#35317: Add the possibility to do prefetches for only a subset of instances
-------------------------------------+-------------------------------------
Reporter: Laurent Lyaudet | Owner: nobody
Type: New feature | Status: new
Component: Database layer | Version: 5.0
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by Laurent Lyaudet):
I have been able to make a very fast proof of concept right now in Django
shell.
Here is the redacted transcript. I will prepare a PR :)
{{{#!python
import random
from django.db import connection
from django.db.models import query
old_prefetch_one_level = query.prefetch_one_level
def new_prefetch_one_level(instances, prefetcher, lookup, level):
if hasattr(lookup, "filter_callback") and lookup.filter_callback is
not None:
instances = [instance for instance in instances if
lookup.filter_callback(instance)]
if len(instances) == 0:
return [], ()
return old_prefetch_one_level(instances, prefetcher, lookup, level)
query.prefetch_one_level = new_prefetch_one_level
from DeleevCore.orders.models import Order, OrderProduct
from django.db.models import Prefetch
some_random_prefetch = Prefetch("items",
queryset=OrderProduct.objects.all())
some_random_prefetch.filter_callback = lambda x: bool(random.randint(0,
1))
orders =
list(Order.objects.all().prefetch_related(some_random_prefetch)[:10])
print(connection.queries)
[{'sql': "SELECT t.oid, typarray FROM pg_type t JOIN pg_namespace ns ON
typnamespace = ns.oid WHERE typname = 'hstore'",
'time': '0.001'},
{'sql': "SELECT typarray FROM pg_type WHERE typname = 'citext'",
'time': '0.000'},
{'sql': 'SELECT @redacted@ FROM "orders_order" ORDER BY
"orders_order"."shipping_time" ASC LIMIT 10',
'time': '0.019'},
{'sql': 'SELECT @redacted@ FROM "orders_orderproduct" WHERE
"orders_orderproduct"."order_id" IN (114407, 84181, 84183, 25369)',
'time': '0.006'}]
Look only 4 ids in last query instead of 10 :)
}}}
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Ticket URL: <https://code.djangoproject.com/ticket/35317#comment:2>
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