DB Schema Designer

Overview

Transforms domain descriptions into optimized database schemas with migrations, indexes, and relationship diagrams.

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1. When to Apply

| Trigger | Behavior | |---------|----------| | New project data modeling | Full schema design workflow | | "database schema", "ERD" | Interactive schema builder | | Adding new entity/table | Incremental schema update |

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2. Design Workflow

Step 1: Domain Analysis

From user description, extract:

• Entities: Nouns → tables/collections
• Relationships: Verbs → foreign keys/references
• Attributes: Properties → columns/fields
• Cardinality: one-to-one, one-to-many, many-to-many

Step 2: Schema Generation

Prisma Example:

model User {
  id        String   @id @default(cuid())
  email     String   @unique
  name      String?
  role      Role     @default(USER)
  posts     Post[]
  createdAt DateTime @default(now())
  updatedAt DateTime @updatedAt

  @@index([email])
}

model Post {
  id        String   @id @default(cuid())
  title     String
  content   String?
  published Boolean  @default(false)
  author    User     @relation(fields: [authorId], references: [id])
  authorId  String
  tags      Tag[]
  createdAt DateTime @default(now())

  @@index([authorId])
  @@index([published, createdAt])
}

Step 3: Index Strategy

## Index Recommendations

| Table | Index | Type | Reason |
|-------|-------|------|--------|
| users | email | UNIQUE | Login lookup |
| posts | authorId | B-TREE | Filter by author |
| posts | (published, createdAt) | COMPOSITE | Published feed query |
| posts | title | GIN (trigram) | Full-text search |

Step 4: Migration Script

Generate migration for chosen ORM/tool.

Step 5: ERD (Mermaid)

erDiagram
    User ||--o{ Post : writes
    Post }o--o{ Tag : has
    User {
        string id PK
        string email UK
        string name
        enum role
    }
    Post {
        string id PK
        string title
        string content
        boolean published
        string authorId FK
    }

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3. Design Principles

1. Start normalized, denormalize when needed — 3NF first, optimize later
2. Always add timestamps — createdAt, updatedAt on every table
3. Use UUIDs/CUIDs for IDs — avoids enumeration attacks
4. Soft delete when data matters — deletedAt instead of DELETE
5. Index foreign keys — always index FK columns
6. Plan for multi-tenancy early — add tenantId if SaaS

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4. Tool Coordination

| Tool | Purpose | |------|---------| | Write | Generate schema files, migrations | | Read | Analyze existing schema | | Bash | Run migrations (npx prisma migrate, alembic upgrade) |

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5. Boundaries

Will:

• Design schemas from domain descriptions
• Generate Prisma/Drizzle/SQLAlchemy models
• Create migration scripts
• Recommend index strategies
• Generate ERD diagrams (Mermaid)

Will Not:

• Set up database servers
• Perform data migration between databases
• Optimize queries (use performance-profiler)
• Design caching strategies