Mysql caches
Documentation
MySQL: Query Cache https://dev.mysql.com/doc/refman/5.7/en/query-cache.html MyISAM: Key Cache https://dev.mysql.com/doc/refman/5.7/en/myisam-key-cache.html InnoDB: Buffer Pool https://dev.mysql.com/doc/refman/5.7/en/innodb-buffer-pool.html
General
Query Cache (<8.0)
NOTE:
It is disabled by default starting in MySQL-5.6
https://dev.mysql.com/blog-archive/mysql-8-0-retiring-support-for-the-query-cache/The query cache is a key-value store of cacheable-queries, and their result.
This interferes with benchmarking, since repeat queries may be much faster to lookup.To avoid the query cache:
# 1. Add a calculated-function to your selected rows - since they are not cacheable # TODO: validate SELECT ..., NOW() FROM ... # 2. Use 'SQL_NO_CACHE' in your query # (This often has not worked for me) SELECT SQL_NO_CACHE ... FROM ...It is implemented as an LRU key-value store for exact (byte-for-byte) query matches.
Each cache entry knows which tables it references for permissions, and cache-invalidation.
Whenever any row is changed in a table, all cache entries using that table are purged.SHOW VARIABLES LIKE 'query_cache_type'; # cache is 'ON/OFF/DEMAND' SHOW VARIABLES LIKE 'query_cache_size'; # max capacity of cacheThe OS Cache
TODO:
research. Apparently most prominent in MyISAM tables?
Table Cache
Used differently by different storage engines.
Engine Specific
InnoDB: Buffer Pool
Stores calculated hashes, row data, write buffers, locks etc.
InnoDB: Data Dictionary
MyISAM: Key Cache
Key Caches/Buffers store index values, and depend on the OS cache for row data.