majiayu000 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 = 'user@example.com';
-- With actual execution stats
EXPLAIN ANALYZE
SELECT * FROM users WHERE email = 'user@example.com';
-- Verbose output with more details
EXPLAIN (ANALYZE, BUFFERS, VERBOSE)
SELECT u.*, o.order_total
FROM users u
JOIN orders o ON u.id = o.user_id
WHERE u.created_at > NOW() - INTERVAL '30 days';
Key Metrics to Watch:
- Seq Scan: Full table scan (usually slow for large tables)
- Index Scan: Using index (good)
- Index Only Scan: Using index without touching table (best)
- Nested Loop: Join method (okay for small datasets)
- Hash Join: Join method (good for larger datasets)
- Merge Join: Join method (good for sorted data)
- Cost: Estimated query cost (lower is better)
- Rows: Estimated rows returned
- Actual Time: Real execution time
2. Index Strategies
Indexes are the most powerful optimization tool.
Index Types:
- B-Tree: Default, good for equality and range queries
- Hash: Only for equality (=) comparisons
- GIN: Full-text search, array queries, JSONB
- GiST: Geometric data, full-text search
- BRIN: Block Range INdex for very large tables with correlation
-- 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);
3. Query Optimization Patterns
Avoid SELECT *:
-- 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;
Use WHERE Clause Efficiently:
-- Bad: Function prevents index usage
SELECT * FROM users WHERE LOWER(email) = 'user@example.com';
-- 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) = 'user@example.com';
-- Or store normalized data
SELECT * FROM users WHERE email = 'user@example.com';
Optimize JOINs:
-- 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;
Optimization Patterns
Pattern 1: Eliminate N+1 Queries
Problem: N+1 Query Anti-Pattern
# Bad: Executes N+1 queries
users = db.query("SELECT * FROM users LIMIT 10")
for user in users:
orders = db.query("SELECT * FROM orders WHERE user_id = ?", user.id)
# Process orders
Solution: Use JOINs or Batch Loading
-- Solution 1: JOIN
SELECT
u.id, u.name,
o.id as order_id, o.total
FROM users u
LEFT JOIN orders o ON u.id = o.user_id
WHERE u.id IN (1, 2, 3, 4, 5);
-- Solution 2: Batch query
SELECT * FROM orders
WHERE user_id IN (1, 2, 3, 4, 5);
# Good: Single query with JOIN or batch load
# Using JOIN
results = db.query("""
SELECT u.id, u.name, o.id as order_id, o.total
FROM users u
LEFT JOIN orders o ON u.id = o.user_id
WHERE u.id IN (1, 2, 3, 4, 5)
""")
# Or batch load
users = db.query("SELECT * FROM users LIMIT 10")
user_ids = [u.id for u in users]
orders = db.query(
"SELECT * FROM orders WHERE user_id IN (?)",
user_ids
)
# Group orders by user_id
orders_by_user = {}
for order in orders:
orders_by_user.setdefault(order.user_id, []).append(order)
Pattern 2: Optimize Pagination
Bad: OFFSET on Large Tables
-- Slow for large offsets
SELECT * FROM users
ORDER BY created_at DESC
LIMIT 20 OFFSET 100000; -- Very slow!
Good: Cursor-Based Pagination
-- Much faster: Use cursor (last seen ID)
SELECT * FROM users
WHERE created_at < '2024-01-15 10:30:00' -- Last cursor
ORDER BY created_at DESC
LIMIT 20;
-- With composite sorting
SELECT * FROM users
WHERE (created_at, id) < ('2024-01-15 10:30:00', 12345)
ORDER BY created_at DESC, id DESC
LIMIT 20;
-- Requires index
CREATE INDEX idx_users_cursor ON users(created_at DESC, id DESC);
Pattern 3: Aggregate Efficiently
Optimize COUNT Queries:
-- Bad: Counts all rows
SELECT COUNT(*) FROM orders; -- Slow on large tables
-- Good: Use estimates for approximate counts
SELECT reltuples::bigint AS estimate
FROM pg_class
WHERE relname = 'orders';
-- Good: Filter before counting
SELECT COUNT(*) FROM orders
WHERE created_at > NOW() - INTERVAL '7 days';
-- Better: Use index-only scan
CREATE INDEX idx_orders_created ON orders(created_at);
SELECT COUNT(*) FROM orders
WHERE created_at > NOW() - INTERVAL '7 days';
Optimize GROUP BY:
-- Bad: Group by then filter
SELECT user_id, COUNT(*) as order_count
FROM orders
GROUP BY user_id
HAVING COUNT(*) > 10;
-- Better: Filter first, then group (if possible)
SELECT user_id, COUNT(*) as order_count
FROM orders
WHERE status = 'completed'
GROUP BY user_id
HAVING COUNT(*) > 10;
-- Best: Use covering index
CREATE INDEX idx_orders_user_status ON orders(user_id, status);
Pattern 4: Subquery Optimization
Transform Correlated Subqueries:
-- Bad: Correlated subquery (runs for each row)
SELECT u.name, u.email,
(SELECT COUNT(*) FROM orders o WHERE o.user_id = u.id) as order_count
FROM users u;
-- Good: JOIN with aggregation
SELECT u.name, u.email, COUNT(o.id) as order_count
FROM users u
LEFT JOIN orders o ON o.user_id = u.id
GROUP BY u.id, u.name, u.email;
-- Better: Use window functions
SELECT DISTINCT ON (u.id)
u.name, u.email,
COUNT(o.id) OVER (PARTITION BY u.id) as order_count
FROM users u
LEFT JOIN orders o ON o.user_id = u.id;
Use CTEs for Clarity:
-- Using Common Table Expressions
WITH recent_users AS (
SELECT id, name, email
FROM users
WHERE created_at > NOW() - INTERVAL '30 days'
),
user_order_counts AS (
SELECT user_id, COUNT(*) as order_count
FROM orders
WHERE created_at > NOW() - INTERVAL '30 days'
GROUP BY user_id
)
SELECT ru.name, ru.email, COALESCE(uoc.order_count, 0) as orders
FROM recent_users ru
LEFT JOIN user_order_counts uoc ON ru.id = uoc.user_id;
Pattern 5: Batch Operations
Batch INSERT:
-- Bad: Multiple individual inserts
INSERT INTO users (name, email) VALUES ('Alice', 'alice@example.com');
INSERT INTO users (name, email) VALUES ('Bob', 'bob@example.com');
INSERT INTO users (name, email) VALUES ('Carol', 'carol@example.com');
-- Good: Batch insert
INSERT INTO users (name, email) VALUES
('Alice', 'alice@example.com'),
('Bob', 'bob@example.com'),
('Carol', 'carol@example.com');
-- Better: Use COPY for bulk inserts (PostgreSQL)
COPY users (name, email) FROM '/tmp/users.csv' CSV HEADER;
Batch UPDATE:
-- Bad: Update in loop
UPDATE users SET status = 'active' WHERE id = 1;
UPDATE users SET status = 'active' WHERE id = 2;
-- ... repeat for many IDs
-- Good: Single UPDATE with IN clause
UPDATE users
SET status = 'active'
WHERE id IN (1, 2, 3, 4, 5, ...);
-- Better: Use temporary table for large batches
CREATE TEMP TABLE temp_user_updates (id INT, new_status VARCHAR);
INSERT INTO temp_user_updates VALUES (1, 'active'), (2, 'active'), ...;
UPDATE users u
SET status = t.new_status
FROM temp_user_updates t
WHERE u.id = t.id;
Advanced Techniques
Materialized Views
Pre-compute expensive queries.
-- Create materialized view
CREATE MATERIALIZED VIEW user_order_summary AS
SELECT
u.id,
u.name,
COUNT(o.id) as total_orders,
SUM(o.total) as total_spent,
MAX(o.created_at) as last_order_date
FROM users u
LEFT JOIN orders o ON u.id = o.user_id
GROUP BY u.id, u.name;
-- Add index to materialized view
CREATE INDEX idx_user_summary_spent ON user_order_summary(total_spent DESC);
-- Refresh materialized view
REFRESH MATERIALIZED VIEW user_order_summary;
-- Concurrent refresh (PostgreSQL)
REFRESH MATERIALIZED VIEW CONCURRENTLY user_order_summary;
-- Query materialized view (very fast)
SELECT * FROM user_order_summary
WHERE total_spent > 1000
ORDER BY total_spent DESC;
Partitioning
Split large tables for better performance.
-- Range partitioning by date (PostgreSQL)
CREATE TABLE orders (
id SERIAL,
user_id INT,
total DECIMAL,
created_at TIMESTAMP
) PARTITION BY RANGE (created_at);
-- Create partitions
CREATE TABLE orders_2024_q1 PARTITION OF orders
FOR VALUES FROM ('2024-01-01') TO ('2024-04-01');
CREATE TABLE orders_2024_q2 PARTITION OF orders
FOR VALUES FROM ('2024-04-01') TO ('2024-07-01');
-- Queries automatically use appropriate partition
SELECT * FROM orders
WHERE created_at BETWEEN '2024-02-01' AND '2024-02-28';
-- Only scans orders_2024_q1 partition
Query Hints and Optimization
-- Force index usage (MySQL)
SELECT * FROM users
USE INDEX (idx_users_email)
WHERE email = 'user@example.com';
-- Parallel query (PostgreSQL)
SET max_parallel_workers_per_gather = 4;
SELECT * FROM large_table WHERE condition;
-- Join hints (PostgreSQL)
SET enable_nestloop = OFF; -- Force hash or merge join
Best Practices
- Index Selectively: Too many indexes slow down writes
- Monitor Query Performance: Use slow query logs
- Keep Statistics Updated: Run ANALYZE regularly
- Use Appropriate Data Types: Smaller types = better performance
- Normalize Thoughtfully: Balance normalization vs performance
- Cache Frequently Accessed Data: Use application-level caching
- Connection Pooling: Reuse database connections
- Regular Maintenance: VACUUM, ANALYZE, rebuild indexes
-- 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;
Common Pitfalls
- Over-Indexing: Each index slows down INSERT/UPDATE/DELETE
- Unused Indexes: Waste space and slow writes
- Missing Indexes: Slow queries, full table scans
- Implicit Type Conversion: Prevents index usage
- OR Conditions: Can't use indexes efficiently
- LIKE with Leading Wildcard:
LIKE '%abc'can't use index - Function in WHERE: Prevents index usage unless functional index exists
Monitoring Queries
-- 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;
Resources
- references/postgres-optimization-guide.md: PostgreSQL-specific optimization
- references/mysql-optimization-guide.md: MySQL/MariaDB optimization
- references/query-plan-analysis.md: Deep dive into EXPLAIN plans
- assets/index-strategy-checklist.md: When and how to create indexes
- assets/query-optimization-checklist.md: Step-by-step optimization guide
- scripts/analyze-slow-queries.sql: Identify slow queries in your database
- scripts/index-recommendations.sql: Generate index recommendations