confirmation: Add database indexes on confirmation fields.

Apparently, we didn't have any database indexes on Confirmation, which
meant that on servers with large numbers of users like zulipchat.com,
new account registration could spend a ton of time effectively doing a
table scan on this table.

confirmation/migrations/0007_add_indexes.py

@@ -0,0 +1,37 @@
+# Generated by Django 2.2.10 on 2020-03-27 09:02
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('confirmation', '0006_realmcreationkey_presume_email_valid'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='confirmation',
+            name='confirmation_key',
+            field=models.CharField(db_index=True, max_length=40),
+        ),
+        migrations.AlterField(
+            model_name='confirmation',
+            name='date_sent',
+            field=models.DateTimeField(db_index=True),
+        ),
+        migrations.AlterField(
+            model_name='confirmation',
+            name='object_id',
+            field=models.PositiveIntegerField(db_index=True),
+        ),
+        migrations.AlterField(
+            model_name='realmcreationkey',
+            name='creation_key',
+            field=models.CharField(db_index=True, max_length=40, verbose_name='activation key'),
+        ),
+        migrations.AlterUniqueTogether(
+            name='confirmation',
+            unique_together={('type', 'confirmation_key')},
+        ),
+    ]

confirmation/models.py

@@ -90,10 +90,10 @@ def confirmation_url(confirmation_key: str, host: str,
 
 class Confirmation(models.Model):
     content_type = models.ForeignKey(ContentType, on_delete=CASCADE)
-    object_id = models.PositiveIntegerField()  # type: int
+    object_id = models.PositiveIntegerField(db_index=True)  # type: int
     content_object = GenericForeignKey('content_type', 'object_id')
-    date_sent = models.DateTimeField()  # type: datetime.datetime
-    confirmation_key = models.CharField(max_length=40)  # type: str
+    date_sent = models.DateTimeField(db_index=True)  # type: datetime.datetime
+    confirmation_key = models.CharField(max_length=40, db_index=True)  # type: str
     realm = models.ForeignKey(Realm, null=True, on_delete=CASCADE)  # type: Optional[Realm]
 
     # The following list is the set of valid types
@@ -110,6 +110,9 @@ class Confirmation(models.Model):
     def __str__(self) -> str:
         return '<Confirmation: %s>' % (self.content_object,)
 
+    class Meta:
+        unique_together = ("type", "confirmation_key")
+
 class ConfirmationType:
     def __init__(self, url_name: str,
                  validity_in_days: int=settings.CONFIRMATION_LINK_DEFAULT_VALIDITY_DAYS) -> None:
@@ -171,7 +174,7 @@ def generate_realm_creation_url(by_admin: bool=False) -> str:
                                kwargs={'creation_key': key}))
 
 class RealmCreationKey(models.Model):
-    creation_key = models.CharField('activation key', max_length=40)
+    creation_key = models.CharField('activation key', db_index=True, max_length=40)
     date_created = models.DateTimeField('created', default=timezone_now)
 
     # True just if we should presume the email address the user enters