Speed up follower query.

lib/pleroma/user.ex

@@ -265,7 +265,7 @@ defmodule Pleroma.User do
 
   def get_followers(%User{id: id, follower_address: follower_address}) do
     q = from u in User,
-      where: ^follower_address in u.following,
+      where: fragment("? <@ ?", ^[follower_address], u.following),
       where: u.id != ^id
 
     {:ok, Repo.all(q)}

priv/repo/migrations/20180325172351_add_follower_address_index_to_users.exs

@@ -4,5 +4,6 @@ defmodule Pleroma.Repo.Migrations.AddFollowerAddressIndexToUsers do
   @disable_ddl_transaction true
   def change do
     create index(:users, [:follower_address], concurrently: true)
+    create index(:users, [:following], concurrently: true, using: :gin)
   end
 end