[PATCH] Add index for patchwork_comment (submission_id,date)

[Stewart Smith]

This (at least theoretically) should speed up displaying comments
on patches/cover letters. It's an index that will return rows
in-order for the query that we always do ("give me the comments
on this submission in date order"), rather than having to have
the database server do a sort for us.

I haven't been able to benchmark something locally that shows
this is an actual improvement, but I don't have as large data
set as various production instances. The query plan does look
a bit nicer though. Although the benefit of index maintenance
versus how long it takes to sort things is a good question.

Signed-off-by: Stewart Smith <stewart at linux.ibm.com>
---
 .../migrations/0027_add_comment_date_index.py | 23 +++++++++++++++++++
 patchwork/models.py                           |  4 ++++
 2 files changed, 27 insertions(+)
 create mode 100644 patchwork/migrations/0027_add_comment_date_index.py

diff --git a/patchwork/migrations/0027_add_comment_date_index.py b/patchwork/migrations/0027_add_comment_date_index.py
new file mode 100644
index 000000000000..0a57a9c3b212
--- /dev/null
+++ b/patchwork/migrations/0027_add_comment_date_index.py
@@ -0,0 +1,23 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.15 on 2018-08-09 14:03
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('patchwork', '0026_add_user_bundles_backref'),
+    ]
+
+    operations = [
+        migrations.AlterModelOptions(
+            name='series',
+            options={'verbose_name_plural': 'Series'},
+        ),
+        migrations.AddIndex(
+            model_name='comment',
+            index=models.Index(fields=['submission', 'date'], name='submission_date_idx'),
+        ),
+    ]
diff --git a/patchwork/models.py b/patchwork/models.py
index d2b88fc48c91..d2389cfdad29 100644
--- a/patchwork/models.py
+++ b/patchwork/models.py
@@ -625,6 +625,10 @@ class Comment(EmailMixin, models.Model):
     class Meta:
         ordering = ['date']
         unique_together = [('msgid', 'submission')]
+        indexes = [
+            models.Index(name='submission_date_idx',
+                         fields=['submission','date'])
+            ]
 
 
 @python_2_unicode_compatible
-- 
2.17.1