Migrations: delete context objects

These objects represent from 30 to 70% of the rows on the objects table,
based on numbers from a few live instances (single-user, small, large.)

As those pseudo-objects prevent creating objects with those actual IDs,
deleting them is a better solution. This could have happened if an
object used another object's ID as its context.

priv/repo/migrations/20220807125023_data_migration_delete_context_objects.exs

@@ -0,0 +1,15 @@
+defmodule Pleroma.Repo.Migrations.DataMigrationDeleteContextObjects do
+  use Ecto.Migration
+
+  require Logger
+
+  @doc "This migration removes objects created exclusively for contexts, containing only an `id` field."
+
+  def change do
+    Logger.warn(
+      "This migration can take a very long time to execute, depending on your database size. Please be patient, Pleroma-tan is doing her best!\n"
+    )
+
+    execute("DELETE FROM objects WHERE (data->>'type') IS NULL;")
+  end
+end