Query Cache Thrashing (Legacy)
infoQuery cache experiencing high invalidation rate, causing more overhead than benefit. Frequent table updates invalidate cached queries, leading to cache thrashing. Note: Query cache is deprecated in MySQL 5.7.20 and removed in MySQL 8.0.
mysql.performance.qcache_lowmem_prunes rate high (>100/sec) with mysql.performance.qcache_inserts rate similar to prune rate, and mysql.performance.qcache.utilization < 50%
If on MySQL 5.7, consider disabling query cache (query_cache_type=0) as it's deprecated and often causes more harm than good in write-heavy workloads. Monitor mysql.performance.qcache_hits / (mysql.performance.qcache_hits + mysql.performance.com_select) to calculate hit ratio - if <20%, disable cache. If on MySQL 8.0+, query cache is not available. Implement application-level caching (Redis, Memcached) for better control and performance.