database-migrations

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# 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

1. **Every change is a migration** — never alter production databases manually
2. **Migrations are forward-only in production** — rollbacks use new forward migrations
3. **Schema and data migrations are separate** — never mix DDL and DML in one migration
4. **Test migrations against production-sized data** — a migration that works on 100 rows may lock on 10M
5. **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

```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
```

### Adding an Index Without Downtime

```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
```

### Renaming a Column (Zero-Downtime)

Never rename directly in production. Use the expand-contract pattern:

```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;
```

### Removing a Column Safely

```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)
```

### Large Data Migrations

```sql
-- BAD: Updates all rows in one transaction (locks table)
UPDATE users SET normalized_email = LOWER(email);

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

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