Add index on succeeded tasks (#164)

* Add index on task table

* also add models file

* add make commands and limit index

* format

django_q/models.py

@@ -5,6 +5,7 @@ from keyword import iskeyword
 from django import get_version
 from django.core.exceptions import ValidationError
 from django.db import models
+from django.db.models import Q
 from django.template.defaultfilters import truncatechars
 from django.urls import reverse
 from django.utils import timezone
@@ -111,6 +112,13 @@ class Task(models.Model):
     class Meta:
         app_label = "django_q"
         ordering = ["-stopped"]
+        indexes = [
+            models.Index(
+                name="success_index",
+                fields=["group", "name", "func"],
+                condition=Q(success=True),
+            ),
+        ]
 
 
 class SuccessManager(models.Manager):