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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