What is ProxySQL and how does it help MySQL deployments?

ProxySQL is a high-performance, open-source MySQL proxy that sits between application servers and MySQL servers, providing several advanced features: (1) Read/Write splitting: automatically routes SELECT queries to read replicas and writes to the primary — no application code changes needed; (2) Connection pooling: multiplexes thousands of application connections into a smaller pool of MySQL connections, reducing MySQL connection overhead; (3) Query routing: route specific queries to specific servers based on regex rules — send reporting queries to a dedicated replica; (4) Query caching: cache query results in ProxySQL memory; (5) Query rewriting: modify queries on the fly (add limits, change parameters) without changing application code; (6) Failover handling: redirect queries away from failed servers automatically; (7) Query mirroring: send a copy of production traffic to a shadow server for testing; (8) Rate limiting and firewall: block or limit specific query patterns; (9) Connection multiplexing: allow 10,000 app connections sharing 200 MySQL connections. ProxySQL is configured via its own admin interface (mysql -u admin -padmin -h 127.0.0.1 -P 6032) and stores config in SQLite. It is used by many high-traffic MySQL deployments (GitHub uses it) and is compatible with Galera Cluster and Group Replication.