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skills/ck-databases/SKILL.md

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+---
+name: ck-databases
+description: Design schemas and write queries for MongoDB and PostgreSQL. Use for database design, SQL/NoSQL queries, aggregation pipelines, indexes, migrations, replication, performance optimization, psql CLI operations.
+---
+
+# ck-databases
+
+Unified guide for working with MongoDB (document-oriented) and PostgreSQL (relational) databases.
+
+## When to Use
+
+- Designing database schemas and data models
+- Writing queries (SQL or MongoDB query language)
+- Building aggregation pipelines or complex joins
+- Optimizing indexes and query performance
+- Implementing database migrations
+- Setting up replication, sharding, or clustering
+- Configuring backups and disaster recovery
+- Analyzing slow queries and performance issues
+- Administering production database deployments
+
+## Don't Use When
+
+- ORM/query-builder API work only — refer to the specific library's docs via `ck-docs-seeker`
+- Infrastructure setup for managed database services — use `ck-devops`
+
+## Database Selection Guide
+
+| Need | Choose |
+|------|--------|
+| ACID transactions, complex joins | PostgreSQL |
+| Flexible document schema | MongoDB |
+| Caching, sessions, queues | Redis |
+| Analytics/OLAP workloads | PostgreSQL with proper indexing |
+| 1-to-few relationships | MongoDB embedded documents |
+| 1-to-many or many-to-many | MongoDB references or PostgreSQL FK |
+
+## MongoDB Best Practices
+
+- Use embedded documents for 1-to-few relationships
+- Reference documents for 1-to-many or many-to-many
+- Index frequently queried and sorted fields
+- Use aggregation pipeline for complex transformations
+- Enable authentication and TLS in production
+- Use Atlas for managed hosting
+
+**Common patterns:**
+- CRUD operations with query operators and atomic updates
+- Aggregation pipeline stages: `$match`, `$group`, `$lookup`, `$project`, `$sort`
+- Index types: single field, compound, text, geospatial, TTL
+- Atlas search for full-text capabilities
+
+## PostgreSQL Best Practices
+
+- Normalize schema to 3NF; denormalize selectively for performance
+- Use foreign keys for referential integrity
+- Index foreign keys and frequently filtered columns
+- Use `EXPLAIN ANALYZE` to optimize queries
+- Regular `VACUUM` and `ANALYZE` maintenance
+- Connection pooling via pgBouncer for web apps
+
+**Key features:**
+- `SELECT`, JOINs, subqueries, CTEs, window functions
+- `psql` meta-commands for administration
+- `EXPLAIN`, query optimization, vacuum, index maintenance
+- User management, backups, replication, point-in-time recovery
+
+## Python Utility Scripts
+
+Available in the skill's `scripts/` directory:
+
+```bash
+# Generate and apply migrations
+python scripts/db_migrate.py --db mongodb --generate "add_user_index"
+python scripts/db_migrate.py --db postgres --apply
+
+# Backup and restore
+python scripts/db_backup.py --db postgres --output /backups/
+
+# Performance analysis
+python scripts/db_performance_check.py --db mongodb --threshold 100ms
+```
+
+## Performance Optimization Checklist
+
+- [ ] Indexes on frequently queried fields
+- [ ] Compound indexes ordered by selectivity
+- [ ] `EXPLAIN ANALYZE` run on slow queries
+- [ ] Connection pooling configured
+- [ ] Query result caching with Redis where appropriate
+- [ ] Pagination instead of full table scans
+- [ ] Read replicas for read-heavy workloads
+- [ ] Partitioning for very large tables (PostgreSQL)
+
+## Security Checklist
+
+- [ ] Authentication enabled (no anonymous access)
+- [ ] TLS/SSL for connections in production
+- [ ] Parameterized queries everywhere (never string concatenation)
+- [ ] Least-privilege database users
+- [ ] Regular backups tested for restore
+- [ ] Audit logging enabled for sensitive operations
+
+## Resources
+
+- MongoDB: https://www.mongodb.com/docs/
+- PostgreSQL: https://www.postgresql.org/docs/