Django: how to use select_related () effectively with Paginator?

Question

I have two related models with 10 million rows each and you want to execute an effective paginated query of 50,000 elements in one of them and access the related data on the other:

class RnaPrecomputed(models.Model):
    id = models.CharField(max_length=22, primary_key=True)
    rna = models.ForeignKey('Rna', db_column='upi', to_field='upi', related_name='precomputed')
    description = models.CharField(max_length=250)


class Rna(models.Model):
    id = models.IntegerField(db_column='id')
    upi = models.CharField(max_length=13, db_index=True, primary_key=True)
    timestamp = models.DateField()
    userstamp = models.CharField(max_length=30)

As you can see, it RnaPrecomputedis connected RNAwith a foreign key. Now I want to get a specific page of 50,000 elements RnaPrecomputedand RNArelated ones. I expect the problem with an N + 1 request if I do this without a call select_related(). Here are the timings:

---

First, for reference, I will not touch on the related model at all:

rna_paginator = paginator.Paginator(RnaPrecomputed.objects.all(), 50000)
message = ""
for object in rna_paginator.page(400).object_list:
    message = message + str(object.id)

accepts:

real    0m12.614s
user    0m1.073s
sys 0m0.188s

---

Now I will try to access data on the associated model:

rna_paginator = paginator.Paginator(RnaPrecomputed.objects.all(), 50000)
message = ""
for object in rna_paginator.page(400).object_list:
    message = message + str(object.rna.upi)

required:

real    2m27.655s
user    1m20.194s
sys 0m4.315s

This is a lot, so maybe I have a problem with N + 1 queries.

---

, select_related(),

rna_paginator = paginator.Paginator(RnaPrecomputed.objects.all().select_related('rna'), 50000)
message = ""
for object in rna_paginator.page(400).object_list:
    message = message + str(object.rna.upi)

:

real    7m9.720s
user    0m1.948s
sys 0m0.337s

, - select_related() 3 , , . , , N + 1 , RnaPrecomputed Django ORM, , RNA?

select_related() ?

Answer 1

, . db_index=True Rna.upi, , ?

select_related count() , select_related object_list.

for object in rna_paginator.page(300).object_list.select_related():
    message = message + str(object.rna.upi)