SQL Optimization Patterns by @wshobson · Skillet

SQL Optimization Patterns

Master SQL query optimization, indexing strategies, and EXPLAIN analysis to dramatically improve database performance and eliminate slow queries. Use when debugging slow queries, designing database schemas, or optimizing application performance.

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Just instructions. No commands, network, or file access.

Files

SKILL.md

sql-optimization-patterns

SQL Optimization Patterns

Transform slow database queries into lightning-fast operations through systematic optimization, proper indexing, and query plan analysis.

When to Use This Skill

Debugging slow-running queries
Designing performant database schemas
Optimizing application response times
Reducing database load and costs
Improving scalability for growing datasets
Analyzing EXPLAIN query plans
Implementing efficient indexes
Resolving N+1 query problems

Core Concepts

1. Query Execution Plans (EXPLAIN)

Understanding EXPLAIN output is fundamental to optimization.

PostgreSQL EXPLAIN:

-- Basic explain
EXPLAIN SELECT * FROM users WHERE email = '[email protected]';

-- With actual execution stats
EXPLAIN ANALYZE
SELECT * FROM users WHERE email = ;


EXPLAIN (ANALYZE, BUFFERS, VERBOSE)
 u., o.order_total
 users u
 orders o  u.id  o.user_id
 u.created_at  NOW()   ;

2 files · 13 KB6 KB

Version history

-- Standard B-Tree index
CREATE INDEX idx_users_email ON users(email);

-- Composite index (order matters!)
CREATE INDEX idx_orders_user_status ON orders(user_id, status);

-- Partial index (index subset of rows)
CREATE INDEX idx_active_users ON users(email)
WHERE status = 'active';

-- Expression index
CREATE INDEX idx_users_lower_email ON users(LOWER(email));

-- Covering index (include additional columns)
CREATE INDEX idx_users_email_covering ON users(email)
INCLUDE (name, created_at);

-- Full-text search index
CREATE INDEX idx_posts_search ON posts
USING GIN(to_tsvector('english', title || ' ' || body));

-- JSONB index
CREATE INDEX idx_metadata ON events USING GIN(metadata);

-- Bad: Fetches unnecessary columns
SELECT * FROM users WHERE id = 123;

-- Good: Fetch only what you need
SELECT id, email, name FROM users WHERE id = 123;

-- Bad: Function prevents index usage
SELECT * FROM users WHERE LOWER(email) = '[email protected]';

-- Good: Create functional index or use exact match
CREATE INDEX idx_users_email_lower ON users(LOWER(email));
-- Then:
SELECT * FROM users WHERE LOWER(email) = '[email protected]';

-- Or store normalized data
SELECT * FROM users WHERE email = '[email protected]';

-- Bad: Cartesian product then filter
SELECT u.name, o.total
FROM users u, orders o
WHERE u.id = o.user_id AND u.created_at > '2024-01-01';

-- Good: Filter before join
SELECT u.name, o.total
FROM users u
JOIN orders o ON u.id = o.user_id
WHERE u.created_at > '2024-01-01';

-- Better: Filter both tables
SELECT u.name, o.total
FROM (SELECT * FROM users WHERE created_at > '2024-01-01') u
JOIN orders o ON u.id = o.user_id;

-- Update statistics
ANALYZE users;
ANALYZE VERBOSE orders;

-- Vacuum (PostgreSQL)
VACUUM ANALYZE users;
VACUUM FULL users;  -- Reclaim space (locks table)

-- Reindex
REINDEX INDEX idx_users_email;
REINDEX TABLE users;

-- Find slow queries (PostgreSQL)
SELECT query, calls, total_time, mean_time
FROM pg_stat_statements
ORDER BY mean_time DESC
LIMIT 10;

-- Find missing indexes (PostgreSQL)
SELECT
    schemaname,
    tablename,
    seq_scan,
    seq_tup_read,
    idx_scan,
    seq_tup_read / seq_scan AS avg_seq_tup_read
FROM pg_stat_user_tables
WHERE seq_scan > 0
ORDER BY seq_tup_read DESC
LIMIT 10;

-- Find unused indexes (PostgreSQL)
SELECT
    schemaname,
    tablename,
    indexname,
    idx_scan,
    idx_tup_read,
    idx_tup_fetch
FROM pg_stat_user_indexes
WHERE idx_scan = 0
ORDER BY pg_relation_size(indexrelid) DESC;

SQL Optimization Patterns

SQL Optimization Patterns

When to Use This Skill

Core Concepts

1. Query Execution Plans (EXPLAIN)

2. Index Strategies

3. Query Optimization Patterns

Detailed patterns and worked examples

Best Practices

Common Pitfalls

Monitoring Queries