[PATCH] Add covering index for /list/ query

[Stewart Smith]

In constructing the list of patches for a project, there are two
main queries that are executed:
1) get a count() of how many patches there are
2) Get the page of results being displayed

In a test dataset of ~11500 LKML patches and ~4000 others, the existing
code would take around 585ms and 858ms with a cold cache and 28ms and
198ms for a warm cache.

By adding a covering index, we get down to 4ms and 255ms for a cold
cache, and 4ms and 143ms for a warm cache!

Additionally, when there's a lot of archived or accepted patches
(I used ~11000 archived out of the 15000 total in my test set)
the query time goes from 28ms and 72ms down to 2ms and 33-40ms!

Signed-off-by: Stewart Smith <stewart at linux.ibm.com>
---
 .../0028_add_list_covering_index.py           | 19 +++++++++++++++++++
 patchwork/models.py                           |  6 ++++++
 2 files changed, 25 insertions(+)
 create mode 100644 patchwork/migrations/0028_add_list_covering_index.py

diff --git a/patchwork/migrations/0028_add_list_covering_index.py b/patchwork/migrations/0028_add_list_covering_index.py
new file mode 100644
index 000000000000..65ebaefbead7
--- /dev/null
+++ b/patchwork/migrations/0028_add_list_covering_index.py
@@ -0,0 +1,19 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.15 on 2018-08-09 17:24
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('patchwork', '0027_add_comment_date_index'),
+    ]
+
+    operations = [
+        migrations.AddIndex(
+            model_name='patch',
+            index=models.Index(fields=['archived', 'patch_project', 'state', 'delegate'], name='patch_list_covering_idx'),
+        ),
+    ]
diff --git a/patchwork/models.py b/patchwork/models.py
index d2389cfdad29..15224ad69cfa 100644
--- a/patchwork/models.py
+++ b/patchwork/models.py
@@ -598,6 +598,12 @@ class Patch(SeriesMixin, Submission):
         if django.VERSION >= (1, 10):
             base_manager_name = 'objects'
 
+        indexes = [
+            # This is a covering index for the /list/ query
+            models.Index(fields=['archived','patch_project','state','delegate'],
+                         name='patch_list_covering_idx'),
+            ]
+
 
 class Comment(EmailMixin, models.Model):
     # parent
-- 
2.17.1