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c33d62adbc
Today the fielddata for global ordinals re-creates docvalues readers of each segment when building the iterator of a single segment. This is required because the lookup of global ordinals needs to access the docvalues's TermsEnum of each segment to retrieve the original terms. This also means that we need to create NxN (where N is the number of segment in the index) docvalues iterators each time we want to collect global ordinal values. This wasn't an issue in previous versions since docvalues readers are stateless before 6.0 so they are reused on each segment but now that docvalues are iterators we need to create a new instance each time we want to access the values. In order to avoid creating too many iterators this change splits the global ordinals fielddata in two classes, one that is used to cache a single instance per directory reader and one that is created from the cached instance that can be used by a single consumer. The latter creates the TermsEnum of each segment once and reuse them to create the segment's iterator. This prevents the creation of all TermsEnums each time we want to access the value of a single segment, hence reducing the number of docvalues iterator to create to Nx2 (one iterator and one lookup per segment).