What is query caching in MySQL?

MySQL had a built-in query cache that cached the complete result set of SELECT queries. On subsequent identical queries (exact same SQL and database state), MySQL returned cached results without re-executing. However, the query cache was removed in MySQL 8.0 because it was a scalability bottleneck: (1) The cache required a global mutex lock on every write, causing contention on write-heavy workloads; (2) Any write to any table in the query invalidated all cached queries involving that table; (3) Modern hardware with fast SSDs and efficient buffer pools made the query cache less beneficial. Alternatives for caching: (1) Application-level cache: use Redis or Memcached to cache query results in your application code — more control over invalidation logic and expiry; (2) InnoDB Buffer Pool: MySQL's own page cache (default 128MB, should be 70-80% of available RAM for dedicated servers) caches disk pages in memory — properly sizing this is more impactful than query cache; (3) ProxySQL Query Cache — proxy layer that caches query results; (4) Read replicas reduce primary write contention. Always cache at the application layer for production systems.