Limited enabling of the query cache

[Elliott Barrere]

Hi there,

We have a large application with far too many queries to allow using the query cache globally. However, there are a few queries in the workload that are perfect for the qcache (limited UPDATES/INSERTS, relatively small, tons of hits).

My question is this: will enabling the query cache in DEMAND mode (sql_cache_type=DEMAND, setting SQL_CACHE only for the queries in question) pose substantial overhead for those thousands of queries that dont need or want caching? What exactly will that overhead be? (does MySQL involve the query cache at all for SELECTS when SQL_CACHE is not set? What is the overhead for UPDATES/INSERTS now?)

Thanks in advance for any help!