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Whenever My SQL performs a SELECT operation, it either increments com_select or the qcache_hits status variables. show status like 'qcache_hits'; --------------- ------------ | Variable_name | Value | --------------- ------------ | Qcache_hits | 1446235295 | --------------- ------------ 1 row in set (0.05 sec) mysql So get the hit ratio by this formula: qcache_hits / (qcache_hits com_select) which gives us .9999 or 99.99%. Check qcache_free_memory status variable, is it also high? Imagine, each query that is cached must be a minimum of query_cache_min_res_unit.

My SQL's query cache is somewhat unique and can be very beneficial to certain applications, such as web apps that run the same queries over and over.

The My SQL query cache is a simple operation and it only caches is not always the same.

The query cache stores a hash of the query along with the result set, this means that the query must be the same byte for byte for it to match in the cache this includes matching case and whitespaces, etc.

Use this command: show variables like 'query%'; ------------------------------ ----------- | Variable_name | Value | ------------------------------ ----------- | query_alloc_block_size | 8192 | | query_cache_limit | 1048576 | | query_cache_min_res_unit | 1024 | | query_cache_size | 256000000 | | query_cache_type | ON | | query_cache_wlock_invalidate | OFF | | query_prealloc_size | 8192 | ------------------------------ ----------- 7 rows in set (0.00 sec) Recall that we described above the maximum size query that the query cache will allow to be cached. If there are very large reports with large result sets, you probably don't want them to clog up your query cache. The biggest problem most untuned query caches will have is either that they (a) aren't big enough, or (b) have fragmentation.

Setting the query cache size can be done by looking at the query cache hit ratio. Sometimes our query cache is quite large; however, we're not getting a high hit ratio.

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So if you think that users are using the same queries over and over again you can turn this option on and then test thoroughly, in some cases it will produce amazing performance improvements.So queries need to be invalidated when data changes.In order to make this fast and easy, it's done on a table-by-table basis; when a table changes, all queries that used that table are removed.So the main things to keep in mind when tuning the query cache are (a) the total size of the query cache (b) the average size of query result sets, and (c) the size of queries above which you don't want to keep cached.Each of these three things can be measured, and once set - assuming your application footprint doesn't change dramatically - should remain fairly static. The query_cache_type variable should be set to ON, and the query_cache_size should be non-zero.