Safe Migrations

The Golden Rule: Backward Compatible First

Never break the running application. Migrations must be compatible with the code running before AND after the migration.

Safe Migration Patterns

Adding a Column (Safe)

-- SAFE: new nullable column, app ignores it until code deploys
ALTER TABLE users ADD COLUMN display_name VARCHAR(255);

-- THEN: deploy code that reads/writes display_name
-- THEN: if needed, backfill values
UPDATE users SET display_name = username WHERE display_name IS NULL;
-- THEN: add NOT NULL constraint (separate migration)
ALTER TABLE users ALTER COLUMN display_name SET NOT NULL;

Renaming a Column (Dangerous → Use This Instead)

-- STEP 1: add new column
ALTER TABLE users ADD COLUMN full_name VARCHAR(255);

-- STEP 2: copy data
UPDATE users SET full_name = first_name || ' ' || last_name;

-- STEP 3: deploy code that reads BOTH columns (backward compat)
-- STEP 4: verify new column has all data
-- STEP 5: drop old columns (in a later migration after old code is gone)
ALTER TABLE users DROP COLUMN first_name;
ALTER TABLE users DROP COLUMN last_name;

Adding an Index (Use CONCURRENTLY)

-- WRONG: locks the table
CREATE INDEX idx_users_email ON users(email);

-- RIGHT: doesn't lock (PostgreSQL)
CREATE INDEX CONCURRENTLY idx_users_email ON users(email);

Dropping a Column (Multi-Step)

-- STEP 1: deploy code that no longer reads/writes the column
-- STEP 2: mark column ignored (shadow drop)
ALTER TABLE users ALTER COLUMN old_col DROP NOT NULL;
-- STEP 3: wait for all app instances to deploy
-- STEP 4: drop the column
ALTER TABLE users DROP COLUMN old_col;

Migration File Structure (Prisma example)

prisma/migrations/
  20240101000000_add_user_display_name/
    migration.sql
  20240102000000_backfill_display_name/
    migration.sql

Rollback Strategy

-- Always write a down migration
-- migration_20240101_up.sql
ALTER TABLE users ADD COLUMN score INTEGER DEFAULT 0;

-- migration_20240101_down.sql
ALTER TABLE users DROP COLUMN score;

Production Checklist

Before running migration in production:

• Tested on staging with production data volume
• Estimated execution time (slow on large tables?)
• Index additions use CONCURRENTLY
• NOT NULL columns have a DEFAULT value
• Down migration written and tested
• Deployment window scheduled (if table lock required)
• Database backup taken
• Monitoring in place to detect query slowdowns post-migration