How can get last nth item of a query on PostgreSql

Question

Question:

How I can get the last 750 records of a query in the Database level?

Here is What I have tried:

# Get last 750 applications

apps = MyModel.active_objects.filter(
    **query_params
).order_by('-created_at').values_list('id', flat=True)[:750]

This query fetches all records that hit the query_params filter and after that return the last 750 records. So I want to do this work at the database level, like mongoDb aggregate queries. Is it possible?

Thanks.

That is doing it at the database level. It does not fetch all records. — Daniel Roseman
– Daniel Roseman, Commented Nov 21, 2019 at 19:21

Navid Zarepak · Accepted Answer · 2019-11-21 19:29:50Z

1

Actually that's not how Django works. The limit part is also done in database level.

Django docs - Limiting QuerySets:

Generally, slicing a QuerySet returns a new QuerySet – it doesn’t evaluate the query.

To see what query is actually being run in the database you can simply print the query like this:

apps = MyModel.active_objects.filter(
           **query_params
       ).order_by('-created_at').values_list('id', flat=True)[:750]
print(apps.query)

The result will be something like this:

SELECT * FROM "app_mymodel" WHERE <...> ORDER BY "app_mymodel"."created_at" DESC LIMIT 750

edited Nov 21, 2019 at 19:29

answered Nov 21, 2019 at 19:24

Navid Zarepak

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1 Comment

Phoenix Over a year ago

Thanks for your explanation.

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