Partial index optimization for namespaces id

changelogs/unreleased/326637-index-namespaces-id-and-parent_id.yml

+---
+title: Partial index optimization for namespaces id
+merge_request: 58220
+author:
+type: performance

db/migrate/20210406063442_create_namespaces_id_parent_id_partial_index.rb

+# frozen_string_literal: true
+
+class CreateNamespacesIdParentIdPartialIndex < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  NAME = 'index_namespaces_id_parent_id_is_null'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :namespaces, :id, where: 'parent_id IS NULL', name: NAME
+  end
+
+  def down
+    remove_concurrent_index :namespaces, :id, name: NAME
+  end
+end

db/schema_migrations/20210406063442

+d29f002f88440a10674b251791fa027cb0ae1c1b0c4fd776a2078e3c94160f17
\ No newline at end of file

db/structure.sql

@@ -23160,6 +23160,8 @@ CREATE UNIQUE INDEX index_namespace_root_storage_statistics_on_namespace_id ON n
 
 CREATE UNIQUE INDEX index_namespace_statistics_on_namespace_id ON namespace_statistics USING btree (namespace_id);
 
+CREATE INDEX index_namespaces_id_parent_id_is_null ON namespaces USING btree (id) WHERE (parent_id IS NULL);
+
 CREATE INDEX index_namespaces_on_created_at ON namespaces USING btree (created_at);
 
 CREATE INDEX index_namespaces_on_custom_project_templates_group_id_and_type ON namespaces USING btree (custom_project_templates_group_id, type) WHERE (custom_project_templates_group_id IS NOT NULL);