Faceting : what are the limitations of Taxonomy (Separate index and hierarchical facets) and SortedSetDocValuesFacetField ( flat facets and no sidecar index) ?

[Kumaran Ramasubramanian]

Hi All,
            We all know that Lucene supports faceting by providing
Taxonomy(Separate index and hierarchical facets) and
SortedSetDocValuesFacetField ( flat facets and no sidecar index).

      Then why did solr and elastic search go for its own implementation ?
 ( that is, solr uses block join & elasticsearch uses aggregations ) Is
there any limitations in lucene's implementation ?


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Kumaran R