Mysql invalidating query
Also note – all queries are removed from cache on table modifications – if there are a lot of queries being cached this might reduce update speed a bit.Fragmentation over time – Over time Query Cache might get fragmented, which reduces performance. FLUSH QUERY CACHE command can be used for query cache defragmentation but it may block query cache for rather long time for large query caches, which might be unsuitable for online applications.My SQL has a great feature called “Query Cache” which is quite helpful for My SQL Performance optimization tasks but there are number of things you need to know. This means it is much more efficient as query which required processing millions of rows now can be instantly summoned from query cache.First let me clarify what My SQL Query Cache is – I’ve seen number of people being confused, thinking My SQL Query Cache is the same as Oracle Query Cache – meaning cache where execution plans are cached. It also means query has to be exactly the same and deterministic, so hit rate would generally be less. Query cache is great for certain applications, typically simple applications deployed on limited scale or applications dealing with small data sets.If you run in default mode you can also use SQL_NO_CACHE to block caching for certain queries, which you know do not need to be cached.Counting query cache efficiency There are few ways you can look at query_cache efficiency. As you can see we have to add Qcache_hits to Com_select to get total number of queries as if query cache hit happens Com_select is not incremented.Might not work with transactions – Different transactions may see different states of the database, depending on the updates they have performed and even depending on snapshot they are working on.
Avoid comment (and space) in the start of the query – Query Cache does simple optimization to check if query can be cached.Query cache catches network packets as they sent from client to the server, which means it can serve responses very fast doing no extra conversion or processing.Works before parsing – One more reason for high performance is Query Cache performs query lookup in the cache before query parsing, so if result is served from query cache, query parsing step is saved.But if you have just 20% Cache hit rate does it mean it is not worth it ?Not really it depends on which queries are cached, as well as overhead query cache provides.