Optimize conversation fetching.

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -96,7 +96,7 @@ def delete(%Object{data: %{"id" => id, "actor" => actor}} = object, local \\ tru
 
   def fetch_activities_for_context(context) do
     query = from activity in Activity,
-      where: fragment("? @> ?", activity.data, ^%{ type: "Create", context: context }),
+      where: fragment("?->>'type' = ? and ?->>'context' = ?", activity.data, "Create", activity.data, ^context),
       order_by: [desc: :inserted_at]
     Repo.all(query)
   end

priv/repo/migrations/20170912114248_add_context_index.exs

@@ -0,0 +1,8 @@
+defmodule Pleroma.Repo.Migrations.AddContextIndex do
+  use Ecto.Migration
+  @disable_ddl_transaction true
+
+  def change do
+    create index(:activities, ["(data->>'type')", "(data->>'context')"], name: :activities_context_index, concurrently: true)
+  end
+end