
About
Database migration best practices for schema changes, data migrations, rollbacks, and zero-downtime deployments across PostgreSQL, MySQL, and common ORMs (Prisma, Drizzle, Kysely, Django, TypeORM, golang-migrate).
name: database-migrations description: Database migration best practices for schema changes, data migrations, rollbacks, and zero-downtime deployments across PostgreSQL, MySQL, and common ORMs (Prisma, Drizzle, Kysely, Django, TypeORM, golang-migrate). origin: ECC
Database Migration Patterns
Safe, reversible database schema changes for production systems.
When to Activate
- Creating or altering database tables
- Adding/removing columns or indexes
- Running data migrations (backfill, transform)
- Planning zero-downtime schema changes
- Setting up migration tooling for a new project
Core Principles
- Every change is a migration — never alter production databases manually
- Migrations are forward-only in production — rollbacks use new forward migrations
- Schema and data migrations are separate — never mix DDL and DML in one migration
- Test migrations against production-sized data — a migration that works on 100 rows may lock on 10M
- Migrations are immutable once deployed — never edit a migration that has run in production
Migration Safety Checklist
Before applying any migration:
- [ ] Migration has both UP and DOWN (or is explicitly marked irreversible)
- [ ] No full table locks on large tables (use concurrent operations)
- [ ] New columns have defaults or are nullable (never add NOT NULL without default)
- [ ] Indexes created concurrently (not inline with CREATE TABLE for existing tables)
- [ ] Data backfill is a separate migration from schema change
- [ ] Tested against a copy of production data
- [ ] Rollback plan documented
PostgreSQL Patterns
Adding a Column Safely
-- GOOD: Nullable column, no lock
ALTER TABLE users ADD COLUMN avatar_url TEXT;
-- GOOD: Column with default (Postgres 11+ is instant, no rewrite)
ALTER TABLE users ADD COLUMN is_active BOOLEAN NOT NULL DEFAULT true;
-- BAD: NOT NULL without default on existing table (requires full rewrite)
ALTER TABLE users ADD COLUMN role TEXT NOT NULL;
-- This locks the table and rewrites every row
Adding an Index Without Downtime
-- BAD: Blocks writes on large tables
CREATE INDEX idx_users_email ON users (email);
-- GOOD: Non-blocking, allows concurrent writes
CREATE INDEX CONCURRENTLY idx_users_email ON users (email);
-- Note: CONCURRENTLY cannot run inside a transaction block
-- Most migration tools need special handling for this
Renaming a Column (Zero-Downtime)
Never rename directly in production. Use the expand-contract pattern:
-- Step 1: Add new column (migration 001)
ALTER TABLE users ADD COLUMN display_name TEXT;
-- Step 2: Backfill data (migration 002, data migration)
UPDATE users SET display_name = username WHERE display_name IS NULL;
-- Step 3: Update application code to read/write both columns
-- Deploy application changes
-- Step 4: Stop writing to old column, drop it (migration 003)
ALTER TABLE users DROP COLUMN username;
Removing a Column Safely
-- Step 1: Remove all application references to the column
-- Step 2: Deploy application without the column reference
-- Step 3: Drop column in next migration
ALTER TABLE orders DROP COLUMN legacy_status;
-- For Django: use SeparateDatabaseAndState to remove from model
-- without generating DROP COLUMN (then drop in next migration)
Large Data Migrations
-- BAD: Updates all rows in one transaction (locks table)
UPDATE users SET normalized_email = LOWER(email);
-- GOOD: Batch update with progress
DO $$
DECLARE
batch_size INT := 10000;
rows_updated INT;
BEGIN
LOOP
UPDATE users
SET normalized_email = LOWER(email)
WHERE id IN (
SELECT id FROM users
WHERE normalized_email IS NULL
LIMIT batch_size
FOR UPDATE SKIP LOCKED
);
GET DIAGNOSTICS rows_updated = ROW_COUNT;
RAISE NOTICE 'Updated % rows', rows_updated;
EXIT WHEN rows_updated = 0;
COMMIT;
END LOOP;
END $$;
Prisma (TypeScript/Node.js)
Workflow
# Create migration from schema changes
npx prisma migrate dev --name add_user_avatar
# Apply pending migrations in production
npx prisma migrate deploy
# Reset database (dev only)
npx prisma migrate reset
# Generate client after schema changes
npx prisma generate
Schema Example
model User {
id String @id @default(cuid())
email String @unique
name String?
avatarUrl String? @map("avatar_url")
createdAt DateTime @default(now()) @map("created_at")
updatedAt DateTime @updatedAt @map("updated_at")
orders Order[]
@@map("users")
@@index([email])
}
Custom SQL Migration
For operations Prisma cannot express (concurrent indexes, data backfills):
# Create empty migration, then edit the SQL manually
npx prisma migrate dev --create-only --name add_email_index
-- migrations/20240115_add_email_index/migration.sql
-- Prisma cannot generate CONCURRENTLY, so we write it manually
CREATE INDEX CONCURRENTLY I
Compatible Tools
Claude CodeCursor
Tags
Backend

