Index for API search of internal projects

Add a new index to support descending searches of internal projects,
which are slow due to sparseness and distribution of the data.

changelogs/unreleased/197957-projects-api-improve-response-time-for-created_at-desc-order-and-v.yml

+---
+title: Improve API response for descending internal project searches
+merge_request: 28038
+author:
+type: performance

db/migrate/20200325183636_add_api_index_for_internal_projects.rb

+# frozen_string_literal: true
+
+class AddApiIndexForInternalProjects < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  INTERNAL_PROJECTS_INDEX_NAME = "index_projects_api_created_at_id_for_vis10"
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :projects, [:created_at, :id],
+      where: "visibility_level = 10 AND pending_delete = false",
+      name: INTERNAL_PROJECTS_INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :projects, INTERNAL_PROJECTS_INDEX_NAME
+  end
+end

db/structure.sql

@@ -9685,6 +9685,8 @@ CREATE INDEX index_projects_api_created_at_id_for_archived ON public.projects US
 
 CREATE INDEX index_projects_api_created_at_id_for_archived_vis20 ON public.projects USING btree (created_at, id) WHERE ((archived = true) AND (visibility_level = 20) AND (pending_delete = false));
 
+CREATE INDEX index_projects_api_created_at_id_for_vis10 ON public.projects USING btree (created_at, id) WHERE ((visibility_level = 10) AND (pending_delete = false));
+
 CREATE INDEX index_projects_api_last_activity_at_id_desc ON public.projects USING btree (last_activity_at, id DESC);
 
 CREATE INDEX index_projects_api_name_id_desc ON public.projects USING btree (name, id DESC);
@@ -12839,5 +12841,6 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200323134519
 20200324115359
 20200325160952
+20200325183636
 \.