Columns
ADD COLUMN using a constrained domain type
Lock mode ACCESS EXCLUSIVE
Risk HIGH
Blocks reads, writes, other DDL
pgfence rule
add-column-constrained-domain ALTER TABLE orders ADD COLUMN status order_status_domain;What this does
When the column's type is a CREATE DOMAIN type that carries a CHECK constraint, Postgres validates that constraint against every existing row as part of the ADD COLUMN, which can force a table rewrite under ACCESS EXCLUSIVE, the same risk profile as an inline CHECK.
Safe rewrite
Add a base-type column first, backfill, then enforce the domain constraint in a follow-up migration
ALTER TABLE orders ADD COLUMN IF NOT EXISTS status text;
-- Backfill and validate in a follow-up migration before switching to the constrained domain.