In the spirit of the Good Practice/Bad Practice series, this post is – apart from my first as contributing engineer at Open Query – a basic level application tip. More advanced application developers will already be aware of the issue.

It is easy to overlook certain “non-deterministic” functions in your queries which will prevent the query cache from caching its results. Common examples include NOW(), CURDATE(), and RAND() (here is the complete list).

Obviously when RAND() is used, you usually don’t expect the same result if the query is repeated, so this isn’t a concern. But other cases, particularly date/time related queries, can actually benefit from the cache, and there is a simple workaround. When the query cache is able to satisfy a query, the overhead (in latency and CPU) of parsing and planning the query is …