Backport partial index to find repositories that have not been checked

This was introduced in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5984.

db/post_migrate/20180629191052_add_partial_index_to_projects_for_last_repository_check_at.rb

+class AddPartialIndexToProjectsForLastRepositoryCheckAt < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  INDEX_NAME = "index_projects_on_last_repository_check_at"
+
+  def up
+    add_concurrent_index(:projects, :last_repository_check_at, where: "last_repository_check_at IS NOT NULL", name: INDEX_NAME)
+  end
+
+  def down
+    remove_concurrent_index(:projects, :last_repository_check_at, where: "last_repository_check_at IS NOT NULL", name: INDEX_NAME)
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
-ActiveRecord::Schema.define(version: 20180626125654) do
+ActiveRecord::Schema.define(version: 20180629191052) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1646,6 +1646,7 @@ ActiveRecord::Schema.define(version: 20180626125654) do
   add_index "projects", ["description"], name: "index_projects_on_description_trigram", using: :gin, opclasses: {"description"=>"gin_trgm_ops"}
   add_index "projects", ["id"], name: "index_projects_on_id_partial_for_visibility", unique: true, where: "(visibility_level = ANY (ARRAY[10, 20]))", using: :btree
   add_index "projects", ["last_activity_at"], name: "index_projects_on_last_activity_at", using: :btree
+  add_index "projects", ["last_repository_check_at"], name: "index_projects_on_last_repository_check_at", where: "(last_repository_check_at IS NOT NULL)", using: :btree
   add_index "projects", ["last_repository_check_failed"], name: "index_projects_on_last_repository_check_failed", using: :btree
   add_index "projects", ["last_repository_updated_at"], name: "index_projects_on_last_repository_updated_at", using: :btree
   add_index "projects", ["name"], name: "index_projects_on_name_trigram", using: :gin, opclasses: {"name"=>"gin_trgm_ops"}