Bug 1419777 - Fix timeouts when loading perfherder compare view (#2973)

We need an index for optimizing fetching datums by push id (mysql is
currently choosing the repository foreign key index, which is not
very selective at all)

treeherder/perf/migrations/0004_add_per_push_performance_datum_index.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.6 on 2017-11-22 18:14
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('model', '0018_superseded_to_textlogstep_enum'),
+        ('perf', '0003_add_performance_signature_alert_change_type'),
+    ]
+
+    operations = [
+        migrations.AlterIndexTogether(
+            name='performancedatum',
+            index_together=set([('repository', 'signature', 'push_timestamp'), ('repository', 'signature', 'push')]),
+        ),
+    ]

treeherder/perf/models.py

@@ -155,7 +155,12 @@ class PerformanceDatum(models.Model):
 
     class Meta:
         db_table = 'performance_datum'
-        index_together = ('repository', 'signature', 'push_timestamp')
+        index_together = [
+            # this should speed up the typical "get a range of performance datums" query
+            ('repository', 'signature', 'push_timestamp'),
+            # this should speed up the compare view in treeherder (we only index on
+            # repository because we currently filter on it in the query)
+            ('repository', 'signature', 'push')]
         unique_together = ('repository', 'job', 'push', 'signature')
 
     def save(self, *args, **kwargs):