For specific query digests, we want to disable the caching behaviour for empty resultsets by setting cache_empty_result to "0".

    {
        rule_id=1
        active=1
        digest="0x12345678ABCDEF" 
        apply=1
        cache_ttl=10000
        cache_empty_result=0
    },

When looking at the stats_mysql_query_digest table, it looks like nothing is cached. There are no records with hostgroup -1, indicating that nothing is served from the cache, even though there are non-empty results.

Looking at the code in MySQL_Session.cpp, it seems to me that it's missing a check for cache_empty_result==0. So, it's possible to enable the caching of empty results for individual rules when the global setting is disabled, but not the other way around.