SQL

/sql - SQL Query Optimization

Analyze and optimize SQL queries.

Usage

/sql "SELECT * FROM users..."    # Analyze query
/sql --explain                   # Generate EXPLAIN plan
/sql --index                     # Suggest indexes
/sql --rewrite                   # Rewrite for performance

What This Skill Does

1. Analyze Query - Parse and understand intent
2. Identify Issues - N+1, missing indexes, bad patterns
3. Suggest Indexes - Based on WHERE/JOIN/ORDER BY
4. Rewrite Query - Optimized version
5. Explain Performance - Why changes help

Query Analysis Report

# SQL Analysis

## Original Query
```sql
SELECT u.*, COUNT(o.id) as order_count
FROM users u
LEFT JOIN orders o ON o.user_id = u.id
WHERE u.created_at > '2024-01-01'
GROUP BY u.id
ORDER BY order_count DESC
LIMIT 100;

Issues Found

1. SELECT * Anti-pattern

Problem: Selects all columns, may fetch unnecessary data Impact: More memory, slower network transfer Fix: Specify only needed columns

2. Missing Index

Problem: No index on users.created_at Impact: Full table scan for date filter Fix: CREATE INDEX idx_users_created_at ON users(created_at);

3. Missing Index

Problem: No index on orders.user_id Impact: Slow JOIN operation Fix: CREATE INDEX idx_orders_user_id ON orders(user_id);

Optimized Query

SELECT
    u.id,
    u.username,
    u.email,
    COUNT(o.id) as order_count
FROM users u
LEFT JOIN orders o ON o.user_id = u.id
WHERE u.created_at > '2024-01-01'
GROUP BY u.id, u.username, u.email
ORDER BY order_count DESC
LIMIT 100;

Suggested Indexes

-- For WHERE clause filtering
CREATE INDEX idx_users_created_at ON users(created_at);

-- For JOIN performance
CREATE INDEX idx_orders_user_id ON orders(user_id);

-- Composite index for common query pattern
CREATE INDEX idx_users_created_at_id ON users(created_at, id);

Expected Improvement

• Query time: ~500ms → ~50ms (10x faster)
• Rows scanned: 100,000 → 5,000

## Common Anti-Patterns

### SELECT *
```sql
-- Bad
SELECT * FROM users WHERE id = 1;

-- Good
SELECT id, username, email FROM users WHERE id = 1;

N+1 Queries

-- Bad: N+1 (1 query + N queries)
SELECT * FROM users;
-- Then for each user:
SELECT * FROM orders WHERE user_id = ?;

-- Good: Single query with JOIN
SELECT u.*, o.*
FROM users u
LEFT JOIN orders o ON o.user_id = u.id;

Missing LIMIT

-- Bad: May return millions of rows
SELECT * FROM logs WHERE level = 'ERROR';

-- Good: Bounded result set
SELECT * FROM logs WHERE level = 'ERROR' LIMIT 1000;

Using OR with different columns

-- Bad: Can't use indexes efficiently
SELECT * FROM users WHERE email = 'x' OR username = 'y';

-- Good: Use UNION
SELECT * FROM users WHERE email = 'x'
UNION
SELECT * FROM users WHERE username = 'y';

LIKE with leading wildcard

-- Bad: Can't use index
SELECT * FROM products WHERE name LIKE '%phone%';

-- Good: Use full-text search
SELECT * FROM products WHERE MATCH(name) AGAINST('phone');

Index Guidelines

When to Create Indexes

• Columns in WHERE clauses
• Columns in JOIN conditions
• Columns in ORDER BY
• Columns in GROUP BY
• Foreign key columns

When NOT to Create Indexes

• Small tables (< 1000 rows)
• Columns with low cardinality (e.g., boolean)
• Columns rarely used in queries
• Tables with heavy writes

Composite Index Order

-- Index on (a, b, c) can be used for:
WHERE a = ?
WHERE a = ? AND b = ?
WHERE a = ? AND b = ? AND c = ?

-- But NOT for:
WHERE b = ?
WHERE c = ?
WHERE b = ? AND c = ?

EXPLAIN Analysis

EXPLAIN ANALYZE SELECT ...;

Key Metrics

Instructions for Claude

When /sql is invoked:

1. Parse query - Understand structure and intent
2. Identify tables - What data is being accessed
3. Check indexes - Are WHERE/JOIN columns indexed?
4. Find anti-patterns - SELECT *, N+1, missing LIMIT
5. Suggest indexes - Based on query patterns
6. Rewrite query - Optimized version
7. Explain changes - Why each change helps
8. Estimate improvement - Expected performance gain