originally from https://github.com/cortexproject/cortex/blob/868898a2921c662dcd4f90683e8b95c927a8edd8/pkg/ingester/index/index.go but modified to support sharding queries.

PostingsForMatchers assembles a single postings iterator against the index reader based on the given matchers.

SelectMatchingProfiles returns a list iterator of profiles matching the given request.

SplitFiltersAndMatchers splits empty matchers off, which are treated as filters, see #220.

IndexReader provides reading access of serialized index data.