Greg Miller created LUCENE-10204:
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Summary: Support iteration of sub-matches in join queries
(ToParentBlockJoinQuery / ToChildBlockJoinQuery)
Key: LUCENE-10204
URL: https://issues.apache.org/jira/browse/LUCENE-10204
Project: Lucene - Core
Issue Type: Improvement
Components: modules/join
Reporter: Greg Miller
It would be nice to be able to iterate over the "sub-matches" in these join
queries for the purpose of faceting (or possibly other use-cases?).
For example, we have a use-case where our query matches on "child" docs, using
a {{ToParentBlockJoinQuery}} to "emit" the associated parents, which are
ultimately added to our match set. But, we want to iterate over the matching
"children" for the purpose of faceting.
To make it concrete, consider searching over a product catalog where "offers"
and "items" are indexed side-by-side, with the offers being represented as
"children" of the parent items. An offer contains information like "condition"
(new vs. used), selling price, etc. for the parent item. If we want to facet on
"condition", we want to observe all children that matched the query to know if
the parent item had a "new" or "used" offer (or both). This requires iterating
over the child matches when faceting, which we cannot do today since the child
hit information isn't retained anywhere.
We can support this by "caching" the child hits in a bitset but there is some
complexity when multiple join queries appear in a query structure (would need
to logically combine various "cached" bitsets using the same boolean operations
as in the original query structure).
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