Thanks for input @Melvyn Sopacua and @Alceu Rodriques de Freitas Junior! I know that it would be easy to just change architecture, but I wanted actually two things here. One was to "get stuff done" and the other was to "understand the problem". The first is easier. If nothing else would have worked I could have paginated or something similar, but the second is more difficult.
The main problem here is the JOIN after `prefetch_related` queries. As the profiling shows something strange happens before serialization and Django is using ridiculous amount of time for the JOIN while SQL queries themselves are fast. Serialization is a bit slow, but not that bad. I sill don't know why the JOIN in this use case by default is so slow. So the "understand the problem" is open :) For the "get stuff done" part I ended up doing the JOIN + serialization by myself. Also adding same ideas that you Melvyn had, e.g. using 'values'. The main part is that I didn't use 'prefetch_related' but made same QuerySets separately and then simple JOIN. Not doing the automatic JOIN following 'prefetch_related' made the biggest difference. While "default" GET e.g. for 50k "Items" took about 1 minute, with updates it's now about 10 seconds. That's good enough for now, although it's sad I had to write code that one shouldn't need to write when using good framework. Added update also to Stack Overflow: https://stackoverflow.com/questions/44461638/django-django-rest-framework-postgresql-queries-and-serialization-is-very-sl Thanks for help. On Sunday, June 11, 2017 at 5:57:22 PM UTC+3, 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... 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