Query optimisation - multiple filter caches?

[Jonathan Woods]

I'm trying to understand how best to integrate directly with Solr
(Java-to-Java in the same JVM) to make the most of its query optimisation -
chiefly, its caching of queries which merely filter rather than rank
results.
 
I notice that SolrIndexSearcher maintains a filter cache and so does
LuceneQueryOptimiser.  Shouldn't they be contributing to/using the same
cache, or are they used for different things?
 
Jon