slightly changes definition so that autovacuum works paralell safe too

priv/repo/migrations/20190122153157_update_activity_visibility.exs

+defmodule Pleroma.Repo.Migrations.UpdateActivityVisibility do
+  use Ecto.Migration
+  @disable_ddl_transaction true
+
+  def up do
+    definition = """
+    create or replace function activity_visibility(actor varchar, recipients varchar[], data jsonb) returns varchar as $$
+    DECLARE
+      fa varchar;
+      public varchar := 'https://www.w3.org/ns/activitystreams#Public';
+    BEGIN
+      SELECT COALESCE(users.follower_address, '') into fa from users where users.ap_id = actor;
+
+      IF data->'to' ? public THEN
+        RETURN 'public';
+      ELSIF data->'cc' ? public THEN
+        RETURN 'unlisted';
+      ELSIF ARRAY[fa] && recipients THEN
+        RETURN 'private';
+      ELSIF not(ARRAY[fa, public] && recipients) THEN
+        RETURN 'direct';
+      ELSE
+        RETURN 'unknown';
+      END IF;
+    END;
+    $$ LANGUAGE plpgsql IMMUTABLE PARALLEL SAFE SECURITY DEFINER;
+    """
+
+    execute(definition)
+
+  end
+
+  def down do
+
+  end
+end