🐬 MySQL & MariaDB

Overview

MySQL is the most widespread open-source relational database, especially in web environments (LAMP stack). MariaDB is a fork after Oracle's acquisition, fully compatible with extensions. Default choice for WordPress, Drupal, Magento, and most PHP applications.

Architecture (server + storage engine)

Based on High Performance MySQL (Schwartz, Zaitsev, Tkachenko):

MySQL Server Layer
  ├── Connection handling (thread-per-connection)
  ├── Query parser & optimizer
  ├── Built-in functions
  └── Storage Engine API
        ├── InnoDB (default, MVCC, ACID)
        ├── MyISAM (legacy, table-level locks)
        ├── MEMORY (in-memory, HEAP)
        └── ... (others)

InnoDB (default engine since MySQL 5.5+)

MVCC — Multi-Version Concurrency Control (snapshot isolation)
REPEATABLE READ (default) — next-key locking prevents phantom reads
Clustered index — primary key = physical data ordering
Buffer pool — cache of data and indexes in RAM (main performance parameter)
Doublewrite buffer — prevents partial page writes

Schema design tips

Prefer smaller data types (MEDIUMINT over INT, TIMESTAMP over DATETIME)
Use NULL carefully (each NULL column increases index complexity)
Use ENUM only for truly small, stable value lists
JSON columns in MySQL 8+ — useful for flexible schema, but not for joins

Deferred join pattern

-- 1. covering index finds PK
-- 2. only then join to full row
SELECT * FROM users
  INNER JOIN (
    SELECT id FROM users
    WHERE status = 'active'
    ORDER BY created_at DESC
    LIMIT 100 OFFSET 1000
  ) AS tmp USING (id);

Join decomposition: Sometimes it's better to split a JOIN into several simple queries (better cache utilization, fewer locks, scaling across servers).

IN() optimization: MySQL sorts values in the IN() list and uses binary search (O(log n)), unlike OR clauses (O(n)).

MariaDB differences from MySQL

Feature	MySQL 8.x	MariaDB 11.x
Storage engine	InnoDB (only)	InnoDB + XtraDB (fork) + Aria + MyRocks
JSON	Native JSON type	JSON alias to LONGTEXT + JSON functions
CTE	WITH (non-recursive + recursive)	WITH (non-recursive + recursive)
Window functions	Yes (8.0+)	Yes (10.2+)
Sequence	No (auto_increment only)	Yes (CREATE SEQUENCE)
Thread pooling	Enterprise only	Built-in
Galera cluster	No (natively)	Yes (native synchronous clustering)

ProxySQL

ProxySQL is an advanced proxy for MySQL with sophisticated routing:

Feature	Description
Query routing	Rules for directing queries (read/write split, sharding)
Connection pooling	Multiplexing thousands of connections into a small pool
Query cache	Result caching in memory (TTL, size limit)
Query rewriting	Rewrite SQL queries in transit
Active monitoring	Backend outage detection, automatic failover

Recommendations — where MySQL is better

Area	MySQL	Competition	Why MySQL
Web applications	De facto standard for WP, Drupal, Magento	PostgreSQL (fewer CMS plugins)	Broadest support in web hosting providers
Read-heavy (SELECT heavy)	InnoDB buffer pool, covering index, adaptive hash	PostgreSQL (MVCC overhead on reads)	Cache-efficient, fast point lookups
Replication	Async replication, Group Replication, InnoDB Cluster	PostgreSQL (streaming replication)	Simpler setup, extensive documentation
Ecosystem	ProxySQL, Orchestrator, Vitess, PlanetScale	PostgreSQL (fewer tools)	Most tooling for cluster management
JSON in MySQL 8+	JSON data type, Multi-Value Indexes	PostgreSQL (jsonb, GIN)	Comparable, Multi-Value Index unique

When to use MySQL / MariaDB

CMS / e-commerce — WordPress, Drupal, Magento, Joomla (all require MySQL)
Read-heavy applications — InnoDB buffer pool efficiently caches frequently read data
Simple replication — Group Replication / InnoDB Cluster for HA
MariaDB for Galera cluster — synchronous multi-master clustering
PHP applications — native PHP MySQL extensions (mysqli, PDO_MySQL)

MySQL / MariaDB licensing

MySQL licensing

Variant	License	Price	Restrictions
MySQL Community (GPL)	GPL v2	$0	If you distribute an application that contains MySQL (e.g., embedded), you must release the entire application under GPL. Web applications (over network) ≠ distribution — GPL does not apply
MySQL Standard (Commercial)	Commercial (Oracle)	~$2,000/server/year	No GPL restrictions, production support, MySQL Enterprise Monitor
MySQL Enterprise	Commercial (Oracle)	~$5,000/server/year	All above + MySQL Enterprise Backup, Audit, Firewall, Thread Pool, Encryption
MySQL Cluster CGE	Commercial (Oracle)	~$10,000/server/year	Distributed multi-master cluster (NDB), telco-grade

When GPL matters: If you embed MySQL into a commercial product (e.g., desktop application with MySQL library). Web applications communicating over TCP/IP are not distribution — GPL does not apply.

MariaDB licensing

Variant	License	Price	Restrictions
MariaDB Community	GPL v2	$0	Same as MySQL Community — GPL, but without Oracle licensing risks
MariaDB Enterprise	Business Source License (BSL)	Subscription (~$2-5k/server/year)	Automatically converts to GPL v2 after 3 years. Includes enterprise features (ColumnStore, Spider, Xpand)
MariaDB SkySQL	Managed (BSL)	Pay-per-use (~$0.10-1.00/hour)	Fully managed DBaaS

Key difference from Oracle MySQL:

MariaDB is an independent fork, not controlled by Oracle
BSL model is more liberal — becomes open source after 3 years
MariaDB does not require commercial license for enterprise features (in MySQL they are enterprise-only)

When to use something else

Complex queries / CTE / window functions → PostgreSQL (more advanced optimizer)
GIS / geospatial data → PostgreSQL + PostGIS
Consistency > speed → PostgreSQL (SSI serializable)
High-throughput writes → Cassandra (MySQL master bottleneck)
Distributed SQL cluster → CockroachDB, Vitess (MySQL compatible sharding)

Sources

References, books, and standards: sources/databases/sources.md

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