Clarify comment

postgres.cc

@@ -414,7 +414,7 @@ schema_pqxx::schema_pqxx(std::string &conninfo, bool no_catalog, bool dump_state
       "AND NOT mz_functions.name like '%recv' " // https://github.com/MaterializeInc/materialize/issues/17870
       "AND mz_functions.name <> 'pg_cancel_backend' " // pg_cancel_backend in this position not yet supported
       "AND mz_functions.name <> 'csv_extract' " // https://github.com/MaterializeInc/materialize/issues/22735
-      "AND (mz_functions.name <> 'sum' OR mz_functions.return_type_id <> (select id from mz_types where name = 'interval')" // sum(interval) not yet supported
+      "AND (mz_functions.name <> 'sum' OR mz_functions.return_type_id <> (select id from mz_types where name = 'interval')" // sum(interval) not yet supported, see https://github.com/MaterializeInc/materialize/issues/18043
       "AND (mz_functions.name <> 'timezone' OR mz_functions.argument_type_ids[1] <> (select id from mz_types where name = 'time')" // timezone with time type is intentionally not supported, see https://github.com/MaterializeInc/materialize/pull/22960
       "AND NOT (" + procedure_is_aggregate + " or " + procedure_is_window + ") ");