api: EventList: change select_related() to prefetch_related()

select_related() creates a single giant query that JOINs the required
tables together in the DB. prefetch_related() does a similar thing,
but at the Django layer - for all referenced models, it makes a
separate query to the DB to fetch them.

This massively, massively simplifies the job the DB has to do:
instead of creating a massive, sparse results table with many
columns, we do 1 query for the events, and then query for only
patches/cover letters/series/projects etc referenced in those 30
events.

Tested with cURL + JSON renderer + Postgres w/ ~100k patches,
request time went from 1.5s to 0.25s, a 6x speedup.

Tested with cURL + JSON renderer + MySQL w/ ~33k patches,
request time went from ~2.2s to ~0.20s, an ~11x speedup.

Signed-off-by: Daniel Axtens <dja@axtens.net>

diff --git a/patchwork/api/event.py b/patchwork/api/event.py
index 0d97af227f0e6f1c245b9d36a8ad73b8a93f6ef1..7e04b716af1a617b7d0a69b66554ee8ae92d319d 100644 (file)
--- a/patchwork/api/event.py
+++ b/patchwork/api/event.py
@@ -96,7 +96,7 @@ class EventList(ListAPIView):
 
     def get_queryset(self):
         return Event.objects.all()\
-            .select_related('project', 'patch', 'series', 'cover',
-                            'previous_state', 'current_state',
-                            'previous_delegate', 'current_delegate',
-                            'created_check')
+            .prefetch_related('project', 'patch', 'series', 'cover',
+                              'previous_state', 'current_state',
+                              'previous_delegate', 'current_delegate',
+                              'created_check')