1.4.1 Database Migrations
A migration is a named change: add column, new index, backfill. Tools (Alembic, Flyway, Prisma) apply them in order and record what ran.
Why it matters
App code and schema must move together. Ad-hoc ALTER TABLE on prod is how you lose a weekend.
How it is used
Expand/contract: add a nullable column, deploy code that writes both old and new, backfill, then drop the old column in a later release. Never rename a live column in one step.