I have the following two cases. Of the two, I am not able to tell which is the best order to take. Is there a performance issue in either of them??
Case 1
Model.objects.filter(item1=4,item2=5).select_related('store').get(id=1)
Case 2
Model.objects.select_related('store').filter(item1=4,item2=5).get(id=1)
>Solution :
You can inspect the query that Django makes by looking at the connections.queries list. So you can print the last query with:
>>> from django.db import connection
>>> print(connection.queries[-1]['sql'])
For the first query, we get:
>>> Model.objects.filter(item1=4,item2=5).select_related('store').get(id=1)
<Model: Model object (1)>
>>> print(connection.queries[-1]['sql'])
SELECT "app_name_model"."id", "app_name_model"."store_id", "app_name_model"."item1", "app_name_model"."item2", "app_name_store"."id" FROM "app_name_model" INNER JOIN "app_name_store" ON ("app_name_model"."store_id" = "app_name_store"."id") WHERE ("app_name_model"."item1" = 4 AND "app_name_model"."item2" = 5 AND "app_name_model"."id" = 1) LIMIT 21
whereas for the latter, it is:
>>> Model.objects.select_related('store').filter(item1=4,item2=5).get(id=1)
<Model: Model object (1)>
>>> print(connection.queries[-1]['sql'])
SELECT "app_name_model"."id", "app_name_model"."store_id", "app_name_model"."item1", "app_name_model"."item2", "app_name_store"."id" FROM "app_name_model" INNER JOIN "app_name_store" ON ("app_name_model"."store_id" = "app_name_store"."id") WHERE ("app_name_model"."item1" = 4 AND "app_name_model"."item2" = 5 AND "app_name_model"."id" = 1) LIMIT 21
Both produce thus exactly the same query:
SELECT "app_name_model"."id", "app_name_model"."store_id", "app_name_model"."item1", "app_name_model"."item2",
"app_name_store"."id"
FROM "app_name_model"
INNER JOIN "app_name_store" ON ("app_name_model"."store_id" = "app_name_store"."id")
WHERE ("app_name_model"."item1" = 4 AND "app_name_model"."item2" = 5 AND "app_name_model"."id" = 1)
Where the parts in boldface is the effect of the .select_related(…) call [Django-doc].