database-migrations

# 数据库迁移模式 为生产系统提供安全、可逆的数据库模式变更。 ## 何时激活 * 创建或修改数据库表 * 添加/删除列或索引 * 运行数据迁移（回填、转换） * 计划零停机模式变更 * 为新项目设置迁移工具 ## 核心原则 1. **每个变更都是一次迁移** — 切勿手动更改生产数据库 2. **迁移在生产环境中是只进不退的** — 回滚使用新的前向迁移 3. **模式迁移和数据迁移是分开的** — 切勿在一个迁移中混合 DDL 和 DML 4. **针对生产规模的数据测试迁移** — 适用于 100 行的迁移可能在 1000 万行时锁定 5. **迁移一旦部署就是不可变的** — 切勿编辑已在生产中运行的迁移 ## 迁移安全检查清单 应用任何迁移之前： * \[ ] 迁移同时包含 UP 和 DOWN（或明确标记为不可逆） * \[ ] 对大表没有全表锁（使用并发操作） * \[ ] 新列有默认值或可为空（切勿添加没有默认值的 NOT NULL） * \[ ] 索引是并发创建的（对于现有表，不与 CREATE TABLE 内联创建） * \[ ] 数据回填是与模式变更分开的迁移 * \[ ] 已针对生产数据副本进行测试 * \[ ] 回滚计划已记录 ## PostgreSQL 模式 ### 安全地添加列 ```sql -- 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 ``` ### 无停机添加索引 ```sql -- 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 ``` ### 重命名列（零停机） 切勿在生产中直接重命名。使用扩展-收缩模式： ```sql -- 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; ``` ### 安全地删除列 ```sql -- 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) ``` ### 大型数据迁移 ```sql -- 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) ### 工作流 ```bash # 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 ``` ### 模式示例 ```prisma 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]) } ``` ### 自定义 SQL 迁移 对于 Prisma 无法表达的操作（并发索引、数据回填）： ```bash # Create empty migration, then edit the SQL manually npx prisma migrate dev --create-only --name add_email_index ``` ```sql -- 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); ``` ## Drizzle (TypeScript/Node.js) ### 工作流 ```bash # 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 ``` ### 模式示例 ```typescript 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(), }); ``` ## Django (Python) ### 工作流 ```bash # 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 ``` ### 数据迁移 ```python 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), ] ``` ### SeparateDatabaseAndState 从 Django 模型中删除列，而不立即从数据库中删除： ```python 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 ), ] ``` ## golang-migrate (Go) ### 工作流 ```bash # 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 ``` ### 迁移文件 ```sql -- 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; ``` ## 零停机迁移策略 对于关键的生产变更，遵循扩展-收缩模式： ``` 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 ``` ### 时间线示例 ``` 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 ``` ## 反模式 | 反模式 | 为何会失败 | 更好的方法 | |-------------|-------------|-----------------| | 在生产中手动执行 SQL | 没有审计追踪，不可重复 | 始终使用迁移文件 | | 编辑已部署的迁移 | 导致环境间出现差异 | 改为创建新迁移 | | 没有默认值的 NOT NULL | 锁定表，重写所有行 | 添加可为空列，回填数据，然后添加约束 | | 在大表上内联创建索引 | 在构建期间阻塞写入 | 使用 CREATE INDEX CONCURRENTLY | | 在一个迁移中混合模式和数据的变更 | 难以回滚，事务时间长 | 分开的迁移 | | 在移除代码之前删除列 | 应用程序在缺失列时出错 | 先移除代码，下一次部署再删除列 |