Add index for mirror_user_id to projects table

Related:
https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/6589

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20190408163745) do
+ActiveRecord::Schema.define(version: 20190423124640) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -2545,6 +2545,7 @@ ActiveRecord::Schema.define(version: 20190408163745) do
     t.index ["last_repository_check_failed"], name: "index_projects_on_last_repository_check_failed", using: :btree
     t.index ["last_repository_updated_at"], name: "index_projects_on_last_repository_updated_at", using: :btree
     t.index ["mirror_last_successful_update_at"], name: "index_projects_on_mirror_last_successful_update_at", using: :btree
+    t.index ["mirror_user_id"], name: "index_projects_on_mirror_user_id", using: :btree
     t.index ["name"], name: "index_projects_on_name_trigram", using: :gin, opclasses: {"name"=>"gin_trgm_ops"}
     t.index ["namespace_id"], name: "index_projects_on_namespace_id", using: :btree
     t.index ["path"], name: "index_projects_on_path", using: :btree

ee/changelogs/unreleased/ab-add-projects-index-for-mirror-user-id.yml

+---
+title: Add index for mirror_user_id to projects table
+merge_request: 11422
+author:
+type: performance

ee/db/migrate/20190423124640_add_index_to_projects_mirror_user_id.rb

+# frozen_string_literal: true
+
+class AddIndexToProjectsMirrorUserId < ActiveRecord::Migration[5.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :projects, :mirror_user_id
+  end
+
+  def down
+    remove_concurrent_index :projects, :mirror_user_id
+  end
+end