Indexes for API search of archived Projects

Add two new indexes to support the default search options on archived
projects, since the archived projects are very sparse in the overall
table, leading to slow queries.

changelogs/unreleased/197955-projects-api-improve-api-response-time-for-archived-true.yml

+---
+title: Improve API response for archived project searchs
+merge_request: 27717
+author:
+type: performance

db/migrate/20200323134519_add_api_indexes_for_archived_projects.rb

+# frozen_string_literal: true
+
+class AddApiIndexesForArchivedProjects < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  PUBLIC_AND_ARCHIVED_INDEX_NAME = "index_projects_api_created_at_id_for_archived_vis20"
+  ARCHIVED_INDEX_NAME = "index_projects_api_created_at_id_for_archived"
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :projects, [:created_at, :id],
+      where: "archived = true AND visibility_level = 20 AND pending_delete = false",
+      name: PUBLIC_AND_ARCHIVED_INDEX_NAME
+
+    add_concurrent_index :projects, [:created_at, :id], where: "archived = true AND pending_delete = false",
+      name: ARCHIVED_INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :projects, ARCHIVED_INDEX_NAME
+
+    remove_concurrent_index_by_name :projects, PUBLIC_AND_ARCHIVED_INDEX_NAME
+  end
+end

db/structure.sql

@@ -9654,6 +9654,10 @@ CREATE UNIQUE INDEX index_project_tracing_settings_on_project_id ON public.proje
 
 CREATE INDEX index_projects_api_created_at_id_desc ON public.projects USING btree (created_at, id DESC);
 
+CREATE INDEX index_projects_api_created_at_id_for_archived ON public.projects USING btree (created_at, id) WHERE ((archived = true) AND (pending_delete = false));
+
+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_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);
@@ -12793,6 +12797,7 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200320123839
 20200323075043
 20200323122201
+20200323134519
 20200324115359
 \.