Set a lock_timeout, split steps, and add columns nullable-then-backfill to keep migration locks short.
Migrations that take and hold strong locks while doing heavy work stall every query behind them and can take production down.
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An ALTER TABLE that rewrites the table or backfills under an ACCESS EXCLUSIVE lock blocks all reads and writes, and a blocked DDL can queue behind one slow query and then block everything else. Keep locks short: set lock_timeout, add columns nullable first, backfill in batches, then add constraints separately.
-- bad (rewrite + backfill under one lock)
ALTER TABLE users ADD COLUMN status text NOT NULL DEFAULT 'active';
-- good
SET lock_timeout = '3s';
ALTER TABLE users ADD COLUMN status text;
-- backfill in batches, then:
ALTER TABLE users ALTER COLUMN status SET DEFAULT 'active';