Add comprehensive skills and documentation for various technologies

.claude/skills/databases/postgresql/SKILL.md

@@ -0,0 +1,69 @@
+# PostgreSQL
+
+## Description
+
+PostgreSQL database patterns including queries, indexing, and optimization.
+
+## When to Use
+
+- PostgreSQL database operations
+- SQL query optimization
+- Schema design
+
+---
+
+## Core Patterns
+
+### Basic Queries
+
+```sql
+-- Select with filtering
+SELECT id, name, email
+FROM users
+WHERE active = true
+ORDER BY created_at DESC
+LIMIT 20 OFFSET 0;
+
+-- Join
+SELECT u.*, COUNT(p.id) as post_count
+FROM users u
+LEFT JOIN posts p ON p.user_id = u.id
+GROUP BY u.id;
+```
+
+### Indexes
+
+```sql
+-- Single column index
+CREATE INDEX idx_users_email ON users(email);
+
+-- Composite index
+CREATE INDEX idx_posts_user_date ON posts(user_id, created_at DESC);
+
+-- Partial index
+CREATE INDEX idx_active_users ON users(email) WHERE active = true;
+```
+
+### Migrations
+
+```sql
+-- Add column with default
+ALTER TABLE users ADD COLUMN role VARCHAR(50) DEFAULT 'user';
+
+-- Add constraint
+ALTER TABLE users ADD CONSTRAINT unique_email UNIQUE (email);
+```
+
+## Best Practices
+
+1. Use indexes for filtered/sorted columns
+2. Use EXPLAIN ANALYZE for slow queries
+3. Avoid SELECT * in production
+4. Use transactions for multiple operations
+5. Use connection pooling
+
+## Common Pitfalls
+
+- **N+1 queries**: Use JOINs or batch loading
+- **Missing indexes**: Add indexes for WHERE/ORDER BY
+- **Large transactions**: Keep transactions short