Index project_group_links.group_id

This column is used when refreshing authorizations and without the
index leads to a sequence scan being performed on this table.

db/migrate/20170504182103_add_index_project_group_links_group_id.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddIndexProjectGroupLinksGroupId < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :project_group_links, :group_id
+  end
+
+  def down
+    remove_concurrent_index :project_group_links, :group_id
+  end
+end

db/schema.rb

@@ -925,6 +925,8 @@ ActiveRecord::Schema.define(version: 20170508190732) do
     t.date "expires_at"
   end
 
+  add_index "project_group_links", ["group_id"], name: "index_project_group_links_on_group_id", using: :btree
+
   create_table "project_import_data", force: :cascade do |t|
     t.integer "project_id"
     t.text "data"