@Alceu Rodrigues de Freitas Junior, there are no too basic questions to ask :) I'm sure I'm missing something, so, please keep asking. But yes, I have considered that.
@Melvyn Sopacue, I opt to use prefetch_related because I'm fetching "Items" that are used as ForeignKey for "Photos" and "Events". Select_related is meant for cases where you have OneToOne of ForeignKey relation (single item) that you need to fetch. If I would be fetching "Photos" I would use selected_related to JOIN "Items" in SQL level. Now that I have fetching "Items" relations to "Photos" and "Events" are "reverse ForeignKey" Many-to-One sets and I need to use prefetch_related. Of course it means that JOIN is done in Python (that is probably what takes the time now), but I cannot see how I could do it with selected_related. Please let me know, if there's a way to utilize select_related with related sets. I'm kind of hoping to find a problem in my implementation or thinking that would somehow make the Python level JOIN behave badly. It's a big JOIN, but still, Python is fast. There must be some kind of extra looping or cloning going on when performing Python level JOIN for prefetched Photos or other related sets. On Sunday, June 11, 2017 at 4:57:22 PM UTC+2, Miika Huusko wrote: > > I have a performance problem with Django + Django REST Framework. The > problem is that it takes Django very, very long time to handle SQL query > response + DRF serialization take quite some time also. It happens when > there are ManyToMany or OneToMany relations and nested objects. Sounds like > "N + 1" problem, but that's not the case. I have 10k to 50k items with > related items and I try to fetch and serialize them for REST API. Request > takes from 20 to 60 seconds and currently I have no idea what is causing > the slowness. > > Note: I asked the same question in Stack Overflow, but haven't found the > answer from there - > https://stackoverflow.com/questions/44461638/django-django-rest-framework-postgresql-queries-and-serialization-is-very-sl > > > * N+1 Problem * > > There are not too many queries and queries themselves are fast (queries at > the end of this post). I'm using `prefetch_related` to limit number of > queries and what I'm seeing from DB queries everything is looking Okay..ish > (?). I get one query for each `prefetch_related` property + the original > query for serialized objects. There are lots and lots of IDs included in > `prefetch_related` queries, but I guess that is inevitable - as many IDs as > there are original items. > > To test SQL queries + data transfer, I run same queries with psql from one > of my EC2 instances to RDS DB with same DB data and wrote the data to file. > Data transfer plus file write on top of that and it is totally between 100 > to 500 ms for bigger SQL queries for different data sets. File write is > extra, but I wanted to ensure that I get all the data I expect. > > I tested timing from EC2 instance with command from like: > > time psql -f fat-query.psql --host=rds_host --port=5432 > --username=user --dbname=dbname > output.txt > > > * Profiling * > > When profiling as shown here > https://www.dabapps.com/blog/api-performance-profiling-django-rest-framework/ > I get results that DB lookups take most of the time while serializing is > not too fast either. As an example I have about 12k "Items" in the my local > PostgreSQL database for one "Project". All "Items" have 1-5 "Events" and > majority of "Items" have also 1-2 "Photos". Fetching and serializing that > data takes around 22 seconds on my laptop. I'm using AWS EC2 + RDS for > deployment and the timing is about the same there. On larger "Item" sets > serialization time is increasing more than DB lookup time, but DB lookups > always take most of the time. With 40k items you'll start to reach 1 min > execution time and different timeouts from Nginx and other parts of the > stack. > > Example with 12k items (models, serializers and queries below) > > Database lookup | 14.0292s > Serialization | 6.0076s > Django request/response | 0.3488s > API view | 0.2170s > Response rendering | 1.1092s > > If I leave Photos and Events out the result is > > Database lookup | 1.2447s > Serialization | 3.9668s > Django request/response | 0.2435s > API view | 0.1320s > Response rendering | 0.8495s > > > * What might cause the slowness? * > > So, the related fields are taking most of the time (many=True). The > profiling I used for testing is making `list` out of queryset before > serializing. Therefore lazy queries are executed before serialization. If I > don't do that, it doesn't change the overall results, but DB lookups are > evaluated when serializing with about the same amount of time. > > Now the problem for me is that all queries that are done are fast if > executed manually. So, I believe SQL queries are fast, but DB lookups from > Django's point of view are very slow. What am I missing here? Or how should > I continue investigations? It feels like now it requires serious effort > from Django to convert SQL query results to Django model instances. That > would imply that there's something wrong with my models, right? > > > At the end, I could turn to caching, but I would assume that handling < > 100k objects should not be an issue for Django if done correctly. > > ---------- > > Setup: Python 2.7.13, Django 1.10.7, DRF 3.6.3 > > Simplified versions of models, views and serializers: > > class List(models.Model): > ... CharFields, DateTimeFields, ForeignKeys etc. ... > > class Item(models.Model): > list = models.ForeignKey(List, on_delete=models.CASCADE, > db_index=True, null=True, related_name='items') > deleted_at = models.DateTimeField(db_index=True, blank=True, > null=True, default=None) > created_by = models.ForeignKey(User, blank=False) > project = models.ForeignKey('projects.Project', > on_delete=models.CASCADE) > ... other CharFields, DateTimeFields, ForeignKeys etc. ... > > class Event(models.Model): > item = models.ForeignKey(Item, on_delete=models.CASCADE, > db_index=True, null=True, related_name='events') > created_by = models.ForeignKey(User, blank=False) > deleted_at = models.DateTimeField(db_index=True, blank=True, > null=True, default=None) > ... other CharFields, DateTimeFields, ForeignKeys etc. ... > > class Photo(models.Model): > item = models.ForeignKey(Item, on_delete=models.CASCADE, > db_index=True, null=True, related_name='photos') > created_by = models.ForeignKey(User, blank=False) > deleted_at = models.DateTimeField(db_index=True, blank=True, > null=True, default=None) > ... other CharFields, DateTimeFields, ForeignKeys etc. ... > > > class PhotoSerializer(serializers.ModelSerializer): > ... other CharFields, DateTimeFields, ForeignKeys etc. ... > > class EventSerializer(serializers.ModelSerializer): > createdBy = PrimaryKeyRelatedStringField(source='created_by', > read_only=True) > createdByFullName = > serializers.CharField(source='created_by.get_full_name', read_only=True) > ... other CharFields, DateTimeFields, ForeignKeys etc. ... > > class ItemSerializer(serializers.ModelSerializer): > listName = serializers.CharField(source='list.name', > read_only=True) > createdBy = PrimaryKeyRelatedStringField(source='created_by', > read_only=True) > createdByFullName = > serializers.CharField(source='created_by.get_full_name', read_only=True) > photos = PhotoSerializer(many=True, read_only=True) > events = EventSerializer(many=True, required=False, > allow_null=True, queryset=Event.objects.all()) > ... other fields ... > > > class ItemListAPIView(ListAPIView): > model = Item > serializer_class = ItemSerializer > > def get_queryset(self): > return > Item.objects.all().filter(project_id=...).filter(deleted_at__isnull=True).prefetch_related( > 'created_by', # ID of user who created item > 'photos', # Photo properties > 'event__created_by', # Full name of the person who > created the event > 'list', # Name of the related list > ) > > Example queries from tests with 14s DB lookup result: > > django.db.backends: (0.196) SELECT "todo_item"."version", ... > everything ... FROM "todo_item" WHERE ("todo_item"."project_id" = 1 AND > "todo_item"."deleted_at" IS NULL) ORDER BY "todo_item"."created_at" DESC; > django.db.backends: (0.001) SELECT "auth_user"."id", ... everything > ... FROM "auth_user" WHERE "auth_user"."id" IN (1, 2, ... some IDs ...); > django.db.backends: (0.148) SELECT "photos_photo"."version", ... > everything ... FROM "photos_photo" WHERE ("photos_photo"."deleted_at" IS > NULL AND "photos_photo"."item_id" IN (1, 2, ... lots of IDs... N)) ORDER BY > "photos_photo"."created_at" DESC; > django.db.backends: (0.078) SELECT "events_event"."created_at", ... > everything ... FROM "events_event" WHERE ("events_event"."deleted_at" IS > NULL AND "events_event"."item_id" IN (1, 2, ... lots of IDs... N)) ORDER BY > "events_event"."created_at" DESC, "events_event"."created_at" DESC; > django.db.backends: (0.157) SELECT "todo_list"."created_at", ... FROM > "todo_list" WHERE "todo_list"."id" IN (1, 2, ... lots of IDs... N) > > -- You received this message because you are subscribed to the Google Groups "Django users" group. To unsubscribe from this group and stop receiving emails from it, send an email to django-users+unsubscr...@googlegroups.com. To post to this group, send email to django-users@googlegroups.com. Visit this group at https://groups.google.com/group/django-users. To view this discussion on the web visit https://groups.google.com/d/msgid/django-users/4e837be3-4832-4a46-be62-c74a4d617bb6%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
