LP:1021131 - dynamic setting query_cache_type but keeping the benefit of keeping away of overhead LOCK if query cache type is 0

As we know, query_cache overhead for LOCK can only be avoided when start-up mysql with query_cache_type is 0. That's what Percona contributed: http://www.percona.com/doc/percona-server/5.5/performance/query_cache_enhance.html?id=percona-server:features:query_cache_enhance#disabling_the_cache_completely

That's a nice work. However, if we want the query cache back, the mysqld must be restarted with query_cache_type=1. That's the problem, and some customers faced the scenarios.

I looked into the code, and found it might be resolved without the limitation of restarted mysqld. If we call is_disabled with a traced query_cache_type and combined with current setting query_cache_type, it's Okay to judge whether we could get the same result as old is_disabled function in Query_cache class.

Detail info could refer to qc.patch, which is based on Percona-5.5.18