Add indexes for authenticated requests to projects API

This supports the default query we run when you hit /api/v4/projects as
an authenticated user.

Relates to https://gitlab.com/gitlab-org/gitlab/issues/36135

changelogs/unreleased/ab-projects-api-indexes-authenticated.yml

+---
+title: Add index for authenticated requests to projects API default endpoint
+merge_request: 19993
+author:
+type: performance

db/migrate/20191112221821_add_indexes_for_projects_api_default_params_authenticated.rb

+# frozen_string_literal: true
+
+class AddIndexesForProjectsApiDefaultParamsAuthenticated < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :projects, %i(created_at id)
+    remove_concurrent_index_by_name :projects, 'index_projects_on_created_at'
+  end
+
+  def down
+    add_concurrent_index :projects, :created_at
+    remove_concurrent_index_by_name :projects, 'index_projects_on_created_at_and_id'
+  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: 2019_11_12_214305) do
+ActiveRecord::Schema.define(version: 2019_11_12_221821) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -3121,7 +3121,7 @@ ActiveRecord::Schema.define(version: 2019_11_12_214305) do
     t.integer "marked_for_deletion_by_user_id"
     t.index "lower((name)::text)", name: "index_projects_on_lower_name"
     t.index ["archived", "pending_delete", "merge_requests_require_code_owner_approval"], name: "projects_requiring_code_owner_approval", where: "((pending_delete = false) AND (archived = false) AND (merge_requests_require_code_owner_approval = true))"
-    t.index ["created_at"], name: "index_projects_on_created_at"
+    t.index ["created_at", "id"], name: "index_projects_on_created_at_and_id"
     t.index ["creator_id"], name: "index_projects_on_creator_id"
     t.index ["description"], name: "index_projects_on_description_trigram", opclass: :gin_trgm_ops, using: :gin
     t.index ["id", "repository_storage", "last_repository_updated_at"], name: "idx_projects_on_repository_storage_last_repository_updated_at"