database-designer

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---
name: "database-designer"
description: "Use when the user asks to design database schemas, plan data migrations, optimize queries, choose between SQL and NoSQL, or model data relationships."
---

# Database Designer - POWERFUL Tier Skill

## Overview

A comprehensive database design skill that provides expert-level analysis, optimization, and migration capabilities for modern database systems. This skill combines theoretical principles with practical tools to help architects and developers create scalable, performant, and maintainable database schemas.

## Core Competencies

### Schema Design & Analysis
- **Normalization Analysis**: Automated detection of normalization levels (1NF through BCNF)
- **Denormalization Strategy**: Smart recommendations for performance optimization
- **Data Type Optimization**: Identification of inappropriate types and size issues
- **Constraint Analysis**: Missing foreign keys, unique constraints, and null checks
- **Naming Convention Validation**: Consistent table and column naming patterns
- **ERD Generation**: Automatic Mermaid diagram creation from DDL

### Index Optimization
- **Index Gap Analysis**: Identification of missing indexes on foreign keys and query patterns
- **Composite Index Strategy**: Optimal column ordering for multi-column indexes
- **Index Redundancy Detection**: Elimination of overlapping and unused indexes
- **Performance Impact Modeling**: Selectivity estimation and query cost analysis
- **Index Type Selection**: B-tree, hash, partial, covering, and specialized indexes

### Migration Management
- **Zero-Downtime Migrations**: Expand-contract pattern implementation
- **Schema Evolution**: Safe column additions, deletions, and type changes
- **Data Migration Scripts**: Automated data transformation and validation
- **Rollback Strategy**: Complete reversal capabilities with validation
- **Execution Planning**: Ordered migration steps with dependency resolution

## Tool Workflow (run these — do not analyze schemas by hand)

All paths relative to this skill folder; sample inputs in `assets/`.

### 1. Analyze the schema

```bash
python3 schema_analyzer.py --input schema.sql --generate-erd --output-format json -o analysis.json
```

Accepts SQL DDL or JSON schema (`assets/sample_schema.sql` / `sample_schema.json`). Output includes normalization findings, missing constraints, naming issues, and a Mermaid ERD — show the ERD to the user and fix flagged issues before optimizing.

### 2. Optimize indexes against real query patterns

```bash
python3 index_optimizer.py --schema assets/sample_schema.json --queries assets/sample_query_patterns.json --analyze-existing --format json -o indexes.json
```

Write the user's hot queries into a query-patterns JSON first (copy `assets/sample_query_patterns.json`). Output is a priority-ordered list of CREATE INDEX recommendations plus redundant-index removals.

### 3. Generate the migration

```bash
python3 migration_generator.py --current current_schema.json --target target_schema.json --zero-downtime --format sql -o migration.sql
```

`--zero-downtime` emits an expand-contract plan; `--validate-only` checks feasibility without generating SQL.

### 4. Verification loop

Re-run step 1 on the *target* schema and assert the issues found in the first pass are gone; run `migration_generator.py --validate-only` before handing over the migration.

## Database Design Principles
→ See references/database-design-reference.md for details

## Best Practices

### Schema Design
1. **Use meaningful names**: Clear, consistent naming conventions
2. **Choose appropriate data types**: Right-sized columns for storage efficiency
3. **Define proper constraints**: Foreign keys, check constraints, unique indexes
4. **Consider future growth**: Plan for scale from the beginning
5. **Document relationships**: Clear foreign key relationships and business rules

### Performance Optimization
1. **Index strategically**: Cover common query patterns without over-indexing
2. **