-1"><a id="__codelineno-7-1" name="__codelineno-7-1" href="#__codelineno-7-1"></a><span class="kn">from</span><span class="w"> </span><span class="nn">core.models</span><span class="w"> </span><span class="kn">import</span> <span class="n">User</span>
</span><span id="__span-7-2"><a id="__codelineno-7-2" name="__codelineno-7-2" href="#__codelineno-7-2"></a><span class="kn">from</span><span class="w"> </span><span class="nn">django.db.models</span><span class="w"> </span><span class="kn">import</span> <span class="n">Count</span>
</span><span id="__span-7-3"><a id="__codelineno-7-3" name="__codelineno-7-3" href="#__codelineno-7-3"></a>
</span><span id="__span-7-4"><a id="__codelineno-7-4" name="__codelineno-7-4" href="#__codelineno-7-4"></a><span class="k">for</span> <span class="n">user</span> <span class="ow">in</span> <span class="n">User</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">annotate</span><span class="p">(</span><span class="n">nb_subscriptions</span><span class="o">=</span><span class="n">Count</span><span class="p">(</span><span class="s2">&quot;subscriptions&quot;</span><span class="p">))[:</span><span class="mi">100</span><span class="p">]:</span>
</span><span id="__span-7-5"><a id="__codelineno-7-5" name="__codelineno-7-5" href="#__codelineno-7-5"></a> <span class="c1"># Et là ça marche, en une seule requête.</span>
</span><span id="__span-7-6"><a id="__codelineno-7-6" name="__codelineno-7-6" href="#__codelineno-7-6"></a> <span class="nb">print</span><span class="p">(</span><span class="n">user</span><span class="o">.</span><span class="n">nb_subscriptions</span><span class="p">)</span>
</span></code></pre></div></p>
<p>Faire une jointure, c'est normal en SQL.
Et pourtant avec Django on les oublie trop facilement.
Posez-vous toujours la question des données que vous pourriez
avoir besoin d'annoter, et vous éviterez beaucoup d'ennuis.</p>
<h2 id="les-aggregations-manquees">Les aggrégations manquées<a class="headerlink" href="#les-aggregations-manquees" title="Permanent link">&para;</a></h2>
<p>Il arrive souvent que l'on veuille une information qui
porte sur un ensemble d'objets de notre db.</p>
<p>Imaginons par exemple que nous voulons connaitre
la somme totale des ventes faites à un comptoir.</p>
<p>Nous avons tous suivi nos cours de programmation,
nous écrivons donc instinctivement :</p>
<div class="language-python highlight"><pre><span></span><code><span id="__span-8-1"><a id="__codelineno-8-1" name="__codelineno-8-1" href="#__codelineno-8-1"></a><span class="kn">from</span><span class="w"> </span><span class="nn">counter.models</span><span class="w"> </span><span class="kn">import</span> <span class="n">Counter</span>
</span><span id="__span-8-2"><a id="__codelineno-8-2" name="__codelineno-8-2" href="#__codelineno-8-2"></a>
</span><span id="__span-8-3"><a id="__codelineno-8-3" name="__codelineno-8-3" href="#__codelineno-8-3"></a><span class="n">foyer</span> <span class="o">=</span> <span class="n">Counter</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s2">&quot;Foyer&quot;</span><span class="p">)</span>
</span><span id="__span-8-4"><a id="__codelineno-8-4" name="__codelineno-8-4" href="#__codelineno-8-4"></a><span class="n">total_amount</span> <span class="o">=</span> <span class="nb">sum</span><span class="p">(</span>
</span><span id="__span-8-5"><a id="__codelineno-8-5" name="__codelineno-8-5" href="#__codelineno-8-5"></a> <span class="n">sale</span><span class="o">.</span><span class="n">amount</span> <span class="o">*</span> <span class="n">sale</span><span class="o">.</span><span class="n">unit_price</span>
</span><span id="__span-8-6"><a id="__codelineno-8-6" name="__codelineno-8-6" href="#__codelineno-8-6"></a> <span class="k">for</span> <span class="n">sale</span> <span class="ow">in</span> <span class="n">foyer</span><span class="o">.</span><span class="n">sellings</span><span class="o">.</span><span class="n">all</span><span class="p">()</span>
</span><span id="__span-8-7"><a id="__codelineno-8-7" name="__codelineno-8-7" href="#__codelineno-8-7"></a><span class="p">)</span>
</span></code></pre></div>
<p>On pourrait penser qu'il n'y a pas de problème.
Après tout, on ne fait qu'une seule requête.
Eh bien si, il y a un problème :
on fait beaucoup de choses en trop.</p>
<p>Concrètement, on demande à la base de données
de renvoyer toutes les informations,
ce qui rallonge inutilement la durée
de l'échange entre le serveur et la db,
puis on perd du temps à convertir ces informations
en objets Python (opération qui a un coût également),
et enfin on reperd du temps à calculer en Python
quelque chose que la db aurait pu calculer
à notre plus bien plus vite.</p>
<p>Nous aurions dû aggréger la requête,
avec la méthode <code>aggregate</code> :</p>
<div class="language-python highlight"><pre><span></span><code><span id="__span-9-1"><a id="__codelineno-9-1" name="__codelineno-9-1" href="#__codelineno-9-1"></a><span class="kn">from</span><span class="w"> </span><span class="nn">counter.models</span><span class="w"> </span><span class="kn">import</span> <span class="n">Counter</span>
</span><span id="__span-9-2"><a id="__codelineno-9-2" name="__codelineno-9-2" href="#__codelineno-9-2"></a><span class="kn">from</span><span class="w"> </span><span class="nn">django.db.models</span><span class="w"> </span><span class="kn">import</span> <span class="n">Sum</span><span class="p">,</span> <span class="n">F</span>
</span><span id="__span-9-3"><a id="__codelineno-9-3" name="__codelineno-9-3" href="#__codelineno-9-3"></a>
</span><span id="__span-9-4"><a id="__codelineno-9-4" name="__codelineno-9-4" href="#__codelineno-9-4"></a><span class="n">foyer</span> <span class="o">=</span> <span class="n">Counter</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s2">&quot;Foyer&quot;</span><span class="p">)</span>
</span><span id="__span-9-5"><a id="__codelineno-9-5" name="__codelineno-9-5" href="#__codelineno-9-5"></a><span class="n">total_amount</span> <span class="o">=</span> <span class="p">(</span>
</span><span id="__span-9-6"><a id="__codelineno-9-6" name="__codelineno-9-6" href="#__codelineno-9-6"></a> <span class="n">foyer</span><span class="o">.</span><span class="n">sellings</span><span class="o">.</span><span class="n">aggregate</span><span class="p">(</span><span class="n">amount</span><span class="o">=</span><span class="n">Sum</span><span class="p">(</span><span class="n">F</span><span class="p">(</span><span class="s2">&quot;amount&quot;</span><span class="p">)</span> <span class="o">*</span> <span class="n">F</span><span class="p">(</span><span class="s2">&quot;unit_price&quot;</span><span class="p">),</span> <span class="n">default</span><span class="o">=</span><span class="mi">0</span><span class="p">))</span>
</span><span id="__span-9-7"><a id="__codelineno-9-7" name="__codelineno-9-7" href="#__codelineno-9-7"></a><span class="p">)[</span><span class="s2">&quot;amount__sum&quot;</span><span class="p">]</span>
</span></code></pre></div>
<p>En effectuant cette requête, la base de données nous renverra exactement
l'information dont nous avons besoin.
Et de notre côté, nous n'aurons pas à faire de traitement en plus.</p>
<h2 id="benchmark">Benchmark<a class="headerlink" href="#benchmark" title="Permanent link">&para;</a></h2>
<h3 id="ce-quil-faut-mesurer">Ce qu'il faut mesurer<a class="headerlink" href="#ce-quil-faut-mesurer" title="Permanent link">&para;</a></h3>
<p>Quand on parle d'interaction avec une base de données,
la question de la performance est cruciale.
Et quand on parle de performance, on en vient
forcément à parler d'optimisation.</p>
<p>Or, pour optimiser, il faut savoir quoi optimiser.
C'est-à-dire qu'il nous faut un benchmark pour
étudier les performances réelles de notre code.
En ce qui concerne des requêtes à une base de données,
deux aspects sont étudiables :</p>
<ul>
<li>le nombre de requêtes qu'une vue ou une fonction
effectue pour son fonctionnement.</li>
<li>le temps d'exécution individuel des requêtes les plus longues.</li>
</ul>
<p>Le premier aspect est celui qui nous intéresse le plus,
puisqu'il est relié au problème le plus fréquent
et le plus facile à mesurer.
Le second aspect, au contraire, est bien moins fréquent
(dans 99% des cas, une requête complexe prendra
moins de temps que deux requêtes, même simples)
et bien plus dur à mesurer (il faut réussir à faire des mesures fiables,
dans un environnement proche de celui de la prod, avec les données de la prod).</p>
<p>Nous considérerons donc que dans la quasi-totalité des cas,
le problème vient du nombre de requêtes, pas du temps d'exécution
d'une requête en particulier.
Partez du principe que moins vous faites de requêtes, mieux c'est,
sans prêter attention au temps d'exécution des requêtes.</p>
<p>Pour quantifier de manière fiables les requêtes effectuées,
il y a quelques outils.</p>
<h3 id="django-debug-toolbar"><code>django-debug-toolbar</code><a class="headerlink" href="#django-debug-toolbar" title="Permanent link">&para;</a></h3>
<p>La <code>django-debug-toolbar</code> est une interface disponible
sur toutes les pages quand vous êtes en mode debug.
Elle s'affiche à droite et vous permet de voir toutes sortes
d'informations, parmi lesquelles le nombre de requêtes effectuées.</p>
<p>Cette interface est très pratique, puisqu'elle va plus loin
que simplement compter les requêtes,
elle vous donne également le SQL qui a été utilisé,
l'endroit du code, avec fichier et numéro de ligne,
où cette requête a été faite et, encore mieux,
elle vous indique quelles requêtes semblent dupliquées.</p>
<p>Quand <code>django-debug-toolbar</code> vous indique qu'une requête
a été dupliquée quatre fois, cinq fois, ou même deux cent fois
(le chiffre peut sembler énorme, mais c'est déjà arrivé),
vous pouvez être sûr qu'il y a là quelque chose à optimiser.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>Le widget de <code>django-debug-toolbar</code> ne s'affiche
que sur les pages html.
Si vous voulez étudier autre chose,
comme une simple fonction,
ou bien comme une vue retournant du JSON,
vous n'aurez donc pas <code>django-debug-toolbar</code>.</p>
</div>
<h3 id="connectionqueries"><code>connection.queries</code><a class="headerlink" href="#connectionqueries" title="Permanent link">&para;</a></h3>
<p>Quand vous voulez examiner les requêtes d'un bout de code
en particulier, Django met à disposition un mécanisme
permettant d'examiner toutes les requêtes qui sont faites :
<code>connection.queries</code></p>
<p>C'est un historique de toutes les requêtes effectuées,
qui est assez simple à utiliser :</p>
<div class="language-python highlight"><pre><span></span><code><span id="__span-10-1"><a id="__codelineno-10-1" name="__codelineno-10-1" href="#__codelineno-10-1"></a><span class="kn">from</span><span class="w"> </span><span class="nn">django.db</span><span class="w"> </span><span class="kn">import</span> <span class="n">connection</span>
</span><span id="__span-10-2"><a id="__codelineno-10-2" name="__codelineno-10-2" href="#__codelineno-10-2"></a><span class="kn">from</span><span class="w"> </span><span class="nn">core.models</span><span class="w"> </span><span class="kn">import</span> <span class="n">User</span>
</span><span id="__span-10-3"><a id="__codelineno-10-3" name="__codelineno-10-3" href="#__codelineno-10-3"></a>
</span><span id="__span-10-4"><a id="__codelineno-10-4" name="__codelineno-10-4" href="#__codelineno-10-4"></a><span class="nb">print</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">connection</span><span class="o">.</span><span class="n">queries</span><span class="p">))</span> <span class="c1"># 0</span>
</span><span id="__span-10-5"><a id="__codelineno-10-5" name="__codelineno-10-5" href="#__codelineno-10-5"></a>
</span><span id="__span-10-6"><a id="__codelineno-10-6" name="__codelineno-10-6" href="#__codelineno-10-6"></a><span class="n">nb_users</span> <span class="o">=</span> <span class="n">User</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">count</span><span class="p">()</span>
</span><span id="__span-10-7"><a id="__codelineno-10-7" name="__codelineno-10-7" href="#__codelineno-10-7"></a>
</span><span id="__span-10-8"><a id="__codelineno-10-8" name="__codelineno-10-8" href="#__codelineno-10-8"></a><span class="nb">print</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">connection</span><span class="o">.</span><span class="n">queries</span><span class="p">))</span> <span class="c1"># 1</span>
</span><span id="__span-10-9"><a id="__codelineno-10-9" name="__codelineno-10-9" href="#__codelineno-10-9"></a><span class="nb">print</span><span class="p">(</span><span class="n">connection</span><span class="o">.</span><span class="n">queries</span><span class="p">)</span> <span class="c1"># affiche toutes les requêtes effectuées</span>
</span></code></pre></div>
<h3 id="assertnumqueries"><code>assertNumQueries</code><a class="headerlink" href="#assertnumqueries" title="Permanent link">&para;</a></h3>
<p>Quand on a mis en place une fonctionnalité,
ou qu'on en a amélioré les performances,
on veut absolument éviter la régression.</p>
<p>Or, une régression ne se manifeste pas forcément
dans l'apparition d'un bug : ça peut aussi
être une augmentation du temps d'exécution, possiblement
causé par une augmentation du nombre de requêtes.</p>
<p>C'est pour ça que django met à disposition un moyen
de tester automatiquement le nombre de requêtes :
<code>assertNumQueries</code>.</p>
<p>Il s'agit d'un gestionnaire de contexte accessible
dans les tests, qui teste le nombre de requêtes
effectuées en son sein.</p>
<p>Par exemple :</p>
<div class="language-python highlight"><pre><span></span><code><span id="__span-11-1"><a id="__codelineno-11-1" name="__codelineno-11-1" href="#__codelineno-11-1"></a><span class="kn">from</span><span class="w"> </span><span class="nn">django.test</span><span class="w"> </span><span class="kn">import</span> <span class="n">TestCase</span>
</span><span id="__span-11-2"><a id="__codelineno-11-2" name="__codelineno-11-2" href="#__codelineno-11-2"></a><span class="kn">from</span><span class="w"> </span><span class="nn">django.shortcuts</span><span class="w"> </span><span class="kn">import</span> <span class="n">reverse</span>
</span><span id="__span-11-3"><a id="__codelineno-11-3" name="__codelineno-11-3" href="#__codelineno-11-3"></a>
</span><span id="__span-11-4"><a id="__codelineno-11-4" name="__codelineno-11-4" href="#__codelineno-11-4"></a>
</span><span id="__span-11-5"><a id="__codelineno-11-5" name="__codelineno-11-5" href="#__codelineno-11-5"></a><span class="k">class</span><span class="w"> </span><span class="nc">FooTest</span><span class="p">(</span><span class="n">TestCase</span><span class="p">):</span>
</span><span id="__span-11-6"><a id="__codelineno-11-6" name="__codelineno-11-6" href="#__codelineno-11-6"></a> <span class="k">def</span><span class="w"> </span><span class="nf">test_nb_queries</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
</span><span id="__span-11-7"><a id="__codelineno-11-7" name="__codelineno-11-7" href="#__codelineno-11-7"></a><span class="w"> </span><span class="sd">&quot;&quot;&quot;Test that the number of db queries is stable.&quot;&quot;&quot;</span>
</span><span id="__span-11-8"><a id="__codelineno-11-8" name="__codelineno-11-8" href="#__codelineno-11-8"></a> <span class="k">with</span> <span class="bp">self</span><span class="o">.</span><span class="n">assertNumQueries</span><span class="p">(</span><span class="mi">6</span><span class="p">):</span>
</span><span id="__span-11-9"><a id="__codelineno-11-9" name="__codelineno-11-9" href="#__codelineno-11-9"></a> <span class="bp">self</span><span class="o">.</span><span class="n">client</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">reverse</span><span class="p">(</span><span class="s2">&quot;foo:bar&quot;</span><span class="p">))</span>
</span></code></pre></div>
<p>Si l'exécution de la route nécessite plus ou moins de six requêtes,
alors le test échoue.
S'il y a eu moins que le nombre de requête attendu, alors tant
mieux, modifiez le test pour coller au nouveau nombre
(sous réserve que tous les autres tests passent, bien sûr).
Si par contre il y a eu plus, alors désolé, vous avez sans doute
introduit une régression.</p>
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