Create optimal indexes for created_at order

This is for the projects API. Note that we can't have one index only
because we cannot scan the index backwards for both `sort=asc,sort=desc`
options. That is because the tie breaker is always `id desc` and hence
the sort order is not suitable for a backward scan for one of the
options.

Relates to https://gitlab.com/gitlab-org/gitlab/issues/36390

changelogs/unreleased/ab-projects-api-created-at-indexes.yml

+---
+title: Create optimal indexes for created_at order (Projects API)
+merge_request: 22623
+author:
+type: performance

db/migrate/20200108155731_create_indexes_for_project_api_created_at_order.rb

+# frozen_string_literal: true
+
+class CreateIndexesForProjectApiCreatedAtOrder < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :projects, %i(visibility_level created_at id), order: { id: :desc }, name: 'index_projects_on_visibility_level_created_at_id_desc'
+    add_concurrent_index :projects, %i(visibility_level created_at id), order: { created_at: :desc, id: :desc }, name: 'index_projects_on_visibility_level_created_at_desc_id_desc'
+    remove_concurrent_index_by_name :projects, 'index_projects_on_visibility_level_and_created_at_and_id'
+  end
+
+  def down
+    add_concurrent_index :projects, %i(visibility_level created_at id), name: 'index_projects_on_visibility_level_and_created_at_and_id'
+    remove_concurrent_index_by_name :projects, 'index_projects_on_visibility_level_created_at_id_desc'
+    remove_concurrent_index_by_name :projects, 'index_projects_on_visibility_level_created_at_desc_id_desc'
+  end
+end

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: 2020_01_08_100603) do
+ActiveRecord::Schema.define(version: 2020_01_08_155731) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -3376,7 +3376,8 @@ ActiveRecord::Schema.define(version: 2020_01_08_100603) do
     t.index ["runners_token"], name: "index_projects_on_runners_token"
     t.index ["runners_token_encrypted"], name: "index_projects_on_runners_token_encrypted"
     t.index ["star_count"], name: "index_projects_on_star_count"
-    t.index ["visibility_level", "created_at", "id"], name: "index_projects_on_visibility_level_and_created_at_and_id"
+    t.index ["visibility_level", "created_at", "id"], name: "index_projects_on_visibility_level_created_at_desc_id_desc", order: { created_at: :desc, id: :desc }
+    t.index ["visibility_level", "created_at", "id"], name: "index_projects_on_visibility_level_created_at_id_desc", order: { id: :desc }
   end
 
   create_table "prometheus_alert_events", force: :cascade do |t|