Mysql 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.

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/

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

Overview

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 cache

Disabling

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 ...