add index for modified to course and subsection grades

and first attempted for subsection
for TNL-6281

lms/djangoapps/grades/migrations/0009_auto_20170111_1507.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('grades', '0008_persistentsubsectiongrade_first_attempted'),
+    ]
+
+    operations = [
+        migrations.AlterIndexTogether(
+            name='persistentcoursegrade',
+            index_together=set([('passed_timestamp', 'course_id'), ('modified', 'course_id')]),
+        ),
+        migrations.AlterIndexTogether(
+            name='persistentsubsectiongrade',
+            index_together=set([('modified', 'course_id', 'usage_key')]),
+        ),
+    ]

lms/djangoapps/grades/migrations/0010_auto_20170112_1156.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('grades', '0009_auto_20170111_1507'),
+    ]
+
+    operations = [
+        migrations.AlterIndexTogether(
+            name='persistentsubsectiongrade',
+            index_together=set([('modified', 'course_id', 'usage_key'), ('first_attempted', 'course_id', 'user_id')]),
+        ),
+    ]

lms/djangoapps/grades/models.py

@@ -253,6 +253,17 @@ class PersistentSubsectionGrade(DeleteGradesMixin, TimeStampedModel):
             # * Course staff can see all grades for a course using (course_id,)
             ('course_id', 'user_id', 'usage_key'),
         ]
+        # Allows querying in the following ways:
+        # (modified): find all the grades updated within a certain timespan
+        # (modified, course_id): find all the grades updated within a timespan for a certain course
+        # (modified, course_id, usage_key): find all the grades updated within a timespan for a subsection
+        #   in a course
+        # (first_attempted, course_id, user_id): find all attempted subsections in a course for a user
+        # (first_attempted, course_id): find all attempted subsections in a course for all users
+        index_together = [
+            ('modified', 'course_id', 'usage_key'),
+            ('first_attempted', 'course_id', 'user_id')
+        ]
 
     # primary key will need to be large for this table
     id = UnsignedBigIntAutoField(primary_key=True)  # pylint: disable=invalid-name
@@ -502,11 +513,14 @@ class PersistentCourseGrade(DeleteGradesMixin, TimeStampedModel):
         # (course_id) for instructors to see all course grades, implicitly created via the unique_together constraint
         # (user_id) for course dashboard; explicitly declared as an index below
         # (passed_timestamp, course_id) for tracking when users first earned a passing grade.
+        # (modified): find all the grades updated within a certain timespan
+        # (modified, course_id): find all the grades updated within a certain timespan for a course
         unique_together = [
             ('course_id', 'user_id'),
         ]
         index_together = [
             ('passed_timestamp', 'course_id'),
+            ('modified', 'course_id')
         ]
 
     # primary key will need to be large for this table