-- 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

-- 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

-- 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;

-- 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)

-- BAD: Updates all rows in one transaction (locks table)
UPDATE users SET normalized_email = LOWER(email);
 
-- GOOD: Batch update with progress
DO $$
DECLARE
  batch_size INT := 10000;
  rows_updated INT;
BEGIN
  LOOP
    UPDATE users
    SET normalized_email = LOWER(email)
    WHERE id IN (
      SELECT id FROM users
      WHERE normalized_email IS NULL
      LIMIT batch_size
      FOR UPDATE SKIP LOCKED
    );
    GET DIAGNOSTICS rows_updated = ROW_COUNT;
    RAISE NOTICE 'Updated % rows', rows_updated;
    EXIT WHEN rows_updated = 0;
    COMMIT;
  END LOOP;
END $$;

# 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

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])
}

# 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 IF NOT EXISTS idx_users_email ON users (email);

# Generate migration from schema changes
npx drizzle-kit generate
 
# Apply migrations
npx drizzle-kit migrate
 
# Push schema directly (dev only, no migration file)
npx drizzle-kit push

import { pgTable, text, timestamp, uuid, boolean } from "drizzle-orm/pg-core";
export const users = pgTable("users", {
  id: uuid("id").primaryKey().defaultRandom(),
  email: text("email").notNull().unique(),
  name: text("name"),
  isActive: boolean("is_active").notNull().default(true),
  createdAt: timestamp("created_at").notNull().defaultNow(),
  updatedAt: timestamp("updated_at").notNull().defaultNow(),
});

# Initialize config file (kysely.config.ts)
kysely init
 
# Create a new migration file
kysely migrate make add_user_avatar
 
# Apply all pending migrations
kysely migrate latest
 
# Rollback last migration
kysely migrate down
 
# Show migration status
kysely migrate list

// migrations/2024_01_15_001_create_user_profile.ts
import { type Kysely, sql } from 'kysely'
// IMPORTANT: Always use Kysely<any>, not your typed DB interface.
// Migrations are frozen in time and must not depend on current schema types.
export async function up(db: Kysely<any>): Promise<void> {
  await db.schema
    .createTable('user_profile')
    .addColumn('id', 'serial', (col) => col.primaryKey())
    .addColumn('email', 'varchar(255)', (col) => col.notNull().unique())
    .addColumn('avatar_url', 'text')
    .addColumn('created_at', 'timestamp', (col) =>
      col.defaultTo(sql`now()`).notNull()
    )
    .execute()
  await db.schema
    .createIndex('idx_user_profile_avatar')
    .on('user_profile')
    .column('avatar_url')
    .execute()
}
export async function down(db: Kysely<any>): Promise<void> {
  await db.schema.dropTable('user_profile').execute()
}

import { Migrator, FileMigrationProvider } from 'kysely'
import { promises as fs } from 'fs'
import * as path from 'path'
// ESM only — CJS can use __dirname directly
import { fileURLToPath } from 'url'
const migrationFolder = path.join(
  path.dirname(fileURLToPath(import.meta.url)),
  './migrations',
)
// `db` is your Kysely<any> database instance
const migrator = new Migrator({
  db,
  provider: new FileMigrationProvider({
    fs,
    path,
    migrationFolder,
  }),
  // WARNING: Only enable in development. Disables timestamp-ordering
  // validation, which can cause schema drift between environments.
  // allowUnorderedMigrations: true,
})
const { error, results } = await migrator.migrateToLatest()
results?.forEach((it) => {
  if (it.status === 'Success') {
    console.log(`migration "${it.migrationName}" executed successfully`)
  } else if (it.status === 'Error') {
    console.error(`failed to execute migration "${it.migrationName}"`)
  }
})
if (error) {
  console.error('migration failed', error)
  process.exit(1)
}

# Generate migration from model changes
python manage.py makemigrations
 
# Apply migrations
python manage.py migrate
 
# Show migration status
python manage.py showmigrations
 
# Generate empty migration for custom SQL
python manage.py makemigrations --empty app_name -n description

from django.db import migrations
def backfill_display_names(apps, schema_editor):
    User = apps.get_model("accounts", "User")
    batch_size = 5000
    users = User.objects.filter(display_name="")
    while users.exists():
        batch = list(users[:batch_size])
        for user in batch:
            user.display_name = user.username
        User.objects.bulk_update(batch, ["display_name"], batch_size=batch_size)
def reverse_backfill(apps, schema_editor):
    pass  # Data migration, no reverse needed
class Migration(migrations.Migration):
    dependencies = [("accounts", "0015_add_display_name")]
    operations = [
        migrations.RunPython(backfill_display_names, reverse_backfill),
    ]

class Migration(migrations.Migration):
    operations = [
        migrations.SeparateDatabaseAndState(
            state_operations=[
                migrations.RemoveField(model_name="user", name="legacy_field"),
            ],
            database_operations=[],  # Don't touch the DB yet
        ),
    ]

# Create migration pair
migrate create -ext sql -dir migrations -seq add_user_avatar
 
# Apply all pending migrations
migrate -path migrations -database "$DATABASE_URL" up
 
# Rollback last migration
migrate -path migrations -database "$DATABASE_URL" down 1
 
# Force version (fix dirty state)
migrate -path migrations -database "$DATABASE_URL" force VERSION

-- migrations/000003_add_user_avatar.up.sql
ALTER TABLE users ADD COLUMN avatar_url TEXT;
CREATE INDEX CONCURRENTLY idx_users_avatar ON users (avatar_url) WHERE avatar_url IS NOT NULL;
 
-- migrations/000003_add_user_avatar.down.sql
DROP INDEX IF EXISTS idx_users_avatar;
ALTER TABLE users DROP COLUMN IF EXISTS avatar_url;

Phase 1: EXPAND
  - Add new column/table (nullable or with default)
  - Deploy: app writes to BOTH old and new
  - Backfill existing data
 
Phase 2: MIGRATE
  - Deploy: app reads from NEW, writes to BOTH
  - Verify data consistency
 
Phase 3: CONTRACT
  - Deploy: app only uses NEW
  - Drop old column/table in separate migration

Day 1: Migration adds new_status column (nullable)
Day 1: Deploy app v2 — writes to both status and new_status
Day 2: Run backfill migration for existing rows
Day 3: Deploy app v3 — reads from new_status only
Day 7: Migration drops old status column