Cache namespace traversal path

Work towards replacing recursive namespace queries with linear queries.
This commit tracks the path to each namespace, and future work will
replace recursive queries with queries that utilize this new column.

db/migrate/20210311001204_change_traversal_ids_to_bigint.rb

+# frozen_string_literal: true
+
+class ChangeTraversalIdsToBigint < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    change_column_type_concurrently :namespaces, :traversal_ids, 'bigint[]'
+  end
+
+  def down
+    undo_change_column_type_concurrently :namespaces, :traversal_ids
+  end
+end

db/schema_migrations/20210311001204

+55b0e53343a5a053097d88842e1b8ebce7a61d168ba14f6866b5800d3b28a4fa
\ No newline at end of file

db/structure.sql

@@ -150,6 +150,15 @@ $$;
 
 COMMENT ON FUNCTION table_sync_function_2be879775d() IS 'Partitioning migration: table sync for audit_events table';
 
+CREATE FUNCTION trigger_26c50fd02f49() RETURNS trigger
+    LANGUAGE plpgsql
+    AS $$
+BEGIN
+  NEW."traversal_ids_for_type_change" := NEW."traversal_ids";
+  RETURN NEW;
+END;
+$$;
+
 CREATE TABLE audit_events (
     id bigint NOT NULL,
     author_id integer NOT NULL,
@@ -24554,6 +24563,8 @@ CREATE TRIGGER table_sync_trigger_b99eb6998c AFTER INSERT OR DELETE OR UPDATE ON
 
 CREATE TRIGGER table_sync_trigger_ee39a25f9d AFTER INSERT OR DELETE OR UPDATE ON audit_events FOR EACH ROW EXECUTE PROCEDURE table_sync_function_2be879775d();
 
+CREATE TRIGGER trigger_26c50fd02f49 BEFORE INSERT OR UPDATE ON namespaces FOR EACH ROW EXECUTE PROCEDURE trigger_26c50fd02f49();
+
 CREATE TRIGGER trigger_has_external_issue_tracker_on_delete AFTER DELETE ON services FOR EACH ROW WHEN ((((old.category)::text = 'issue_tracker'::text) AND (old.active = true) AND (old.project_id IS NOT NULL))) EXECUTE PROCEDURE set_has_external_issue_tracker();
 
 CREATE TRIGGER trigger_has_external_issue_tracker_on_insert AFTER INSERT ON services FOR EACH ROW WHEN ((((new.category)::text = 'issue_tracker'::text) AND (new.active = true) AND (new.project_id IS NOT NULL))) EXECUTE PROCEDURE set_has_external_issue_tracker();

spec/models/namespace/traversal_hierarchy_spec.rb

@@ -62,6 +62,10 @@ RSpec.describe Namespace::TraversalHierarchy, type: :model do
       Namespace.update_all(traversal_ids: [])
     end
 
+    before do
+      Namespace.update_all(traversal_ids: [])
+    end
+
     it { is_expected.to match_array Namespace.all }
 
     context 'when lock is true' do