MySQL Patterns

Quick reference for MySQL best practices. For detailed guidance, use the mysql-database-reviewer agent.

When to Activate

• Writing SQL queries or migrations
• Designing database schemas
• Troubleshooting slow queries
• Setting up connection pooling
• Implementing multi-tenant isolation

Quick Reference

Index Cheat Sheet

Query Pattern	Index Type	Example
WHERE col = value	B-tree (default)	CREATE INDEX idx ON t (col)
WHERE col > value	B-tree	CREATE INDEX idx ON t (col)
WHERE a = x AND b > y	Composite	CREATE INDEX idx ON t (a, b)
MATCH(...) AGAINST(...)	FULLTEXT	CREATE FULLTEXT INDEX ft ON t (col)
Long string prefix	Prefix	CREATE INDEX idx ON t (col(50))
Geographic data	SPATIAL	CREATE SPATIAL INDEX idx ON t (col)

Data Type Quick Reference

Use Case	Correct Type	Avoid
IDs	bigint unsigned	int, random UUID as PK
Business code	varchar(64)	char, text
Strings	varchar(n)	text for short strings
Timestamps	datetime	timestamp (2038 problem)
Money	decimal(10,2)	float, double
Flags	tinyint	varchar, boolean literal
Status	tinyint	enum (hard to modify)
JSON data	json	text for structured data

Common Patterns

Composite Index Order:

-- Equality columns first, then range columns
CREATE INDEX idx_status_created ON t_order (status, created_at);
-- Works for: WHERE status = 'pending' AND created_at > '2024-01-01'
-- Does NOT work for: WHERE created_at > '2024-01-01' alone

Prefix Index (Long Strings):

CREATE INDEX idx_url ON t_page (url(100));
-- Index only first 100 characters

Generated Column + Index (JSON):

ALTER TABLE t_product
ADD COLUMN brand varchar(100) GENERATED ALWAYS AS (attributes->>'$.brand') STORED;
CREATE INDEX idx_brand ON t_product (brand);

UPSERT (ON DUPLICATE KEY):

INSERT INTO t_setting (user_code, `key`, `value`)
VALUES ('u123', 'theme', 'dark')
ON DUPLICATE KEY UPDATE
  `value` = VALUES(`value`),
  updated_at = NOW();

Cursor Pagination:

SELECT * FROM t_product WHERE id > ? ORDER BY id LIMIT 20;
-- O(1) vs OFFSET which is O(n)

Queue Processing (MySQL 8.0+):

START TRANSACTION;
SELECT * FROM t_job
WHERE status = 'pending'
ORDER BY created_at LIMIT 1
FOR UPDATE SKIP LOCKED;
-- Process job...
UPDATE t_job SET status = 'processing' WHERE id = ?;
COMMIT;

Batch Insert:

INSERT INTO t_event (user_code, action) VALUES
  ('u1', 'click'),
  ('u2', 'view'),
  ('u3', 'click');
-- 1 round trip instead of 3

Anti-Pattern Detection

-- Find tables without primary key
SELECT t.table_name
FROM information_schema.tables t
LEFT JOIN information_schema.key_column_usage k
  ON t.table_schema = k.table_schema
  AND t.table_name = k.table_name
  AND k.constraint_name = 'PRIMARY'
WHERE t.table_schema = DATABASE()
  AND k.constraint_name IS NULL
  AND t.table_type = 'BASE TABLE';

-- Find slow queries (performance_schema)
SELECT DIGEST_TEXT, COUNT_STAR,
  ROUND(AVG_TIMER_WAIT/1000000000, 2) as avg_ms
FROM performance_schema.events_statements_summary_by_digest
WHERE AVG_TIMER_WAIT > 100000000  -- > 100ms
ORDER BY AVG_TIMER_WAIT DESC LIMIT 10;

-- Find unused indexes (sys schema)
SELECT * FROM sys.schema_unused_indexes
WHERE object_schema = DATABASE();

-- Find redundant indexes
SELECT * FROM sys.schema_redundant_indexes
WHERE table_schema = DATABASE();

-- Check table fragmentation
SELECT table_name, data_free,
  ROUND(data_free/(data_length+index_length+data_free)*100, 2) as frag_pct
FROM information_schema.tables
WHERE table_schema = DATABASE() AND data_free > 1000000
ORDER BY data_free DESC;

Configuration Template

-- Connection limits
SET GLOBAL max_connections = 200;
SET GLOBAL max_user_connections = 50;

-- Timeouts
SET GLOBAL wait_timeout = 300;
SET GLOBAL interactive_timeout = 600;
SET GLOBAL max_execution_time = 30000;  -- 30s query timeout (MySQL 5.7.8+)

-- Slow query log
SET GLOBAL slow_query_log = 'ON';
SET GLOBAL long_query_time = 1;
SET GLOBAL log_queries_not_using_indexes = 'ON';

-- InnoDB settings (my.cnf recommended)
-- innodb_buffer_pool_size = 70% of RAM
-- innodb_log_file_size = 256M
-- innodb_flush_log_at_trx_commit = 1

Table Template

CREATE TABLE `t_example` (
  `id` bigint unsigned NOT NULL AUTO_INCREMENT COMMENT 'PK',
  `example_code` varchar(64) NOT NULL COMMENT 'Business code (UUID)',

  -- Business fields here
  `status` tinyint NOT NULL DEFAULT 1 COMMENT '0-inactive, 1-active',

  -- Audit fields (5 fields)
  `created_at` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT 'Creation time',
  `updated_at` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT 'Update time',
  `created_by` varchar(64) NOT NULL DEFAULT '' COMMENT 'Creator (user_code)',
  `updated_by` varchar(64) NOT NULL DEFAULT '' COMMENT 'Modifier (user_code)',
  `deleted_at` datetime DEFAULT NULL COMMENT 'Soft delete marker',

  PRIMARY KEY (`id`),
  UNIQUE KEY `uk_example_code` (`example_code`),
  KEY `idx_status` (`status`),
  KEY `idx_created_at` (`created_at`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

EXPLAIN Analysis

EXPLAIN ANALYZE SELECT * FROM t_order WHERE user_code = 'u123';

Indicator	Problem	Solution
type: ALL	Full table scan	Add index
type: index	Full index scan	Check WHERE
Using filesort	Sorting not indexed	Add ORDER BY index
Using temporary	Temp table	Optimize GROUP BY
High rows	Poor selectivity	Review conditions

Related

• Agent: mysql-database-reviewer - Full database review workflow

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Quick reference for MySQL 5.7+ / 8.0+