Add index on group_id column concurrently

This fixes previously added migration which caused timeouts on
big events table.

db/migrate/20190826100605_add_group_column_to_events.rb

 # frozen_string_literal: true
 class AddGroupColumnToEvents < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
   DOWNTIME = false
-  def change
-    add_reference :events, :group, index: true, foreign_key: { to_table: :namespaces, on_delete: :cascade }
+  disable_ddl_transaction!
+
+  def up
+    add_column(:events, :group_id, :bigint) unless column_exists?(:events, :group_id)
+    add_concurrent_index(:events, :group_id)
+    add_concurrent_foreign_key(:events, :namespaces, column: :group_id, on_delete: :cascade)
+  end
+
+  def down
+    remove_column(:events, :group_id) if column_exists?(:events, :group_id)
   end
 end

db/schema.rb

@@ -3864,7 +3864,7 @@ ActiveRecord::Schema.define(version: 2019_09_05_223900) do
   add_foreign_key "epics", "users", column: "assignee_id", name: "fk_dccd3f98fc", on_delete: :nullify
   add_foreign_key "epics", "users", column: "author_id", name: "fk_3654b61b03", on_delete: :cascade
   add_foreign_key "epics", "users", column: "closed_by_id", name: "fk_aa5798e761", on_delete: :nullify
-  add_foreign_key "events", "namespaces", column: "group_id", on_delete: :cascade
+  add_foreign_key "events", "namespaces", column: "group_id", name: "fk_61fbf6ca48", on_delete: :cascade
   add_foreign_key "events", "projects", on_delete: :cascade
   add_foreign_key "events", "users", column: "author_id", name: "fk_edfd187b6f", on_delete: :cascade
   add_foreign_key "external_pull_requests", "projects", on_delete: :cascade