feat: performance tuning credentials admin (#2684)

this adds a single index to  UserCredential.credential_id. You would
think this would be implicitly indexed by the  presence of a
unique_together, but it isn't. I don't want to get overly exuberant with
indices because we have a lot of costly writes  during grading already,
but I can see  using SQL `EXPLAIN`  that  making this change does
convert the django admin page from `using  filesort` to `using index`,
so theoretically this should speed it up. If it doesn't,  I will try a
different approach.

Author: deborahgu

credentials/apps/credentials/migrations/0031_usercredential_explicit_index.py

@@ -0,0 +1,18 @@
+# Generated by Django 4.2.17 on 2025-01-15 21:43
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("credentials", "0030_revoke_certificates_management_command"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="usercredential",
+            name="credential_id",
+            field=models.PositiveIntegerField(db_index=True),
+        ),
+    ]

credentials/apps/credentials/models.py

@@ -172,7 +172,7 @@ class UserCredential(TimeStampedModel):
         limit_choices_to={"model__in": ("coursecertificate", "programcertificate", "credlybadgetemplate")},
         on_delete=models.CASCADE,
     )
-    credential_id = models.PositiveIntegerField()
+    credential_id = models.PositiveIntegerField(db_index=True)
     credential = GenericForeignKey("credential_content_type", "credential_id")
 
     username = models.CharField(max_length=255, db_index=True)