# 🐬 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):

```text
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

```sql
-- 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.en.md](sources/databases/sources.en.md)

### Recommended reading

| Book | Authors | ISBN | Description |
|------|---------|------|-------------|
| High Performance MySQL (4th ed.) | Schwartz, Zaitsev, Tkachenko | 978-1492075292 | Comprehensive guide to MySQL architecture, optimization, and monitoring |

*Last revision: 2026-06-03*