Fix slow Patch counting query

Stephen Rothwell noticed (way back in September - sorry Stephen!) that
the following query is really slow on OzLabs:

SELECT COUNT(*) AS "__count" FROM "patchwork_patch"
    INNER JOIN "patchwork_submission" ON
        ("patchwork_patch"."submission_ptr_id" = "patchwork_submission"."id")
    WHERE ("patchwork_submission"."project_id" = 14 AND
           "patchwork_patch"."state_id" IN
	       (SELECT U0."id" AS Col1 FROM "patchwork_state" U0
                WHERE U0."action_required" = true
		ORDER BY U0."ordering" ASC));

I think this is really slow because we have to join the patch and
submission table to get the project id, which we need to filter the
patches.

Duplicate the project id in the patch table itself, which allows us to
avoid the JOIN.

The new query reads as:
SELECT COUNT(*) AS "__count" FROM "patchwork_patch"
    WHERE ("patchwork_patch"."patch_project_id" = 1 AND
           "patchwork_patch"."state_id" IN
	       (SELECT U0."id" AS Col1 FROM "patchwork_state" U0
	        WHERE U0."action_required" = true
		ORDER BY U0."ordering" ASC));

Very simple testing on a small, artifical Postgres instance (3
projects, 102711 patches), shows speed gains of ~1.5-5x for this
query. Looking at Postgres' cost estimates (EXPLAIN) of the first
query vs the second query, we see a ~1.75x improvement there too.

I suspect the gains will be bigger on OzLabs.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Daniel Axtens <dja@axtens.net>

5 files changed, 47 insertions, 1 deletions

diff --git a/patchwork/migrations/0024_patch_patch_project.py b/patchwork/migrations/0024_patch_patch_project.py
new file mode 100644
index 0000000..76d8f14
--- /dev/null
+++ b/patchwork/migrations/0024_patch_patch_project.py
@@ -0,0 +1,39 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.10 on 2018-03-08 01:51
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+    # per migration 16, but note this seems to be going away
+    # in new PostgreSQLs (https://stackoverflow.com/questions/12838111/south-cannot-alter-table-because-it-has-pending-trigger-events#comment44629663_12838113)
+    atomic = False
+
+    dependencies = [
+        ('patchwork', '0023_timezone_unify'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='patch',
+            name='patch_project',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, to='patchwork.Project'),
+            preserve_default=False,
+        ),
+
+        # as with 10, this will break if you use non-default table names
+        migrations.RunSQL('''UPDATE patchwork_patch SET patch_project_id =
+                               (SELECT project_id FROM patchwork_submission
+                                WHERE patchwork_submission.id =
+                                        patchwork_patch.submission_ptr_id);'''
+        ),
+
+        migrations.AlterField(
+            model_name='patch',
+            name='patch_project',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, to='patchwork.Project'),
+        ),
+
+    ]
diff --git a/patchwork/models.py b/patchwork/models.py
index ff1d7dc..f91b994 100644
--- a/patchwork/models.py
+++ b/patchwork/models.py
@@ -433,6 +433,10 @@ class Patch(SeriesMixin, Submission):
     archived = models.BooleanField(default=False)
     hash = HashField(null=True, blank=True)
 
+    # duplicate project from submission in subclass so we can count the
+    # patches in a project without needing to do a JOIN.
+    patch_project = models.ForeignKey(Project, on_delete=models.CASCADE)
+
     objects = PatchManager()
 
     @staticmethod
diff --git a/patchwork/parser.py b/patchwork/parser.py
index 803e985..805037c 100644
--- a/patchwork/parser.py
+++ b/patchwork/parser.py
@@ -1004,6 +1004,7 @@ def parse_mail(mail, list_id=None):
         patch = Patch.objects.create(
             msgid=msgid,
             project=project,
+            patch_project=project,
             name=name[:255],
             date=date,
             headers=headers,
diff --git a/patchwork/tests/utils.py b/patchwork/tests/utils.py
index f0a71fb..eb14a7d 100644
--- a/patchwork/tests/utils.py
+++ b/patchwork/tests/utils.py
@@ -176,6 +176,8 @@ def create_patch(**kwargs):
         'diff': SAMPLE_DIFF,
     }
     values.update(kwargs)
+    if 'patch_project' not in values:
+        values['patch_project'] = values['project']
 
     return Patch.objects.create(**values)
 
diff --git a/patchwork/views/__init__.py b/patchwork/views/__init__.py
index 3baf299..f8d23a3 100644
--- a/patchwork/views/__init__.py
+++ b/patchwork/views/__init__.py
@@ -270,7 +270,7 @@ def generic_list(request, project, view, view_args=None, filter_settings=None,
             context['filters'].set_status(filterclass, setting)
 
     if patches is None:
-        patches = Patch.objects.filter(project=project)
+        patches = Patch.objects.filter(patch_project=project)
 
     # annotate with tag counts
     patches = patches.with_tag_counts(project)