@Melvyn Sopacua, yes, that is very true that auth_user SELECT can be avoided. Thanks for pointing that out! I don't know what I had put it like that. Unfortunately, it's the related sets and big JOINs that are the problem and generating the "slowness".
The reason I'm providing all "Items" and other relates stuff with one request is that I want to make customer (that's also me and my good frieds) life easier. I want to keep all the difficult and heavy work on server side. Customer needs all data because of offline requirements and therefore would need to loop through pagination or perform the same big JOIN if "Photos" and other related objects would be requested separately. Doing that in mobile devices is going to take longer than on Ubuntu server even if it takes longer that I would expect with Python + Django. That said, it's good to point out opinions on design patterns. I haven't listed all background information for decisions. I'm not quite sure what you mean by "hash photos and use that as identifier". What I could do - and believe what you mean(?) - is to provide list of "Photo" IDs (or hashes) for "Item" and ask customer to fetch "Photo" details separately with another request(s). Or - easier for API - leave "Photo" IDs out of the "Item" information completely and ask customer to make the JOIN with separately requested "Photos". JOIN could be done with hashes or IDs. Either way it would be customer who would make the JOIN. I might go there, but still I don't believe that Django could not handle my meager data faster than 20 seconds :) If that would be the case it really would be sad. 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... 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