Major Description
From this thread:
There's really a few points here, which may be different issues:
1. Either facet queries aren't using the filterCache correctly, or the stats don't reflect actual usage. (Or it's psychic.) Somehow, "lookups" only ever gets incremented when "hits" does, yielding a 100% cache hit rate at all times.
2. Facet queries appear to use the filterCache as a queryResultCache. Meaning, only identical facet queries cause filterCache "hits" to increase. Interestingly, disabling the queryResultCache still results in facet queries doing inserts into the filterCache, but no longer allows stats-reported usage of those entries.
If the stats are right and facet queries aren't actually using the filterCache for anything except possible future searches, then there should be a mechanism for disabling facet query filterCache usage to avoid filling the filterCache with low usage queries. Honestly though, that sounds more like something for the queryResultCache than filterCache anyway.
If facet queries are using the filterCache for performance within a single query, I'd suggest that facet queries should have their own named cache specifically for that use, rather than try to share a task load (size, regenerator) with the generic filterCache.