diff --git a/activities/migrations/0012_in_reply_to_index.py b/activities/migrations/0012_in_reply_to_index.py
new file mode 100644
index 0000000..48519d5
--- /dev/null
+++ b/activities/migrations/0012_in_reply_to_index.py
@@ -0,0 +1,20 @@
+# Generated by Django 4.1.4 on 2023-03-05 17:33
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("activities", "0011_postinteraction_value_alter_postinteraction_type"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="post",
+            name="in_reply_to",
+            field=models.CharField(
+                blank=True, db_index=True, max_length=500, null=True
+            ),
+        ),
+    ]
diff --git a/activities/models/post.py b/activities/models/post.py
index bbcfecc..22ad292 100644
--- a/activities/models/post.py
+++ b/activities/models/post.py
@@ -269,7 +269,7 @@ class Post(StatorModel):
 
     # The Post it is replying to as an AP ID URI
     # (as otherwise we'd have to pull entire threads to use IDs)
-    in_reply_to = models.CharField(max_length=500, blank=True, null=True)
+    in_reply_to = models.CharField(max_length=500, blank=True, null=True, db_index=True)
 
     # The identities the post is directly to (who can see it if not public)
     to = models.ManyToManyField(