Using select_related in django view increases query times

Question

I thought of trying to use select_realted in Django view to boost the performance. And I compare the results before using select_realted and after using it.

Although I see the reduction of significant number of queries, the time however rises. So I am not sure whether to use select_related in every view or not use them.

I just want to know when to use them and when not to.

My view is :

Before:

class ProductAPIView(ListAPIView):
    permission_classes = [AllowAny]
    serializer_class = ProductSerializer
    queryset = Product.objects.all()

After:

class ProductAPIView(ListAPIView):
    permission_classes = [AllowAny]
    serializer_class = ProductSerializer
    #queryset = Product.objects.all()

    queryset = Product.objects.select_related('merchant','brand','collection','sub_category')
    pagination_class = CustomPagination

My models:

class Product(models.Model):        
    merchant = models.ForeignKey(Seller,on_delete=models.CASCADE,blank=True,null=True)
    category = models.ManyToManyField(Category, blank=False)
    sub_category = models.ForeignKey(Subcategory, on_delete=models.CASCADE,blank=True,null=True)
    brand = models.ForeignKey(Brand, on_delete=models.CASCADE)
    collection = models.ForeignKey(Collection, on_delete=models.CASCADE)
    featured = models.BooleanField(default=False)  # is product featured?

The images before and after the use of select_related.

After:

Here we can see that the number of queries are reduced to 124, but the time is increased to 227. So, should i be using select_related? When to use it or when not to use it??

there is no point in using `select_realted` if you're not using the fetched relations. — Vishal Singh, Jun 06 '21 at 16:03
https://docs.djangoproject.com/en/3.2/ref/models/querysets/#select-related — Vishal Singh, Jun 06 '21 at 16:04

score 1 · Answer 1 · answered Jun 06 '21 at 18:59

the select_related in Django is actually JOIN in SQL.
select_related will fetch your related fields as well with your object in only one query.
for example imagine if you have a Book Model and you have this following code:

# Django will execute another query for getting author name
book = Book.objects.get(id=1)  
author = book.author.name
# 2 queries executed

In this example you have executed only one query but for getting the author.name Django will execute another query. But if you use select_related Django will get the author too in just one query.

# Django won't execute another query for getting author name
book = Book.objects.select_related('author').get(id=1)  
author = book.author.name
# 1 query executed

Note that you don't need to select all of your related objects in select_related if you don't need them.
Please check out this link in stackoverflow to know what's the differences between select_related and prefetch_related and when to use them.
also check out this link to know more about n + 1 query problems.

Using select_related in django view increases query times

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