Database Design

Database Design & Schema Management

When to Use

• Designing a new database schema or adding tables
• Reviewing an existing schema for correctness, performance, or security
• Planning migrations (especially for production data)
• Designing indexes for query patterns
• Defining retention/deletion policies for sensitive data
• Evaluating SQLite vs PostgreSQL for a given use case

SQLite vs PostgreSQL Decision

Rule: If multi-tenant or handling financial/PII data → PostgreSQL. No exceptions.

Schema Design Checklist

1. Table Design

□ Every table has a clear, singular purpose
□ Primary key defined (prefer INTEGER AUTOINCREMENT or UUID, not natural keys)
□ Foreign keys declared with ON DELETE behavior (CASCADE, SET NULL, RESTRICT)
□ NOT NULL on every column that should never be empty
□ DEFAULT values for columns with sensible defaults
□ No VARCHAR without length limit on user-facing input
□ Created_at and updated_at timestamps on every mutable table
□ No redundant data that can be derived from joins

2. Naming Conventions

□ Tables: plural snake_case (users, audit_events, context_chunks)
□ Columns: singular snake_case (user_id, created_at, token_count)
□ Foreign keys: {referenced_table_singular}_id (user_id, blob_hash)
□ Indexes: idx_{table}_{columns} (idx_users_email, idx_chunks_hash_index)
□ Constraints: {type}_{table}_{columns} (uq_aliases_alias_namespace, fk_chunks_hash)

3. Normalization

• 1NF: No repeating groups, no arrays in columns (use junction tables)
• 2NF: Every non-key column depends on the full primary key
• 3NF: No transitive dependencies (column A → column B → column C)
• Denormalize intentionally only for proven performance needs, document why

4. Index Strategy

□ Primary keys are automatically indexed
□ Foreign keys need explicit indexes (PostgreSQL does NOT auto-index FK columns)
□ Add indexes for WHERE, JOIN, ORDER BY columns in hot queries
□ Composite indexes: leftmost column must be the most selective
□ Covering indexes for read-heavy queries (include all SELECT columns)
□ Partial indexes for queries that filter on a constant (WHERE status = 'active')
□ Do NOT index columns with low cardinality (boolean, enum with 3 values)
□ Do NOT index columns that are rarely queried

5. Multi-Tenant Isolation

□ Tenant ID column on every tenant-scoped table
□ Every query includes WHERE tenant_id = ? (no exceptions)
□ Composite unique constraints include tenant_id: UNIQUE(alias, tenant_id)
□ Row-level security (PostgreSQL) or application-level enforcement
□ Test: Tenant A cannot read Tenant B's data (explicit cross-tenant test)
□ Indexes include tenant_id as prefix for tenant-scoped queries

6. Sensitive Data

□ PII columns identified and documented
□ Encryption-at-rest for SSN, EIN, account numbers (application-level Fernet or PG pgcrypto)
□ No PII in indexes (encrypted columns cannot be indexed — use blind index pattern)
□ Retention policy defined: how long, auto-delete mechanism, audit trail for deletions
□ Audit log table: who accessed what, when, from where (append-only, no DELETE permission)
□ Backup encryption for database dumps containing PII

Migration Strategy

Alembic (Python/SQLAlchemy)

# Initialize
alembic init alembic

# Create migration from model changes
alembic revision --autogenerate -m "add user_roles table"

# Apply
alembic upgrade head

# Rollback
alembic downgrade -1

Migration Safety Rules

□ Never DROP COLUMN in production without a deprecation period
□ Add columns as NULLABLE first, backfill, then add NOT NULL constraint
□ Rename via: add new column → copy data → drop old column (not ALTER RENAME)
□ Large data migrations: batch in chunks (1000-10000 rows) to avoid lock contention
□ Always test migration AND rollback on a copy of production data
□ Migrations must be idempotent (safe to run twice)
□ No data-dependent migrations in the same transaction as schema changes

SQLite Migrations (No Alembic)

SQLite has limited ALTER TABLE support. For schema changes:

1. Create new table with desired schema
2. Copy data from old table
3. Drop old table
4. Rename new table to old name
5. Recreate indexes and triggers

Wrap in a transaction. Test on a copy first.

Schema Review Protocol

When reviewing an existing schema:

1. Read the migration files or CREATE TABLE statements
2. Map entity relationships — draw the FK graph mentally
3. Check for missing indexes on FK columns and WHERE clause columns
4. Check for missing constraints — can invalid data be inserted?
5. Check tenant isolation — is tenant_id on every tenant-scoped table? In every query?
6. Check for PII — is sensitive data identified and protected?
7. Check for orphans — can rows exist without their parent? (missing ON DELETE CASCADE)
8. Check for N+1 patterns — are related entities fetched in loops instead of JOINs?
9. Output: findings table with severity (Critical/High/Medium/Low) and recommended fix

Anti-Patterns

Integration

• tdd: Write schema tests first (can I insert valid data? rejected invalid data?)
• code-quality: N+1 queries and missing indexes are performance findings