database

database

Overview

Relational database design, query optimization, migration safety, connection pooling in serverless environments, and ORM usage across PostgreSQL, Prisma, and Drizzle.

When to Use

Activate for tasks involving database schema design, migrations, indexing, relational models, ORM queries, transactions, or query performance tuning.

Rules & Patterns

Negative Constraints (What NOT to Do)

1. NEVER do SELECT * in production: Always select explicit columns required by the caller to minimize memory bandwidth and lock footprint.
2. NEVER run destructive migrations without backward compatibility: Always follow expand-and-contract (Phase 1: add new column as nullable; Phase 2: backfill; Phase 3: make non-nullable & remove old column).
3. NEVER execute queries in loops (The N+1 Anti-Pattern): Always use batch loading (inArray, DataLoader, or relational include / JOIN).
4. NEVER leave foreign keys without indexes: In PostgreSQL/MySQL, child foreign key columns must always have an index to prevent table-level locking on cascade deletes.
5. NEVER perform multi-entity writes without a database transaction: Any operation touching multiple records must use prisma.$transaction or db.transaction.
6. NEVER open unpooled database connections in Serverless / Edge functions: Serverless scale-outs will instantly exhaust PostgreSQL's max_connections.

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Zero-Downtime Migrations (Expand-and-Contract)

When modifying schemas with zero downtime:

1. Phase 1 (Expand): Add the new column as NULLABLE (or with a default value). Deploy the application code that reads from old column and writes to both old and new.
2. Phase 2 (Backfill): Run an asynchronous batch migration job in chunks (e.g. 1000 rows at a time) to populate data from old column to new column.
3. Phase 3 (Contract): Update application code to read and write exclusively from the new column.
4. Phase 4 (Cleanup): Once traffic is fully shifted, remove the old column and mark the new column as NOT NULL in a separate migration.