Log level warn is being used at . In case an index can't be used because of some filter not supported by that index, a warning is being locked. Though is a normal scenario as that query may have been created keeping another index in mind.
select [jcr:path], [jcr:score], [rep:facet(jcr:content/metadata/x)], [rep:facet(jcr:content/metadata/y)] from [z] as
a where isdescendantnode(a, '/content/p')
Lets assume we have facets configured on index definition z on properties which are not indexed in nt:base leading to warning log.