Joining Across a Relationship & The N + 1 Problem

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Look at the queries on this page: there are 8... or for you there might be 11, 10 or 9: it depends on how many unique questions these 10 answers are related to.

The N+1 Problem

Whatever your number is, that's a lot of queries for such a simple page! The cause of all of this is the N+1 problem.

Look at the queries in the profiler. The first is for the answers: where status is approved, ordered by the most votes DESC, limit 10. Simple enough. Then, each time we render an answer, we also render that answer's question text. The moment we do that, Doctrine makes a second query from the question table to get that answer's question data: so in this case WHERE id = 463. Then we render the second answer... and make another query to get its question data... which is this third query.

Ultimately, we end up with 1 query to get the 10 answers plus 10 more queries: one for each answer's question. That's the N + 1 problem. Well, if two answers share the same question, you might have less than 11 queries - but it's still not great.

This is a classic problem that's really easy to trigger when using a nice system like Doctrine. In AnswerController, we simply query for the answers.

... lines 1 - 12
class AnswerController extends AbstractController
{
... lines 15 - 17
public function popularAnswers(AnswerRepository $answerRepository)
{
$answers = $answerRepository->findMostPopular();
... lines 21 - 24
}
... lines 26 - 48
}

Then, as we loop over them and render _answer.html.twig, we innocently render answer.questionText and answer.question.slug.

<li class="mb-4">
{% if showQuestion|default(false) %}
<a
href="{{ path('app_question_show', {
slug: answer.question.slug
}) }}"
class="mb-1 link-secondary"
>
<strong>Question:</strong>
{{ answer.questionText|u.truncate(80, '...') }}
</a>
{% endif %}
... lines 13 - 46
</li>

It doesn't look like much, but those lines trigger an extra query.

The point is: we end up with a lot of queries on this page and, in theory, we shouldn't need so many! Let's think: in a normal database, how would we solve this? Thinking about the query, we could select the most popular answers and then INNER JOIN over to the question table to grab that data all at once. Yup, one query to return both the answer and question data.

Joining in a QueryBuilder

Can we add a join with Doctrine? Of course! Head over to AnswerRepository, to the findMostPopular() method. It's this simple: ->innerJoin() passing this answer.question and then question.

... lines 1 - 15
class AnswerRepository extends ServiceEntityRepository
{
... lines 18 - 43
public function findMostPopular(): array
{
return $this->createQueryBuilder('answer')
->addCriteria(self::createApprovedCriteria())
->orderBy('answer.votes', 'DESC')
->innerJoin('answer.question', 'question')
->setMaxResults(10)
->getQuery()
->getResult();
}
}

Remember: answer is the alias we're using for our Answer entity. So the answer.question part refers to the question property on the Answer class. This basically tells Doctrine:

Hey! I want you to do an inner join across the answer.question relationship.

We don't need to tell Doctrine how to join like you would in a normal query... we don't need to say "JOIN question ON answer.question_id = question.id". Nope! Doctrine looks at the $question property in Answer, sees that it's a relationship over to the question table and then generates the SQL needed automatically. It's awesome!

The second argument isn't important yet, but this becomes the "alias" to the Question entity, just like how answer is the alias to the Answer entity.

The 2 Reasons to Join

Ok, so let's try this! Close the profiler, refresh and... hmm. We have the same number of queries! So... that didn't work.

Open up the profiler. If you look at the first query... cool! There's the inner join! And it's perfect: Doctrine generated the exact SQL needed. So then... why do we still have all these extra queries? Shouldn't Doctrine be able to get all the question data from the first?

Yes... but the problem is that, while we did join over to the question table... we didn't actually select any question data. It's still only selecting from answer. This is more obvious if we look at the formatted query. It joins to question, but only selects from answer.

This leads us to an important point! There are two reasons that you might use a JOIN in a query. The first is when you want to select more data, and that's our situation: we want to select all the answer and question data.

The second situation is when you want to join across a relationship... not to select more data, but to filter or order the results based on something in the joined table. We'll see that in a minute.

Selecting Data on a Joined Table

The point is: if you want to select more data, then you need to actually say that in the query. You do that with ->addSelect() and then the alias to the entity: question.

... lines 1 - 15
class AnswerRepository extends ServiceEntityRepository
{
... lines 18 - 43
public function findMostPopular(): array
{
return $this->createQueryBuilder('answer')
->addCriteria(self::createApprovedCriteria())
->orderBy('answer.votes', 'DESC')
->innerJoin('answer.question', 'question')
->addSelect('question')
->setMaxResults(10)
->getQuery()
->getResult();
}
}

Two important things here. First, notice that I'm not saying question.id, question.slug or even question.*: I'm just saying question. This tells Doctrine to grab everything from question.

Second, even though we're now selecting more data, this does not change what this method returns: it will still return an array of Answer objects. But now, each Answer object will already have the Question data preloaded into it.

I'll prove it. Refresh the page. Yup! It still works exactly like before, because that method still returns an array of Answer objects! But our query count is down to 1!

Because we're now grabbing the question data in the first query, when we try to render the question for each answer, Doctrine realizes that it already has that data and avoids the query. That's the fix for the N+1 problem.

What about the other reason for joining... where you want to join across a relationship in order to filter the results... like to only return answers whose question is published.

Let's talk about that next by adding a search to our most popular answers page.

Leave a comment!

What PHP libraries does this tutorial use?

// composer.json
{
    "require": {
        "php": "^7.4.1 || ^8.0.0",
        "ext-ctype": "*",
        "ext-iconv": "*",
        "babdev/pagerfanta-bundle": "^3.3", // v3.3.0
        "composer/package-versions-deprecated": "^1.11", // 1.11.99.3
        "doctrine/doctrine-bundle": "^2.1", // 2.4.2
        "doctrine/doctrine-migrations-bundle": "^3.0", // 3.1.1
        "doctrine/orm": "^2.7", // 2.9.5
        "knplabs/knp-markdown-bundle": "^1.8", // 1.9.0
        "knplabs/knp-time-bundle": "^1.11", // v1.16.1
        "pagerfanta/doctrine-orm-adapter": "^3.3", // v3.3.0
        "pagerfanta/twig": "^3.3", // v3.3.0
        "sensio/framework-extra-bundle": "^6.0", // v6.1.5
        "stof/doctrine-extensions-bundle": "^1.4", // v1.6.0
        "symfony/asset": "5.3.*", // v5.3.4
        "symfony/console": "5.3.*", // v5.3.7
        "symfony/dotenv": "5.3.*", // v5.3.7
        "symfony/flex": "^1.3.1", // v1.15.1
        "symfony/framework-bundle": "5.3.*", // v5.3.7
        "symfony/monolog-bundle": "^3.0", // v3.7.0
        "symfony/runtime": "5.3.*", // v5.3.4
        "symfony/stopwatch": "5.3.*", // v5.3.4
        "symfony/twig-bundle": "5.3.*", // v5.3.4
        "symfony/webpack-encore-bundle": "^1.7", // v1.12.0
        "symfony/yaml": "5.3.*", // v5.3.6
        "twig/extra-bundle": "^2.12|^3.0", // v3.3.1
        "twig/string-extra": "^3.3", // v3.3.1
        "twig/twig": "^2.12|^3.0" // v3.3.2
    },
    "require-dev": {
        "doctrine/doctrine-fixtures-bundle": "^3.3", // 3.4.0
        "symfony/debug-bundle": "5.3.*", // v5.3.4
        "symfony/maker-bundle": "^1.15", // v1.33.0
        "symfony/var-dumper": "5.3.*", // v5.3.7
        "symfony/web-profiler-bundle": "5.3.*", // v5.3.5
        "zenstruck/foundry": "^1.1" // v1.13.1
    }
}