Resolve "Inverse index on namespaces parent_id, id"

changelogs/unreleased/330086-inverse-index-on-namespaces-parent_id-id.yml

+---
+title: Partial index optimization for namespaces id
+merge_request: 61098
+author:
+type: performance

db/post_migrate/20210506064413_create_namespaces_id_parent_id_inverse_partial_index.rb

+# frozen_string_literal: true
+
+class CreateNamespacesIdParentIdInversePartialIndex < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  NAME = 'index_namespaces_id_parent_id_is_not_null'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :namespaces, :id, where: 'parent_id IS NOT NULL', name: NAME
+  end
+
+  def down
+    remove_concurrent_index :namespaces, :id, name: NAME
+  end
+end

db/schema_migrations/20210506064413

+f400225e6caa854f825422b9799e61ea557ab4bd3e4a33dc3cd3193ed3ce1db2
\ No newline at end of file

db/structure.sql

@@ -23454,6 +23454,8 @@ 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_not_null ON namespaces USING btree (id) WHERE (parent_id IS NOT NULL);
+
 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);