[PATCH 03/11] Add index for patchwork_comment (submission_id,date)

[Stephen Finucane]

On Fri, 2018-08-10 at 18:00 +1000, Stewart Smith wrote:
> 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.

Indexes scare me purely because there aren't clear "this is when you do
this" and "then is when you don't do this" guidelines :) However, I
trust your DB knowledge here and it should be easy to revisit this so

Reviewed-by: Stephen Finucane <stephen at that.guru>

Note that this migration will need to be renamed to 0028 and the
Series.Meta modification hunk removed, owing to that belonging in patch
1. I've done this locally without issue.

> 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