Remove unused indexes

These indexes were always listed as unused and several grow quite large.
The most significant impact is the activities_visibility_index which takes many hours to rebuild when restoring the server from backup even on fast hardware.

priv/repo/migrations/20230422154018_drop_unused_indexes.exs

@@ -0,0 +1,88 @@
+defmodule Pleroma.Repo.Migrations.DropUnusedIndexes do
+  use Ecto.Migration
+
+  @disable_ddl_transaction true
+  @disable_migration_lock true
+
+  def up do
+    drop_if_exists(
+      index(:activities, ["(data->>'actor')", "inserted_at desc"], name: :activities_actor_index)
+    )
+
+    drop_if_exists(index(:activities, ["(data->'to')"], name: :activities_to_index))
+
+    drop_if_exists(index(:activities, ["(data->'cc')"], name: :activities_cc_index))
+
+    drop_if_exists(index(:activities, ["(split_part(actor, '/', 3))"], name: :activities_hosts))
+
+    drop_if_exists(
+      index(:activities, ["(data->'object'->>'inReplyTo')"], name: :activities_in_reply_to)
+    )
+
+    drop_if_exists(
+      index(:activities, ["((data #> '{\"object\",\"likes\"}'))"], name: :activities_likes)
+    )
+
+    drop_if_exists(
+      index(:activities, ["activity_visibility(actor, recipients, data)", "id DESC NULLS LAST"],
+        name: :activities_visibility_index,
+        where: "data->>'type' = 'Create'"
+      )
+    )
+  end
+
+  def down do
+    create_if_not_exists(
+      index(:activities, ["(data->>'actor')", "inserted_at desc"],
+        name: :activities_actor_index,
+        concurrently: true
+      )
+    )
+
+    create_if_not_exists(
+      index(:activities, ["(data->'to')"],
+        name: :activities_to_index,
+        using: :gin,
+        concurrently: true
+      )
+    )
+
+    create_if_not_exists(
+      index(:activities, ["(data->'cc')"],
+        name: :activities_cc_index,
+        using: :gin,
+        concurrently: true
+      )
+    )
+
+    create_if_not_exists(
+      index(:activities, ["(split_part(actor, '/', 3))"],
+        name: :activities_hosts,
+        concurrently: true
+      )
+    )
+
+    create_if_not_exists(
+      index(:activities, ["(data->'object'->>'inReplyTo')"],
+        name: :activities_in_reply_to,
+        concurrently: true
+      )
+    )
+
+    create_if_not_exists(
+      index(:activities, ["((data #> '{\"object\",\"likes\"}'))"],
+        name: :activities_likes,
+        using: :gin,
+        concurrently: true
+      )
+    )
+
+    create_if_not_exists(
+      index(:activities, ["activity_visibility(actor, recipients, data)", "id DESC NULLS LAST"],
+        name: :activities_visibility_index,
+        where: "data->>'type' = 'Create'",
+        concurrently: true
+      )
+    )
+  end
+end