SYSE.34:5.2 - Keep one write authority during coexistence

The selected authority remains D. Old applications write D directly. The new application’s adapter validates L/T, joins them into D and writes D; it does not independently write derived columns.

A row-level BEFORE INSERT OR UPDATE trigger computes L/T from NEW.delivery_address and returns the modified row. The exact PostgreSQL 18 trigger behavior supplies same-transaction execution and returned-row semantics. This construction excludes other address-writing triggers, replicated writers that bypass this trigger and external side effects of these row updates. If those premises are not established, this mechanism is not yet qualified.

The authorized operator briefly fences new address writes and drains in-flight writers while installing the schema and trigger. Ordinary application roles must not bypass the mechanism or independently write the derived columns. The PostgreSQL 18 privilege rules require inspection of table grants, inherited rights, ownership and superuser access; revoking a column privilege does not cancel a broad table grant. Resume old writers only after the actual configuration satisfies those conditions.