When to use query_cache in MySQL?

Question

I have read the query_cache as a very important tool to improve query performance.

A general recommendation would be to have it enabled to on-demand (using SELECT SQL_CACHE, with a query_cache_type = 2 config setting).

• What type of circumstances would make the query_cache more optimal in MySQL?

Answer 1

The usefullness of the query-cache is mainly dependent on 4 things:

• How good are the chances for repetition of (nearly) identical queries?
• How often is data used in the repeated queries changed?
• Do those repeated queries have small result-sets?
• How long do the repeated queries take without query-caching?

A simple example: say you have a big table with 10 billion rows and you query a difficult "count" operation on it. It will last very long, give you a tiny result-set. Now if you repeatedly execute that query without changing the table too often, the benefit of query-caching is simply huge.

I hope the principles are somewhat clearer now.