reuse DocValues iterators in SolrDocumentFetcher/udvas

[magibney]

No description provided.

[magibney]

This is very upstreamable IMO. It basically ports a similar dv iter reuse technique (already used for streaming, iirc) to SolrDocumentFetcher (to be used in conjunction with useDocValuesAsStored). I mention this because it might be possible to merge this to fs/branch_9_3 and fs/branch_9_2 instead, pursue upstream to ultimately end up on fs/branch_9x, and if that takes too long, we can always forward-port.

[justinrsweeney]

LGTM! One note in terms of upstreaming that probably isn't worth the effort right now. I wonder if we could consolidate approaches between this and the streaming. I haven't dug into it much, but I think this

fullstory-solr/solr/core/src/java/org/apache/solr/handler/export/StringFieldWriter.java

Line 85 in 5465b01

// Reuse the last DocValues object if possible

might be where it is handled in streaming.

Maybe not a way to overlap, but just a thought.

Also +1 on merging to branch_9_3 and branch_9_2 and then upstreaming the change to get on 9x.